def __init__(self, simulation, **kwargs):
## Initialize the replica
super(RaftReplica, self).__init__(simulation, **kwargs)
## Initialize Raft Specific settings
self.state = State.FOLLOWER
self.currentTerm = 0
self.votedFor = None
self.log = MultiObjectWriteLog()
self.cache = {}
## Policies
self.read_policy = ReadPolicy.get(kwargs.get('read_policy', READ_POLICY))
self.aggregate_writes = kwargs.get('aggregate_writes', AGGREGATE_WRITES)
## Timers for work
eto = kwargs.get('election_timeout', ELECTION_TIMEOUT)
hbt = kwargs.get('heartbeat_interval', HEARTBEAT_INTERVAL)
self.timeout = ElectionTimer.fromReplica(self, eto)
self.heartbeat = Timer(self.env, hbt, self.on_heartbeat_timeout)
## Leader state
self.nextIndex = None
self.matchIndex = None
def gossip(self):
"""
Randomly select a neighbor and exchange information about the state
of the latest entries in the log since the last anti-entropy delay.
"""
# Gossip to one node at each location
for location in self.locations:
# Don't gossip to nodes in self!
if location == self.location: continue
# Select a random target to gossip to
target = random.choice(list(self.remotes(location)))
# Log the gossip that's happening
self.sim.logger.debug("{} gossiping {} entries to {}".format(
self, len(self.ae_cache), target))
entries = tuple([
Write(version.name, self, version) for version in self.ae_cache
])
# Send all the values in the cache.
self.send(target, Gossip(entries, len(self.ae_cache), -1))
# Empty the cache on gossip
self.ae_cache = []
# Reset the anti-entropy timer
self.ae_timer = Timer(self.env, self.ae_delay, self.gossip)
self.ae_timer.start()
def get_anti_entropy_timeout(self):
"""
Creates the anti-entropy timeout.
In the future this could be random timeout not fixed.
"""
self.timeout = Timer(self.env, self.ae_delay, self.gossip)
return self.timeout.start()
def handle_session(self):
"""
Starts a session timer if one isn't running, otherwise resets the
currently running session timer on an additional access.
"""
if not self.session:
self.session = Timer(
self.env, self.session_timeout,
partial(self.on_session_timeout, self.env.now))
else:
self.session = self.session.reset()
def run(self):
"""
We have to check in at every heartbeat interval. If we own a tag then
send a heartbeat message, otherwise just keep quiescing.
"""
while True:
if self.state == State.OWNER:
self.heartbeat = Timer(self.env, self.heartbeat_interval,
self.on_heartbeat_timeout)
yield self.heartbeat.start()
else:
yield self.env.timeout(self.heartbeat_interval)
def gossip(self):
"""
Randomly select a neighbor and exchange information about the state
of the latest entries in the log since the last anti-entropy delay.
"""
# Gossip to one node at each location
for location in self.locations:
# Don't gossip to nodes in self!
if location == self.location: continue
# Select a random target to gossip to
target = random.choice(list(self.remotes(location)))
# Log the gossip that's happening
self.sim.logger.debug(
"{} gossiping {} entries to {}".format(
self, len(self.ae_cache), target
)
)
entries = tuple([
Write(version.name, self, version)
for version in self.ae_cache
])
# Send all the values in the cache.
self.send(target, Gossip(entries, len(self.ae_cache), -1))
# Empty the cache on gossip
self.ae_cache = []
# Reset the anti-entropy timer
self.ae_timer = Timer(self.env, self.ae_delay, self.gossip)
self.ae_timer.start()
def __init__(self, simulation, **kwargs):
## Initialize the replica
super(RaftReplica, self).__init__(simulation, **kwargs)
## Initialize Raft Specific settings
self.state = State.FOLLOWER
self.currentTerm = 0
self.votedFor = None
self.log = MultiObjectWriteLog()
self.cache = {}
## Policies
self.read_policy = ReadPolicy.get(kwargs.get("read_policy", READ_POLICY))
self.aggregate_writes = kwargs.get("aggregate_writes", AGGREGATE_WRITES)
## Timers for work
eto = kwargs.get("election_timeout", ELECTION_TIMEOUT)
hbt = kwargs.get("heartbeat_interval", HEARTBEAT_INTERVAL)
self.timeout = ElectionTimer.fromReplica(self, eto)
self.heartbeat = Timer(self.env, hbt, self.on_heartbeat_timeout)
## Leader state
self.nextIndex = None
self.matchIndex = None
def get_anti_entropy_timeout(self):
"""
Creates the anti-entropy timeout.
In the future this could be random timeout not fixed.
"""
self.timeout = Timer(self.env, self.ae_delay, self.gossip)
return self.timeout.start()
def handle_session(self):
"""
Starts a session timer if one isn't running, otherwise resets the
currently running session timer on an additional access.
"""
if not self.session:
self.session = Timer(
self.env, self.session_timeout,
partial(self.on_session_timeout, self.env.now)
)
else:
self.session = self.session.reset()
def on_state_change(self):
"""
Does the same stuff as super, but also - if leader; starts the anti
entropy interval to do gossiping.
"""
super(FloatedRaftReplica, self).on_state_change()
if self.state in (State.FOLLOWER, State.CANDIDATE):
if hasattr(self, 'ae_timer') and self.ae_timer is not None:
# Cancel the anti-entropy timer.
self.ae_timer.stop()
self.ae_timer = None
elif self.state == State.LEADER:
self.ae_timer = Timer(self.env, self.ae_delay, self.gossip)
self.ae_timer.start()
elif self.state == State.READY:
# This happens on the call to super, just ignore for now.
pass
else:
raise SimulationException(
"Unknown Floating Raft State: {!r} set on {}".format(
self.state, self))
def run(self):
"""
We have to check in at every heartbeat interval. If we own a tag then
send a heartbeat message, otherwise just keep quiescing.
"""
while True:
if self.state == State.OWNER:
self.heartbeat = Timer(
self.env, self.heartbeat_interval, self.on_heartbeat_timeout
)
yield self.heartbeat.start()
else:
yield self.env.timeout(self.heartbeat_interval)
def on_state_change(self):
"""
Does the same stuff as super, but also - if leader; starts the anti
entropy interval to do gossiping.
"""
super(FloatedRaftReplica, self).on_state_change()
if self.state in (State.FOLLOWER, State.CANDIDATE):
if hasattr(self, 'ae_timer') and self.ae_timer is not None:
# Cancel the anti-entropy timer.
self.ae_timer.stop()
self.ae_timer = None
elif self.state == State.LEADER:
self.ae_timer = Timer(self.env, self.ae_delay, self.gossip)
self.ae_timer.start()
elif self.state == State.READY:
# This happens on the call to super, just ignore for now.
pass
else:
raise SimulationException(
"Unknown Floating Raft State: {!r} set on {}".format(self.state, self)
)
class FloatedRaftReplica(RaftReplica):
def __init__(self, simulation, **kwargs):
## Initialize the replica
super(FloatedRaftReplica, self).__init__(simulation, **kwargs)
# Anti entropy settings
self.ae_delay = kwargs.get('anti_entropy_delay', ANTI_ENTROPY_DELAY)
self.ae_timer = None
self.ae_cache = []
@memoized
def locations(self):
"""
Returns all the locations in the network with Raft nodes.
"""
return set([
node.location for node in self.neighbors(self.consistency)
])
def quorum(self):
"""
Returns only nodes in the same location to do Raft consensus with.
"""
# Filter only connections that are in the same consistency group
for node in self.neighbors(self.consistency):
if node.location == self.location:
yield node
# Don't forget to yield self!
yield self
def remotes(self, location=None):
"""
Returns only nodes that are not in the same location to float writes
to using anti-entropy. This method is only used by the leader.
Can also specify a specific location to fetch the remotes for. Note
that specifying your current location will not return nodes.
"""
# Filter only connections that are in the same consistency group
for node in self.neighbors(self.consistency):
if node.location != self.location:
if location is not None and node.location != location:
continue
yield node
def gossip(self):
"""
Randomly select a neighbor and exchange information about the state
of the latest entries in the log since the last anti-entropy delay.
"""
# Gossip to one node at each location
for location in self.locations:
# Don't gossip to nodes in self!
if location == self.location: continue
# Select a random target to gossip to
target = random.choice(list(self.remotes(location)))
# Log the gossip that's happening
self.sim.logger.debug(
"{} gossiping {} entries to {}".format(
self, len(self.ae_cache), target
)
)
entries = tuple([
Write(version.name, self, version)
for version in self.ae_cache
])
# Send all the values in the cache.
self.send(target, Gossip(entries, len(self.ae_cache), -1))
# Empty the cache on gossip
self.ae_cache = []
# Reset the anti-entropy timer
self.ae_timer = Timer(self.env, self.ae_delay, self.gossip)
self.ae_timer.start()
######################################################################
## Event Handlers
######################################################################
def on_state_change(self):
"""
Does the same stuff as super, but also - if leader; starts the anti
entropy interval to do gossiping.
"""
super(FloatedRaftReplica, self).on_state_change()
if self.state in (State.FOLLOWER, State.CANDIDATE):
if hasattr(self, 'ae_timer') and self.ae_timer is not None:
# Cancel the anti-entropy timer.
self.ae_timer.stop()
self.ae_timer = None
elif self.state == State.LEADER:
self.ae_timer = Timer(self.env, self.ae_delay, self.gossip)
self.ae_timer.start()
elif self.state == State.READY:
# This happens on the call to super, just ignore for now.
pass
else:
raise SimulationException(
"Unknown Floating Raft State: {!r} set on {}".format(self.state, self)
)
def on_gossip_rpc(self, message):
"""
Handles the receipt of a gossip from another node. Expects multiple
accesses (Write events) as entries. Goes through all and compares the
versions, replying False only if there is an error or a conflict.
"""
entries = message.value.entries
# Go through the entries from the RPC and write to local cluster.
for access in entries:
access.version.gossiped = True
self.write(access)
# Should we return with what's in our cache?
# Respond to the sender
self.send(message.source, GossipResponse([], 0, True, -1))
def on_response_rpc(self, message):
"""
Just receives the acknowledgment of the response.
"""
pass
def on_ae_response_rpc(self, msg):
"""
Does the same stuff that the super handler does, but also caches
commits to gossip about them later!
"""
rpc = msg.value
if self.state == State.LEADER:
if rpc.success:
self.nextIndex[msg.source] = rpc.lastLogIndex + 1
self.matchIndex[msg.source] = rpc.lastLogIndex
else:
# Decrement next index and retry append entries
self.nextIndex[msg.source] -= 1
self.send_append_entries(msg.source)
# Decide if we can commit the entry
for n in xrange(self.log.lastApplied, self.log.commitIndex, -1):
commit = Election(self.matchIndex.keys())
for k, v in self.matchIndex.iteritems():
commit.vote(k, v >= n)
if commit.has_passed() and self.log[n][1] == self.currentTerm:
# Commit all versions from the last log entry to now.
for idx in xrange(self.log.commitIndex, n+1):
if self.log[idx][0] is None: continue
# Cache the version to anti-entropy!
version = self.log[idx][0]
if not hasattr(version, 'gossiped') or not version.gossiped:
self.ae_cache.append(version)
self.log[idx][0].update(self, commit=True)
# Set the commit index and break
self.log.commitIndex = n
break
elif self.state == State.CANDIDATE:
# Decide whether or not to step down.
if rpc.term >= self.currentTerm:
## Become a follower
self.state = State.FOLLOWER
## Log the failed election
self.sim.logger.info(
"{} has stepped down as candidate".format(self)
)
return
elif self.state == State.FOLLOWER:
# Ignore AE messages if we are the follower.
return
else:
raise RaftRPCException(
"Append entries response in unknown state: '{}'".format(self.state)
)
def read(self, name, **kwargs):
"""
When a tag replica performs a read it has to decide whether or not to
read locally or to make a remote read across the cluster.
Convert the read into an access, then check if we own the object.
If we do, then return the latest commit.
If we don't and no one else does either, attempt to acquire the tag.
If we don't and someone else does then either drop, wait, or remote.
Current implementation: #2, MR, no remote access.
If someone else owns tag, reads are dropped.
TODO: Remote vs Local Reads
"""
# Create the read event using super.
access = super(TagReplica, self).read(name, **kwargs)
# Record the number of attempts for the access
if access.is_local_to(self): access.attempts += 1
# Increase the session on access.
self.handle_session()
# Are we the owner of this tag?
if self.owns(access.name):
# TODO: Change to last commit!
version = self.log[access.name].lastVersion
# If the version is None, bail since we haven't read anything
if version is None: return access.drop(empty=True)
# Update the version, complete the read, and log the access
access.update(version, completed=True)
access.log(self)
# Return, we're done reading!
return access
# Is there a different owner for the tag?
owner = self.find_owner(access.name)
if owner is not None:
# Right now just drop the read on its face.
self.sim.logger.info("ownership conflict: dropped {} at {}".format(
access, self))
return access.drop()
# We're going to acquire the tag!
else:
# Log the access from this particular replica.
access.log(self)
# We're going to have some read latency, retry the read.
retry = Timer(self.env, self.heartbeat_interval,
lambda: self.read(access)).start()
if access.attempts <= 1 and self.state != State.TAGGING:
# Request the ownership of the tag
self.acquire(access.name)
return access
class TagReplica(ConsensusReplica):
def __init__(self, simulation, **kwargs):
## Timers for work
self.session_timeout = kwargs.get('session_timeout', SESSION_TIMEOUT)
self.heartbeat_interval = kwargs.get('heartbeat_interval',
HEARTBEAT_INTERVAL)
self.session = None
self.heartbeat = None
## Initialze the tag specific settings
self.epoch = 0
self.log = defaultdict(WriteLog)
self.view = defaultdict(set)
## Owner state
self.nextIndex = None
self.matchIndex = None
## Initialize the replica
super(TagReplica, self).__init__(simulation, **kwargs)
self.state = State.READY
######################################################################
## Core Methods (Replica API)
######################################################################
def read(self, name, **kwargs):
"""
When a tag replica performs a read it has to decide whether or not to
read locally or to make a remote read across the cluster.
Convert the read into an access, then check if we own the object.
If we do, then return the latest commit.
If we don't and no one else does either, attempt to acquire the tag.
If we don't and someone else does then either drop, wait, or remote.
Current implementation: #2, MR, no remote access.
If someone else owns tag, reads are dropped.
TODO: Remote vs Local Reads
"""
# Create the read event using super.
access = super(TagReplica, self).read(name, **kwargs)
# Record the number of attempts for the access
if access.is_local_to(self): access.attempts += 1
# Increase the session on access.
self.handle_session()
# Are we the owner of this tag?
if self.owns(access.name):
# TODO: Change to last commit!
version = self.log[access.name].lastVersion
# If the version is None, bail since we haven't read anything
if version is None: return access.drop(empty=True)
# Update the version, complete the read, and log the access
access.update(version, completed=True)
access.log(self)
# Return, we're done reading!
return access
# Is there a different owner for the tag?
owner = self.find_owner(access.name)
if owner is not None:
# Right now just drop the read on its face.
self.sim.logger.info("ownership conflict: dropped {} at {}".format(
access, self))
return access.drop()
# We're going to acquire the tag!
else:
# Log the access from this particular replica.
access.log(self)
# We're going to have some read latency, retry the read.
retry = Timer(self.env, self.heartbeat_interval,
lambda: self.read(access)).start()
if access.attempts <= 1 and self.state != State.TAGGING:
# Request the ownership of the tag
self.acquire(access.name)
return access
def write(self, name, **kwargs):
"""
When a replica performs a write it needs to decide if it can write to
the tag locally, can acquire a tag for this object, or if it has to do
something else like drop, wait, or remote write.
If the access is local:
- if the replica owns the tag, append and complete
- if someone else owns the tag then drop, wait, or remote
- if no one owns the tag, then attempt to acquire it
If access is remote:
- if we own the tag, then append but do not complete (at local)
- if someone else owns the tag, log and forward to owner
- if no one owns the tag then respond false
"""
# Create the read event using super.
access = super(TagReplica, self).write(name, **kwargs)
# Increase the session on access.
self.handle_session()
# Determine if the write is local or remote
if access.is_local_to(self):
# Record the number of attempts for the access
access.attempts += 1
# Fetch the latest version from the log.
latest = self.log[access.name].lastVersion
# Perform the write
if latest is None:
version = namespace(access.name)(self)
else:
version = latest.nextv(self)
# Update the access with the latest version
access.update(version)
else:
# If there is no version, raise an exception
if access.version is None:
raise AccessError(
"Attempting a remote write on {} without a version!".
format(self))
# Save the version variable for use below.
version = access.version
# Log the access at this replica
access.log(self)
# Are we the owner of this tag?
if self.owns(access.name):
# Perform the append entries
self.log[name].append(version, self.epoch)
# Update the version to track visibility latency
version.update(self)
# Complete the access if it was local
if access.is_local_to(self): access.complete()
# Now do AppendEntries
# Also interrupt the heartbeat since we just sent AppendEntries
if not settings.simulation.aggregate_writes:
self.send_append_entries()
if self.heartbeat: self.heartbeat.stop()
return access
# Is there a different owner for the tag?
owner = self.find_owner(name)
if owner is not None:
# Right now just drop the write on its face.
self.sim.logger.info("ownership conflict: dropped {} at {}".format(
access, self))
return access.drop()
# We're going to acquire the tag!
else:
# We're going to have some write latency, retry the write.
retry = Timer(self.env, self.heartbeat_interval,
lambda: self.write(access)).start()
# Request the ownership of the tag
self.acquire(access.name)
return access
def run(self):
"""
We have to check in at every heartbeat interval. If we own a tag then
send a heartbeat message, otherwise just keep quiescing.
"""
while True:
if self.state == State.OWNER:
self.heartbeat = Timer(self.env, self.heartbeat_interval,
self.on_heartbeat_timeout)
yield self.heartbeat.start()
else:
yield self.env.timeout(self.heartbeat_interval)
######################################################################
## Helper Methods
######################################################################
def owns(self, name):
"""
Returns True if the name is in the current view for that owner.
"""
return name in self.view[self]
def find_owner(self, name):
"""
Looks up the owner of the name in the current view.
Returns None if there is no owner fo the tag.
"""
for owner, tag in self.view.items():
if name in tag:
return owner
return None
def acquire(self, tag):
"""
Sends out the acquire tag RPC
"""
# Construct the tag to send out
if not isinstance(tag, (set, frozenset)):
tag = frozenset([tag])
# Make sure to request the tag we already have
tag = frozenset(self.view[self] | tag)
# Request tag with all current tags
self.send_tag_request(tag)
# Log the tag acquisition
self.sim.logger.info("{} is atempting to acquire tag {}".format(
self, self.tag))
def release(self, tag=None):
"""
Sends out the release tag RPC
"""
# Release all currently held tags
if tag is None: tag = self.view[self]
# Construct the tag to send out (if specified)
if not isinstance(tag, (set, frozenset)):
tag = frozenset([tag])
# Request the difference of the tags we already have
tag = frozenset(self.view[self] - tag)
# Request tag with all current tags
self.send_tag_request(tag)
# Log the tag release
self.sim.logger.info("{} is atempting to release tag {}".format(
self, tag))
def handle_session(self):
"""
Starts a session timer if one isn't running, otherwise resets the
currently running session timer on an additional access.
"""
if not self.session:
self.session = Timer(
self.env, self.session_timeout,
partial(self.on_session_timeout, self.env.now))
else:
self.session = self.session.reset()
def get_log_state(self, tag=None):
"""
Constructs a log state object for append entries responses, either
for the current tag or simply the current view.
"""
if tag is None:
tag = [obj for view in self.view.values() for obj in view]
return {
obj: LogState(self.log[obj].lastApplied, self.log[obj].lastTerm,
self.log[obj].commitIndex)
for obj in tag
}
def send_tag_request(self, tag):
"""
Broadcasts a tag request for the passed in tag.
"""
# Change state to tagging and save tag locally
self.state = State.TAGGING
self.tag = tag
# Request the entire tag in your current view.
tagset = {owner.id: tagset for owner, tagset in self.view.items()}
tagset[self.id] = self.tag
# Send the tag request RPC to each neighbor
rpc = RequestTag(self.epoch, tagset, self)
for neighbor in self.neighbors():
self.send(neighbor, rpc)
def send_append_entries(self, target=None):
"""
Helper function to send append entries to quorum or a specific node.
Note: fails silently if target is not in the neighbors list.
"""
# ownership check
if not self.state == State.OWNER:
return
# Go through follower list.
for node, objs in self.nextIndex.iteritems():
# Filter based on the target supplied.
if target is not None and node != target:
continue
# Construct the entries, or empty for heartbeat
# The tag contains the state of each item to be sent
entries = defaultdict(list)
tag = defaultdict(LogState)
for obj, nidx in objs.items():
# A rule directly from the Raft paper
if self.log[obj].lastApplied >= nidx:
entries[obj] = self.log[obj][nidx:]
# Compute the previous log index and term
prevLogIndex = nidx - 1
prevLogTerm = self.log[obj][prevLogIndex].term
commitIndex = self.log[obj].commitIndex
# Create the tag state
tag[obj] = LogState(prevLogIndex, prevLogTerm, commitIndex)
# Send the append entries message
self.send(node, AppendEntries(self.epoch, self.id, tag, entries))
######################################################################
## Event Handlers
######################################################################
def on_state_change(self):
"""
Setting the state decides how the Tag node will interact.
"""
# Do state specific tag modifications
if self.state == State.READY:
self.votes = None
self.tag = None
# Remove owner state
self.nextIndex = None
self.matchIndex = None
# Also interrupt the heartbeat
if self.heartbeat: self.heartbeat.stop()
elif self.state == State.TAGGING:
# Convert to tag acquisition/release
self.epoch += 1
# Create election and vote for self
self.votes = Election([node.id for node in self.quorum()])
self.votes.vote(self.id)
# Also interrupt the heartbeat
if self.heartbeat: self.heartbeat.stop()
elif self.state == State.OWNER:
# Create the next index and match index
self.nextIndex = {
node: {
obj: self.log[obj].lastApplied + 1
for obj in self.view[self]
}
for node in self.neighbors()
}
self.matchIndex = {
node: {obj: 0
for obj in self.view[self]}
for node in self.neighbors()
}
else:
raise SimulationException(
"Unknown Tag Replica State: {!r} set on {}".format(
state, self))
def on_heartbeat_timeout(self):
"""
Time to send a heartbeat message to all tags.
"""
if not self.state == State.OWNER:
return
# Send heartbeat or aggregated writes
self.send_append_entries()
def on_session_timeout(self, started):
"""
If the session times out then go ahead and release the tag.
"""
duration = self.env.now - started
self.sim.logger.info("session on {} terminated at {} ({} ms)".format(
self.id, self.env.now, duration))
self.sim.results.update('session length', (self.id, duration))
self.session = None
self.release()
def on_request_tag_rpc(self, msg):
"""
Respond to a request for a tag acquisition from a server.
"""
rpc = msg.value
accept = True
# The requested epoch must be less than or greater than local.
if rpc.epoch < self.epoch: accept = False
# Ensure that no one else owns the tag in your current view.
for candidate, tagset in rpc.tag.items():
# Short circuit
if not accept: break
for tag in tagset:
owner = self.find_owner(tag)
if owner is not None and owner.id != candidate:
accept = False
break
# Log the vote decision
amsg = "accepted" if accept else "did not accept"
lmsg = "{} {} tag [{}] for {}".format(
self, amsg, ",".join(rpc.tag[rpc.candidate.id]), rpc.candidate.id)
self.sim.logger.info(lmsg)
# Send the vote response back to the tag requester
return self.send(msg.source, TagResponse(self.epoch, accept))
def on_tag_response_rpc(self, msg):
"""
Handle the votes from tag requests to other nodes.
"""
rpc = msg.value
if self.state == State.TAGGING:
# If the epoch is greater than the current epoch
if rpc.epoch > self.epoch:
# Retry the tag request
self.epoch = rpc.epoch
self.send_tag_request(self.tag)
self.sim.logger.info("{} retrying tag request for {}".format(
self, self.tag))
# Exit: no more work required!
return
# Update the current election
self.votes.vote(msg.source.id, rpc.accept)
if self.votes.has_passed():
# Update our local tag and become owner.
if self.tag:
self.state = State.OWNER
self.view[self] = set(self.tag)
else:
self.state = State.READY
# Send out the ownership change append entries
self.send_append_entries()
# Log the new tag owner
self.sim.logger.info("{} tag goes to: {}".format(
self, self.view[self]))
# Record tag length over time
self.sim.results.update(
'tag size', (self.id, self.env.now, len(self.view[self])))
elif self.state in (State.READY, State.OWNER):
# Ignore vote responses if we've changed our state
return
else:
raise TagRPCException(
"Tag request response in unknown state: '{}'".format(
self.state))
def on_append_entries_rpc(self, msg):
rpc = msg.value
# reply false if the epoch < current epoch
if rpc.epoch < self.epoch:
self.sim.logger.info(
"{} doesn't accept append entries in epoch {} for epoch {}".
format(self, self.epoch, rpc.epoch))
# Send back the request that you made originally.
return self.send(
msg.source,
AEResponse(self.epoch, {obj: False
for obj in rpc.tag.keys()}, rpc.tag,
Reason.EPOCH))
# Update the view to match the view of the append entries
# Update the epoch to match the rpc of the append entries
self.view[msg.source] = set(rpc.tag.keys())
if self.epoch < rpc.epoch:
self.epoch = rpc.epoch
# Now for each object in the RPC, perform Raft-like append entries.
# The success tracking is a complete tracking for all objects, will
# return false even if we need to update the log for only one thing.
# We will reply back with a state object that has per-object details.
success = defaultdict(bool)
state = defaultdict(LogState)
for obj, prev in rpc.tag.items():
entries = rpc.entries[obj]
objlog = self.log[obj]
# If log doesn't contain an entry at prev index matching epoch.
if objlog.lastApplied < prev.index or objlog[
prev.index].term != prev.epoch:
# Perform the logging of this state failure
if objlog.lastApplied < prev.index:
self.sim.logger.info(
"{} doesn't accept append to {} index {} where last applied is {}"
.format(self, obj, prev.index, objlog.lastApplied))
else:
self.sim.logger.info(
"{} doesn't accept append to {} due to epoch mismatch: {} vs {}"
.format(self, obj, prev.epoch,
objlog[prev.index].term))
# Mark that there is a problem and continue
success[obj] = False
state[obj] = LogState(objlog.lastApplied, objlog.lastTerm,
objlog.lastCommit)
continue
# At this point the entries are accepted because of continue statements
if entries:
if objlog.lastApplied >= prev.index:
# If existing entry conflicts with a new one (same index, different epochs)
# Delete the existing entry and all that follow it.
if objlog[prev.index].term != prev.epoch:
objlog.truncate(prev.index)
if objlog.lastApplied > prev.index:
# Better look into what's happening here!
raise TagRPCException(
"{} is possibly receiving duplicate append entries".
format(self))
# Append any new entries not already in the log.
for entry in entries:
# Add the entry/epoch to the log
objlog.append(*entry)
# Update the versions to compute visibilities
entry[0].update(self)
# Log the last write from the append entries
self.sim.logger.debug(
"appending {} entries to {} log on {} (term {}, commit {})"
.format(len(entries), obj, self, objlog.lastTerm,
objlog.commitIndex))
# Update the commit index and save the state of the object.
if prev.commit > objlog.commitIndex:
objlog.commitIndex = min(prev.commit, objlog.lastApplied)
success[obj] = True
state[obj] = LogState(objlog.lastApplied, objlog.lastTerm,
objlog.lastCommit)
# Return the response back to the owner
reason = Reason.OK if all(success.values()) else Reason.LOG
return self.send(msg.source,
AEResponse(self.epoch, success, state, reason))
def on_ae_response_rpc(self, msg):
"""
Handles acknowledgment of append entries messages.
"""
rpc = msg.value
retry = False
if self.state == State.OWNER:
# Update state of followers in the tag group
for obj, success in rpc.success.items():
if success:
self.nextIndex[msg.source][obj] = rpc.tag[obj].index + 1
self.matchIndex[msg.source][obj] = rpc.tag[obj].index
else:
# If the epoch is not the same, update accordingly.
if rpc.epoch > self.epoch:
self.epoch = rpc.epoch
# If the failure was because of the epoch, simply retry.
if rpc.reason == Reason.EPOCH:
retry = True
# Otherwise decrement the next index and to retry
elif rpc.reason == Reason.LOG:
self.nextIndex[msg.source][obj] -= 1
retry = True
else:
raise TagRPCException(
"Unknown append entries failure reason: {}".format(
rpc.reason))
# Determine if we can commit the entry
for obj, state in rpc.tag.items():
log = self.log[obj]
for n in xrange(log.lastApplied, log.commitIndex, -1):
commit = Election(self.matchIndex.keys())
for node, objs in self.matchIndex.items():
match = objs[obj]
commit.vote(node, match >= n)
if commit.has_passed() and log[n].term == self.epoch:
# Commit all versions from the last log to now.
for idx in xrange(log.commitIndex, n + 1):
if not log[idx].version: continue
log[idx].version.update(self, commit=True)
# Set the commit index and break
log.commitIndex = n
break
# If retry, send append entries back to the source.
if retry: self.send_append_entries(msg.source)
elif self.state == State.TAGGING:
# Determine if we need to retry the tagging again.
if rpc.epoch > self.epoch:
# Retry the tag request
self.epoch = rpc.epoch
self.send_tag_request(self.tag)
self.sim.logger.info("{} retrying tag request for {}".format(
self, self.tag))
return
elif self.state == State.READY:
# Ignore AE messages if we're not an owner anymore.
return
else:
raise TagRPCException("Response in unknown state: '{}'".format(
self.state))
def on_remote_access(self, msg):
"""
Handles remote writes to and from the replicas.
"""
access = msg.value.access
# Ensure that we own the object
if not self.owns(access.name):
return self.send(msg.source,
AccessResponse(self.epoch, False, access))
# If we do own the object, then respond:
method = {
'read': self.read,
'write': self.write,
}[access.type]
# Call the remote method with the access.
method(access)
return self.send(msg.source, AccessResponse(self.epoch, True, access))
def on_access_response_rpc(self, msg):
"""
Handles responses to remote accesses.
"""
rpc = msg.value
if rpc.success:
rpc.access.complete()
class FloatedRaftReplica(RaftReplica):
def __init__(self, simulation, **kwargs):
## Initialize the replica
super(FloatedRaftReplica, self).__init__(simulation, **kwargs)
# Anti entropy settings
self.ae_delay = kwargs.get('anti_entropy_delay', ANTI_ENTROPY_DELAY)
self.ae_timer = None
self.ae_cache = []
@memoized
def locations(self):
"""
Returns all the locations in the network with Raft nodes.
"""
return set(
[node.location for node in self.neighbors(self.consistency)])
def quorum(self):
"""
Returns only nodes in the same location to do Raft consensus with.
"""
# Filter only connections that are in the same consistency group
for node in self.neighbors(self.consistency):
if node.location == self.location:
yield node
# Don't forget to yield self!
yield self
def remotes(self, location=None):
"""
Returns only nodes that are not in the same location to float writes
to using anti-entropy. This method is only used by the leader.
Can also specify a specific location to fetch the remotes for. Note
that specifying your current location will not return nodes.
"""
# Filter only connections that are in the same consistency group
for node in self.neighbors(self.consistency):
if node.location != self.location:
if location is not None and node.location != location:
continue
yield node
def gossip(self):
"""
Randomly select a neighbor and exchange information about the state
of the latest entries in the log since the last anti-entropy delay.
"""
# Gossip to one node at each location
for location in self.locations:
# Don't gossip to nodes in self!
if location == self.location: continue
# Select a random target to gossip to
target = random.choice(list(self.remotes(location)))
# Log the gossip that's happening
self.sim.logger.debug("{} gossiping {} entries to {}".format(
self, len(self.ae_cache), target))
entries = tuple([
Write(version.name, self, version) for version in self.ae_cache
])
# Send all the values in the cache.
self.send(target, Gossip(entries, len(self.ae_cache), -1))
# Empty the cache on gossip
self.ae_cache = []
# Reset the anti-entropy timer
self.ae_timer = Timer(self.env, self.ae_delay, self.gossip)
self.ae_timer.start()
######################################################################
## Event Handlers
######################################################################
def on_state_change(self):
"""
Does the same stuff as super, but also - if leader; starts the anti
entropy interval to do gossiping.
"""
super(FloatedRaftReplica, self).on_state_change()
if self.state in (State.FOLLOWER, State.CANDIDATE):
if hasattr(self, 'ae_timer') and self.ae_timer is not None:
# Cancel the anti-entropy timer.
self.ae_timer.stop()
self.ae_timer = None
elif self.state == State.LEADER:
self.ae_timer = Timer(self.env, self.ae_delay, self.gossip)
self.ae_timer.start()
elif self.state == State.READY:
# This happens on the call to super, just ignore for now.
pass
else:
raise SimulationException(
"Unknown Floating Raft State: {!r} set on {}".format(
self.state, self))
def on_gossip_rpc(self, message):
"""
Handles the receipt of a gossip from another node. Expects multiple
accesses (Write events) as entries. Goes through all and compares the
versions, replying False only if there is an error or a conflict.
"""
entries = message.value.entries
# Go through the entries from the RPC and write to local cluster.
for access in entries:
access.version.gossiped = True
self.write(access)
# Should we return with what's in our cache?
# Respond to the sender
self.send(message.source, GossipResponse([], 0, True, -1))
def on_response_rpc(self, message):
"""
Just receives the acknowledgment of the response.
"""
pass
def on_ae_response_rpc(self, msg):
"""
Does the same stuff that the super handler does, but also caches
commits to gossip about them later!
"""
rpc = msg.value
if self.state == State.LEADER:
if rpc.success:
self.nextIndex[msg.source] = rpc.lastLogIndex + 1
self.matchIndex[msg.source] = rpc.lastLogIndex
else:
# Decrement next index and retry append entries
self.nextIndex[msg.source] -= 1
self.send_append_entries(msg.source)
# Decide if we can commit the entry
for n in xrange(self.log.lastApplied, self.log.commitIndex, -1):
commit = Election(self.matchIndex.keys())
for k, v in self.matchIndex.iteritems():
commit.vote(k, v >= n)
if commit.has_passed() and self.log[n][1] == self.currentTerm:
# Commit all versions from the last log entry to now.
for idx in xrange(self.log.commitIndex, n + 1):
if self.log[idx][0] is None: continue
# Cache the version to anti-entropy!
version = self.log[idx][0]
if not hasattr(version,
'gossiped') or not version.gossiped:
self.ae_cache.append(version)
self.log[idx][0].update(self, commit=True)
# Set the commit index and break
self.log.commitIndex = n
break
elif self.state == State.CANDIDATE:
# Decide whether or not to step down.
if rpc.term >= self.currentTerm:
## Become a follower
self.state = State.FOLLOWER
## Log the failed election
self.sim.logger.info(
"{} has stepped down as candidate".format(self))
return
elif self.state == State.FOLLOWER:
# Ignore AE messages if we are the follower.
return
else:
raise RaftRPCException(
"Append entries response in unknown state: '{}'".format(
self.state))
class EventualReplica(Replica):
def __init__(self, simulation, **kwargs):
super(EventualReplica, self).__init__(simulation, **kwargs)
# Eventually consistent settings
self.ae_delay = kwargs.get("anti_entropy_delay", AE_DELAY)
self.n_neighbors = kwargs.get("num_neighbors", NEIGHBORS)
# Deprecated
self.do_gossip = kwargs.get("do_gossip", DO_GOSSIP)
self.do_rumoring = kwargs.get("do_rumoring", DO_RUMORING)
self.log = MultiObjectWriteLog() # the write log of the replica
self.timeout = None # anti entropy timer
######################################################################
## Properties
######################################################################
######################################################################
## Core Methods (Replica API)
######################################################################
def read(self, name, **kwargs):
"""
Eventually consistent replicas simply return the latest version for
the name that they have in their store. This easily could be stale or
forked depending on writes elsewhere in the cluster.
"""
# Create the read event using super.
access = super(EventualReplica, self).read(name, **kwargs)
# Record the number of attempts for the access
if access.is_local_to(self):
access.attempts += 1
# Fetch the latest version from the log
version = self.log.get_latest_version(access.name)
# If version is None then we haven't read anything; bail!
if version is None:
return access.drop(empty=True)
# Eventual nodes read locally and immediately, so complete the read.
access.update(version, completed=True)
# Log the access from this particular replica.
access.log(self)
return access
def write(self, name, **kwargs):
"""
Performs a write to the object with the given name by first creating
the access event using super. Note that other access events can be
passed into the write method in the case of remote writes.
The access will define if the write is local or not.
If local: write to the latest local version and complete.
If remote: append write to log if latest version of object else error.
After local vs. remote do the following:
1. append the write to the log as (version, id)
2. cache the latest access for gossip or rumoring
3. update the version for visibility latency
4. call the rumor handler
Note this method can raise an error if not writing the latest version.
"""
# Create the write event using super.
access = super(EventualReplica, self).write(name, **kwargs)
# Determine if the write is local or remote
if access.is_local_to(self):
# Record the number of attempts for the access
access.attempts += 1
# Fetch the latest version from the log
latest = self.log.get_latest_version(access.name)
# Perform the write
if latest is None:
version = namespace(access.name)(self)
else:
version = latest.nextv(self)
# Update the access with the latest version and complete
access.update(version, completed=True)
else:
# If there is no version, raise an exception
if access.version is None:
raise AccessError("Attempting a remote write on {} without a version!".format(self))
# Save the version variable for use below
version = access.version
current = self.log.get_latest_version(access.name)
# Ensure that the version is the latest.
if current is not None and version <= current:
raise AccessError("Attempting unordered write of {} after write of {}".format(version, current))
# At this point we've dealt with local vs. remote
# Append the latest version to the local data store
self.log.append(version, 0)
# Handle the access according to eventual rules
version.update(self) # Update the version to track visibility latency
access.log(self) # Log the access from this particular replica.
self.rumor(access) # Rumor the access on demand
# Return the access for subclass access.
return access
def run(self):
"""
The run method basically implements an anti-entropy timer.
"""
while True:
yield self.get_anti_entropy_timeout()
######################################################################
## Helper Methods
######################################################################
def gossip(self):
"""
Pairwise gossip protocol by randomly selecting a neighbor and
exchanging information about the state of the latest objects in the
cache since the last anti-entropy delay.
TODO: how to gossip to strong consistency nodes?
"""
# If gossiping is not allowed, forget about it.
if not self.do_gossip:
return
# Perform pairwise anti-entropy sessions with n_neighbors
for target in self.get_anti_entropy_neighbors():
# Send the latest version of ALL objects.
entries = [self.log.get_latest_version(name).access for name in self.log.namespace]
gossip = Gossip(tuple(entries), len(entries))
self.send(target, gossip)
def rumor(self, access):
"""
Performs on access rumor mongering
"""
# if rumoring is not allowed, forget about it.
if not self.do_rumoring:
return
# Send the access to n other neighbors (excluding the origin)
for target in self.get_anti_entropy_neighbors():
rumor = Rumor(access)
self.send(target, rumor)
def get_anti_entropy_timeout(self):
"""
Creates the anti-entropy timeout.
In the future this could be random timeout not fixed.
"""
self.timeout = Timer(self.env, self.ae_delay, self.gossip)
return self.timeout.start()
def select_anti_entropy_neighbor(self):
"""
Implements the anti-entropy neighbor selection policy. By default this
is simply uniform random selection of all the eventual neighbors.
"""
return random.choice(self.neighbors(self.consistency))
def get_anti_entropy_neighbors(self):
"""
Selects the neighbors to perform anti-entropy with.
"""
for _ in xrange(self.n_neighbors):
yield self.select_anti_entropy_neighbor()
def update_forte_children(self, current, remote):
"""
This unfortunately named method is a recursive function that updates
all the children of the remote version with the new forte number and
returns the newly correct current version.
The idea here is that if the current version has a lower forte number
then we should update the children of the remote (higher forte) in
order to make sure that the latest branch is current.
This method provides backpressure from Raft to Eventual.
"""
def update_forte(forte, version, current):
"""
Recursive update the forte number for a particular version.
"""
# Update all the version's children with its forte number.
for child in version.children:
# Only update children that are in the current log.
if child in self.log:
# Update child forte to parent and detect current
child.forte = forte
if child > current:
current = child
# Recurse on grandchildren
current = update_forte(forte, child, current)
# Return the maximal version (using forte numbers) discovered.
return current
# This function only needs be called if we're in federated versioning.
if settings.simulation.versioning != "federated":
return current
# If the current is greater than the remote, return it.
if current is None or current >= remote:
return current
# Check the forte number on the remote and update the children.
if remote.forte > current.forte:
strong = update_forte(remote.forte, remote, current)
if strong > current:
# Put the strong version at the end of the log and return it
# as the new current version (or latest for this object)
if strong in self.log:
self.log.remove(strong)
self.log.append(strong, strong.forte)
return strong
else:
# This really shouldn't happen?!
self.sim.logger.warning("Attempting to move {} to end when not in log!")
# Last resort, return the current version.
return current
######################################################################
## Event Handlers
######################################################################
def on_gossip_rpc(self, message):
"""
Handles the receipt of a gossip from another node. Expects multiple
accesses (Write events) as entries. Goes through all and compares the
versions, replying False only if there is an error or a conflict.
"""
entries = message.value.entries
updates = []
# Go through the entries from the RPC and update log
for access in entries:
# Get the latest version from the log then update with forte
current = self.log.get_latest_version(access.name)
current = self.update_forte_children(current, access.version)
# If the access is greater than our current version, write it!
if current is None or access.version > current:
self.write(access)
# Is the the remote behind us? If so, send the latest version!
elif access.version < current:
updates.append(current.access)
else:
# Presumably the version are equal, so do nothing.
continue
# Success here just means whether or not we're responding with updates
success = True if updates else False
# Respond to the sender with the latest versions from our log
self.send(message.source, GossipResponse(updates, len(updates), success))
def on_gossip_response_rpc(self, message):
"""
Handles the response to pairwise gossiping, updating entries from the
responder's cache to the local log and latest version cache.
"""
entries = message.value.entries
for access in entries:
current = self.log.get_latest_version(access.name)
current = self.update_forte_children(current, access.version)
# This is a new version or a later version than our current.
if current is None or access.version > current:
self.write(access)
def on_rumor_rpc(self, message):
"""
Handles the rumor message from the originator of the rumor.
"""
access = message.value.access
current = self.log.get_latest_version(access.name)
current = self.update_forte_children(current, access.version)
# Is the rumored version later than our current?
if current is None or access.version > current:
# Write the access which will rumor it out again
self.write(access)
# Respond True to the origin of the rumor
response = RumorResponse(None, True)
elif access.version < current:
# Respond False to the origin with the later version
response = RumorResponse(current.access, False)
else:
# Simply acknowledge receipt
response = RumorResponse(None, True)
# Send the response back to the source
self.send(message.source, response)
def on_rumor_response_rpc(self, message):
"""
Handles the rumor acknowledgment
"""
response = message.value
if not response.success:
# This means that a later value has come in!
current = self.log.get_latest_version(response.access.name)
current = self.update_forte_children(current, access.version)
# If their response is later than our version, write it.
if current is None or response.access.version > current:
self.write(response.access)
class RaftReplica(ConsensusReplica):
def __init__(self, simulation, **kwargs):
## Initialize the replica
super(RaftReplica, self).__init__(simulation, **kwargs)
## Initialize Raft Specific settings
self.state = State.FOLLOWER
self.currentTerm = 0
self.votedFor = None
self.log = MultiObjectWriteLog()
self.cache = {}
## Policies
self.read_policy = ReadPolicy.get(kwargs.get("read_policy", READ_POLICY))
self.aggregate_writes = kwargs.get("aggregate_writes", AGGREGATE_WRITES)
## Timers for work
eto = kwargs.get("election_timeout", ELECTION_TIMEOUT)
hbt = kwargs.get("heartbeat_interval", HEARTBEAT_INTERVAL)
self.timeout = ElectionTimer.fromReplica(self, eto)
self.heartbeat = Timer(self.env, hbt, self.on_heartbeat_timeout)
## Leader state
self.nextIndex = None
self.matchIndex = None
######################################################################
## Core Methods (Replica API)
######################################################################
def recv(self, event):
"""
Before dispatching the message to an RPC specific handler, there are
some message-wide checks that need to occur. In this case the term
must be inspected and if the replica is behind, become follower.
"""
message = event.value
rpc = message.value
# If RPC request or response contains term > currentTerm
# Set currentTerm to term and convert to follower.
if rpc.term > self.currentTerm:
self.state = State.FOLLOWER
self.currentTerm = rpc.term
# Record the received message and dispatch to event handler
return super(RaftReplica, self).recv(event)
def read(self, name, **kwargs):
"""
Raft nodes perform a local read of the most recent commited version
for the name passed in. Because the committed version could be stale
(a new version is still waiting for 2 phase commit) a fork is possible
but the Raft group will maintain full linearizability.
"""
# Create the read event using super.
access = super(RaftReplica, self).read(name, **kwargs)
# Record the number of attempts for the access
if access.is_local_to(self):
access.attempts += 1
# NOTE: Formerly, this was ALWAYS read commit not read latest, now
# it is set by the read policy on the replica. We previously noted that
# read committed was one of the key differences from eventual.
version = self.read_via_policy(access.name)
# If the version is None, that we haven't read anything!
if version is None:
return access.drop(empty=True)
# Because this is a local read committed, complete the read.
access.update(version, completed=True)
# Log the access from this particular replica.
access.log(self)
return access
def write(self, name, **kwargs):
"""
The write can be initiated on any replica server, including followers.
Step one is to create the access event using super, which will give us
the ability to detect local vs. remote writes.
If the write is local:
- create a new version from the latest write.
- if follower: send a RemoteWrite with new version to the leader (write latency)
store a cache copy so that followers can read their own writes.
cached copy of the write goes away on AppendEntries.
- if leader: append to log and complete (no leader latency)
If the write is remote:
- if follower: log warning and forward to leader
- if leader: append to log but do not complete (complete at local)
Check the committed vs. latest new versions.
After local vs. remote do the following:
1. update the version for visibility latency
2. if leader send append entries
"""
access = super(RaftReplica, self).write(name, **kwargs)
# Determine if the write is local or remote
if access.is_local_to(self):
# Record the number of attempts for the access
access.attempts += 1
# Write a new version to the latest read by policy
version = self.write_via_policy(access.name)
# Update the access with the latest version
access.update(version)
# Log the access from this particular replica.
access.log(self)
if self.state == State.LEADER:
# Append to log and complete if leader and local
self.append_via_policy(access, complete=True)
else:
# Store the version in the cache and send remote write.
self.cache[access.name] = version
return self.send_remote_write(access)
else:
# Log the access from this particular replica.
access.log(self)
# If there is no version, raise an exception
if access.version is None:
raise AccessError("Attempting a remote write on {} without a version!".format(self))
# Save the version variable for use below.
version = access.version
if self.state == State.LEADER:
# Append to log but do not complete since its remote
self.append_via_policy(access, complete=False)
else:
# Remote write occurred from client to a follower
self.sim.logger.info("remote write on follower node: {}".format(self))
# Store the version in the cache and send remote write.
self.cache[access.name] = version
return self.send_remote_write(access)
# At this point we've dealt with local vs. remote, we should be the leader
assert self.state == State.LEADER
# Update the version to track visibility latency
forte = True if settings.simulation.forte_on_append else False
version.update(self, forte=forte)
# Now do AppendEntries
# Also interrupt the heartbeat since we just sent AppendEntries
if not self.aggregate_writes:
self.send_append_entries()
self.heartbeat.stop()
return access
def run(self):
"""
Implements the Raft consensus protocol and elections.
"""
while True:
if self.state in {State.FOLLOWER, State.CANDIDATE}:
yield self.timeout.start()
elif self.state == State.LEADER:
yield self.heartbeat.start()
else:
raise SimulationException("Unknown Raft State: {!r} on {}".format(self.state, self))
######################################################################
## Helper Methods
######################################################################
def send_append_entries(self, target=None):
"""
Helper function to send append entries to quorum or a specific node.
Note: fails silently if target is not in the neighbors list.
"""
# Leader check
if not self.state == State.LEADER:
return
# Go through follower list.
for node, nidx in self.nextIndex.iteritems():
# Filter based on the target supplied.
if target is not None and node != target:
continue
# Construct the entries, or empty for heartbeat
entries = []
if self.log.lastApplied >= nidx:
entries = self.log[nidx:]
# Compute the previous log index and term
prevLogIndex = nidx - 1
prevLogTerm = self.log[prevLogIndex].term
# Send the heartbeat message
self.send(
node, AppendEntries(self.currentTerm, self.id, prevLogIndex, prevLogTerm, entries, self.log.commitIndex)
)
def send_remote_write(self, access):
"""
Helper function to send a remote write from a follower to leader.
"""
# Find the leader to perform the remote write.
leader = self.get_leader_node()
# If not leader, then drop the write
if not leader:
self.sim.logger.info("no leader: dropped write at {}".format(self))
return access.drop()
# Send the remote write to the leader
self.send(leader, RemoteWrite(self.currentTerm, access))
return access
def get_leader_node(self):
"""
Searches for the leader amongst the neighbors. Raises an exception if
there are multiple leaders, which is an extreme edge case.
"""
leaders = [node for node in self.quorum() if node.state == State.LEADER]
if len(leaders) > 1:
raise SimulationException("MutipleLeaders?!")
elif len(leaders) < 1:
return None
else:
return leaders[0]
def read_via_policy(self, name):
"""
This method returns a version from either the log or the cache
according to the read policy set on the replica server as follows:
- COMMIT: return the latest commited version (ignoring cache)
- LATEST: return latest version in log or in cache
This method raises an exception on bad read policies.
"""
# If the policy is read committed, return the latest committed version
if self.read_policy == ReadPolicy.COMMIT:
return self.log.get_latest_commit(name)
# If the policy is latest, read the latest and compare to cache.
if self.read_policy == ReadPolicy.LATEST:
# Get the latest version from the log (committed or not)
version = self.log.get_latest_version(name)
# If name in the cache and the cache version is greater, return it.
if name in self.cache and version is not None:
if self.cache[name] > version:
return self.cache[name]
# Return the latest version
return version
# If we've reached this point, we don't know what to do!
raise SimulationException("Unknown read policy!")
def write_via_policy(self, name):
"""
This method returns a new version incremented from either from the
log or from the cache according to the read policy. It also handles
any "new" writes, e.g. to objects that haven't been written yet.
"""
# Fetch the version from the log or the cache according to the
# read policy. This implements READ COMMITTED/READ LATEST
latest = self.read_via_policy(name)
# Perform the write
if latest is None:
return namespace(name)(self)
return latest.nextv(self)
def append_via_policy(self, access, complete=False):
"""
This method is the gatekeeper for the log and can implement policies
like "don't admit forks". It must drop the access if it doesn't meet
the policy, and complete it if specified.
NOTE: This is a leader-only method (followers have entries appended
to their logs via AppendEntries) and will raise an exception if the
node is not the leader.
"""
if self.state != State.LEADER:
raise RaftRPCException("Append via policies called on a follower replica!")
# The default policy is just append anything
# NOTE: subclasses (as in Federated) can modify this
self.log.append(access.version, self.currentTerm)
# Complete the access if specified by the caller.
if complete:
access.complete()
# Indicate that we've successfully appended to the log
return True
######################################################################
## Event Handlers
######################################################################
def on_state_change(self):
"""
When the state on a replica changes the internal state of the replica
must also change, particularly the properties that define how the node
interacts with RPC messages and client reads/writes.
"""
if self.state in (State.FOLLOWER, State.CANDIDATE):
self.votedFor = None
self.nextIndex = None
self.matchIndex = None
elif self.state == State.CANDIDATE:
pass
elif self.state == State.LEADER:
self.nextIndex = {node: self.log.lastApplied + 1 for node in self.quorum() if node != self}
self.matchIndex = {node: 0 for node in self.quorum() if node != self}
elif self.state == State.READY:
# This happens on the call to super, just ignore for now.
pass
else:
raise SimulationException("Unknown Raft State: {!r} set on {}".format(self.state, self))
def on_heartbeat_timeout(self):
"""
Callback for when a heartbeat timeout occurs, for AppendEntries RPC.
"""
if not self.state == State.LEADER:
return
# Send heartbeat or aggregated writes
self.send_append_entries()
def on_election_timeout(self):
"""
Callback for when an election timeout occurs, e.g. become candidate.
"""
# Set state to candidate
self.state = State.CANDIDATE
# Create Election and vote for self
self.currentTerm += 1
self.votes = Election([node.id for node in self.quorum()])
self.votes.vote(self.id)
self.votedFor = self.id
# Inform the rest of the quorum you'd like their vote.
rpc = RequestVote(self.currentTerm, self.id, self.log.lastApplied, self.log.lastTerm)
for follower in self.quorum():
if follower == self:
continue
self.send(follower, rpc)
# Log the newly formed candidacy
self.sim.logger.info("{} is now a leader candidate".format(self))
def on_request_vote_rpc(self, msg):
"""
Callback for RequestVote RPC call.
"""
rpc = msg.value
if rpc.term >= self.currentTerm:
if self.votedFor is None or self.votedFor == rpc.candidateId:
if self.log.as_up_to_date(rpc.lastLogTerm, rpc.lastLogIndex):
self.sim.logger.info("{} voting for {}".format(self, rpc.candidateId))
self.timeout.stop()
self.votedFor = rpc.candidateId
return self.send(msg.source, VoteResponse(self.currentTerm, True))
return self.send(msg.source, VoteResponse(self.currentTerm, False))
def on_vote_response_rpc(self, msg):
"""
Callback for AppendEntries and RequestVote RPC response.
"""
rpc = msg.value
if self.state == State.CANDIDATE:
# Update the current election
self.votes.vote(msg.source.id, rpc.voteGranted)
if self.votes.has_passed():
## Become the leader
self.state = State.LEADER
self.timeout.stop()
## Send the leadership change append entries
self.send_append_entries()
## Log the new leader
self.sim.logger.info("{} has become raft leader".format(self))
return
elif self.state in (State.FOLLOWER, State.LEADER):
# Ignore vote responses if we've already been elected.
return
else:
raise RaftRPCException("Vote response in unknown state: '{}'".format(self.state))
def on_append_entries_rpc(self, msg):
"""
Callback for the AppendEntries RPC call.
"""
rpc = msg.value
# Stop the election timeout
self.timeout.stop()
# Reply false if term < current term
if rpc.term < self.currentTerm:
self.sim.logger.info("{} doesn't accept write on term {}".format(self, self.currentTerm))
return self.send(msg.source, AEResponse(self.currentTerm, False, self.log.lastApplied, self.log.lastCommit))
# Reply false if log doesn't contain an entry at prevLogIndex whose
# term matches previous log term.
if self.log.lastApplied < rpc.prevLogIndex or self.log[rpc.prevLogIndex][1] != rpc.prevLogTerm:
if self.log.lastApplied < rpc.prevLogIndex:
self.sim.logger.info(
"{} doesn't accept write on index {} where last applied is {}".format(
self, rpc.prevLogIndex, self.log.lastApplied
)
)
else:
self.sim.logger.info(
"{} doesn't accept write for term mismatch {} vs {}".format(
self, rpc.prevLogTerm, self.log[rpc.prevLogIndex][1]
)
)
return self.send(msg.source, AEResponse(self.currentTerm, False, self.log.lastApplied, self.log.lastCommit))
# At this point AppendEntries RPC is accepted
if rpc.entries:
if self.log.lastApplied >= rpc.prevLogIndex:
# If existing entry conflicts with new one (same index, different terms)
# Delete the existing entry and all that follow it.
if self.log[rpc.prevLogIndex][1] != rpc.prevLogTerm:
self.log.truncate(rpc.prevLogIndex)
if self.log.lastApplied > rpc.prevLogIndex:
# Otherwise this could be a message that is sent again
# raise RaftRPCException(
# "{} is possibly receiving a duplicate append entries!".format(self)
# )
self.sim.logger.warn("{} is possibly receiving a duplicate append entries!".format(self))
return self.send(
msg.source, AEResponse(self.currentTerm, True, self.log.lastApplied, self.log.lastCommit)
)
# Append any new entries not already in the log.
for entry in rpc.entries:
# Add the entry/term to the log
self.log.append(*entry)
self.sim.logger.debug("appending {} to {} on {}".format(entry[0], entry[1], self))
# Update the versions to compute visibilities
entry[0].update(self)
# Log the last write from the append entries.
self.sim.logger.debug(
"{} writes {} at idx {} (term {}, commit {})".format(
self, self.log.lastVersion, self.log.lastApplied, self.log.lastTerm, self.log.commitIndex
)
)
# If leaderCommit > commitIndex, update commit Index
if rpc.leaderCommit > self.log.commitIndex:
self.log.commitIndex = min(rpc.leaderCommit, self.log.lastApplied)
# Return success response.
return self.send(msg.source, AEResponse(self.currentTerm, True, self.log.lastApplied, self.log.lastCommit))
def on_ae_response_rpc(self, msg):
"""
Handles acknowledgment of append entries message.
"""
rpc = msg.value
if self.state == State.LEADER:
if rpc.success:
self.nextIndex[msg.source] = rpc.lastLogIndex + 1
self.matchIndex[msg.source] = rpc.lastLogIndex
else:
# Decrement next index and retry append entries
# Ensure to floor the nextIndex to 1 (the start of the log).
nidx = self.nextIndex[msg.source] - 1
self.nextIndex[msg.source] = max(nidx, 1)
self.send_append_entries(msg.source)
# Decide if we can commit the entry
for n in xrange(self.log.lastApplied, self.log.commitIndex, -1):
commit = Election(self.matchIndex.keys())
for k, v in self.matchIndex.iteritems():
commit.vote(k, v >= n)
if commit.has_passed() and self.log[n][1] == self.currentTerm:
# Commit all versions from the last log entry to now.
for idx in xrange(self.log.commitIndex, n + 1):
if self.log[idx][0] is None:
continue
forte = True if settings.simulation.forte_on_commit else False
self.log[idx][0].update(self, commit=True, forte=forte)
# Set the commit index and break
self.log.commitIndex = n
break
elif self.state == State.CANDIDATE:
# Decide whether or not to step down.
if rpc.term >= self.currentTerm:
## Become a follower
self.state = State.FOLLOWER
## Log the failed election
self.sim.logger.info("{} has stepped down as candidate".format(self))
return
elif self.state == State.FOLLOWER:
# Ignore AE messages if we are the follower.
return
else:
raise RaftRPCException("Append entries response in unknown state: '{}'".format(self.state))
def on_remote_write_rpc(self, message):
"""
Unpacks the version from the remote write and initiates a local write.
"""
# Write the access from the remote replica
access = message.value.version
self.write(access)
# Check if the access was dropped (e.g. the write failed)
success = not access.is_dropped()
# Send the write response
self.send(message.source, WriteResponse(self.currentTerm, success, access))
def on_write_response_rpc(self, message):
"""
Completes the write if the remote write was successful.
"""
rpc = message.value
if rpc.success:
rpc.access.complete()
class EventualReplica(Replica):
def __init__(self, simulation, **kwargs):
super(EventualReplica, self).__init__(simulation, **kwargs)
# Eventually consistent settings
self.ae_delay = kwargs.get('anti_entropy_delay', AE_DELAY)
self.n_neighbors = kwargs.get('num_neighbors', NEIGHBORS)
# Deprecated
self.do_gossip = kwargs.get('do_gossip', DO_GOSSIP)
self.do_rumoring = kwargs.get('do_rumoring', DO_RUMORING)
self.log = MultiObjectWriteLog() # the write log of the replica
self.timeout = None # anti entropy timer
######################################################################
## Properties
######################################################################
######################################################################
## Core Methods (Replica API)
######################################################################
def read(self, name, **kwargs):
"""
Eventually consistent replicas simply return the latest version for
the name that they have in their store. This easily could be stale or
forked depending on writes elsewhere in the cluster.
"""
# Create the read event using super.
access = super(EventualReplica, self).read(name, **kwargs)
# Record the number of attempts for the access
if access.is_local_to(self): access.attempts += 1
# Fetch the latest version from the log
version = self.log.get_latest_version(access.name)
# If version is None then we haven't read anything; bail!
if version is None: return access.drop(empty=True)
# Eventual nodes read locally and immediately, so complete the read.
access.update(version, completed=True)
# Log the access from this particular replica.
access.log(self)
return access
def write(self, name, **kwargs):
"""
Performs a write to the object with the given name by first creating
the access event using super. Note that other access events can be
passed into the write method in the case of remote writes.
The access will define if the write is local or not.
If local: write to the latest local version and complete.
If remote: append write to log if latest version of object else error.
After local vs. remote do the following:
1. append the write to the log as (version, id)
2. cache the latest access for gossip or rumoring
3. update the version for visibility latency
4. call the rumor handler
Note this method can raise an error if not writing the latest version.
"""
# Create the write event using super.
access = super(EventualReplica, self).write(name, **kwargs)
# Determine if the write is local or remote
if access.is_local_to(self):
# Record the number of attempts for the access
access.attempts += 1
# Fetch the latest version from the log
latest = self.log.get_latest_version(access.name)
# Perform the write
if latest is None:
version = namespace(access.name)(self)
else:
version = latest.nextv(self)
# Update the access with the latest version and complete
access.update(version, completed=True)
else:
# If there is no version, raise an exception
if access.version is None:
raise AccessError(
"Attempting a remote write on {} without a version!".format(self)
)
# Save the version variable for use below
version = access.version
current = self.log.get_latest_version(access.name)
# Ensure that the version is the latest.
if current is not None and version <= current:
raise AccessError(
"Attempting unordered write of {} after write of {}".format(version, current)
)
# At this point we've dealt with local vs. remote
# Append the latest version to the local data store
self.log.append(version, 0)
# Handle the access according to eventual rules
version.update(self) # Update the version to track visibility latency
access.log(self) # Log the access from this particular replica.
self.rumor(access) # Rumor the access on demand
# Return the access for subclass access.
return access
def run(self):
"""
The run method basically implements an anti-entropy timer.
"""
while True:
yield self.get_anti_entropy_timeout()
######################################################################
## Helper Methods
######################################################################
def gossip(self):
"""
Pairwise gossip protocol by randomly selecting a neighbor and
exchanging information about the state of the latest objects in the
cache since the last anti-entropy delay.
TODO: how to gossip to strong consistency nodes?
"""
# If gossiping is not allowed, forget about it.
if not self.do_gossip:
return
# Perform pairwise anti-entropy sessions with n_neighbors
for target in self.get_anti_entropy_neighbors():
# Send the latest version of ALL objects.
entries = [
self.log.get_latest_version(name).access
for name in self.log.namespace
]
gossip = Gossip(tuple(entries), len(entries))
self.send(target, gossip)
def rumor(self, access):
"""
Performs on access rumor mongering
"""
# if rumoring is not allowed, forget about it.
if not self.do_rumoring:
return
# Send the access to n other neighbors (excluding the origin)
for target in self.get_anti_entropy_neighbors():
rumor = Rumor(access)
self.send(target, rumor)
def get_anti_entropy_timeout(self):
"""
Creates the anti-entropy timeout.
In the future this could be random timeout not fixed.
"""
self.timeout = Timer(self.env, self.ae_delay, self.gossip)
return self.timeout.start()
def select_anti_entropy_neighbor(self):
"""
Implements the anti-entropy neighbor selection policy. By default this
is simply uniform random selection of all the eventual neighbors.
"""
return random.choice(self.neighbors(self.consistency))
def get_anti_entropy_neighbors(self):
"""
Selects the neighbors to perform anti-entropy with.
"""
for _ in xrange(self.n_neighbors):
yield self.select_anti_entropy_neighbor()
def update_forte_children(self, current, remote):
"""
This unfortunately named method is a recursive function that updates
all the children of the remote version with the new forte number and
returns the newly correct current version.
The idea here is that if the current version has a lower forte number
then we should update the children of the remote (higher forte) in
order to make sure that the latest branch is current.
This method provides backpressure from Raft to Eventual.
"""
def update_forte(forte, version, current):
"""
Recursive update the forte number for a particular version.
"""
# Update all the version's children with its forte number.
for child in version.children:
# Only update children that are in the current log.
if child in self.log:
# Update child forte to parent and detect current
child.forte = forte
if child > current: current = child
# Recurse on grandchildren
current = update_forte(forte, child, current)
# Return the maximal version (using forte numbers) discovered.
return current
# This function only needs be called if we're in federated versioning.
if settings.simulation.versioning != "federated":
return current
# If the current is greater than the remote, return it.
if current is None or current >= remote: return current
# Check the forte number on the remote and update the children.
if remote.forte > current.forte:
strong = update_forte(remote.forte, remote, current)
if strong > current:
# Put the strong version at the end of the log and return it
# as the new current version (or latest for this object)
if strong in self.log:
self.log.remove(strong)
self.log.append(strong, strong.forte)
return strong
else:
# This really shouldn't happen?!
self.sim.logger.warning(
"Attempting to move {} to end when not in log!"
)
# Last resort, return the current version.
return current
######################################################################
## Event Handlers
######################################################################
def on_gossip_rpc(self, message):
"""
Handles the receipt of a gossip from another node. Expects multiple
accesses (Write events) as entries. Goes through all and compares the
versions, replying False only if there is an error or a conflict.
"""
entries = message.value.entries
updates = []
# Go through the entries from the RPC and update log
for access in entries:
# Get the latest version from the log then update with forte
current = self.log.get_latest_version(access.name)
current = self.update_forte_children(current, access.version)
# If the access is greater than our current version, write it!
if current is None or access.version > current:
self.write(access)
# Is the the remote behind us? If so, send the latest version!
elif access.version < current:
updates.append(current.access)
else:
# Presumably the version are equal, so do nothing.
continue
# Success here just means whether or not we're responding with updates
success = True if updates else False
# Respond to the sender with the latest versions from our log
self.send(message.source, GossipResponse(updates, len(updates), success))
def on_gossip_response_rpc(self, message):
"""
Handles the response to pairwise gossiping, updating entries from the
responder's cache to the local log and latest version cache.
"""
entries = message.value.entries
for access in entries:
current = self.log.get_latest_version(access.name)
current = self.update_forte_children(current, access.version)
# This is a new version or a later version than our current.
if current is None or access.version > current:
self.write(access)
def on_rumor_rpc(self, message):
"""
Handles the rumor message from the originator of the rumor.
"""
access = message.value.access
current = self.log.get_latest_version(access.name)
current = self.update_forte_children(current, access.version)
# Is the rumored version later than our current?
if current is None or access.version > current:
# Write the access which will rumor it out again
self.write(access)
# Respond True to the origin of the rumor
response = RumorResponse(None, True)
elif access.version < current:
# Respond False to the origin with the later version
response = RumorResponse(current.access, False)
else:
# Simply acknowledge receipt
response = RumorResponse(None, True)
# Send the response back to the source
self.send(message.source, response)
def on_rumor_response_rpc(self, message):
"""
Handles the rumor acknowledgment
"""
response = message.value
if not response.success:
# This means that a later value has come in!
current = self.log.get_latest_version(response.access.name)
current = self.update_forte_children(current, access.version)
# If their response is later than our version, write it.
if current is None or response.access.version > current:
self.write(response.access)
def write(self, name, **kwargs):
"""
When a replica performs a write it needs to decide if it can write to
the tag locally, can acquire a tag for this object, or if it has to do
something else like drop, wait, or remote write.
If the access is local:
- if the replica owns the tag, append and complete
- if someone else owns the tag then drop, wait, or remote
- if no one owns the tag, then attempt to acquire it
If access is remote:
- if we own the tag, then append but do not complete (at local)
- if someone else owns the tag, log and forward to owner
- if no one owns the tag then respond false
"""
# Create the read event using super.
access = super(TagReplica, self).write(name, **kwargs)
# Increase the session on access.
self.handle_session()
# Determine if the write is local or remote
if access.is_local_to(self):
# Record the number of attempts for the access
access.attempts += 1
# Fetch the latest version from the log.
latest = self.log[access.name].lastVersion
# Perform the write
if latest is None:
version = namespace(access.name)(self)
else:
version = latest.nextv(self)
# Update the access with the latest version
access.update(version)
else:
# If there is no version, raise an exception
if access.version is None:
raise AccessError(
"Attempting a remote write on {} without a version!".
format(self))
# Save the version variable for use below.
version = access.version
# Log the access at this replica
access.log(self)
# Are we the owner of this tag?
if self.owns(access.name):
# Perform the append entries
self.log[name].append(version, self.epoch)
# Update the version to track visibility latency
version.update(self)
# Complete the access if it was local
if access.is_local_to(self): access.complete()
# Now do AppendEntries
# Also interrupt the heartbeat since we just sent AppendEntries
if not settings.simulation.aggregate_writes:
self.send_append_entries()
if self.heartbeat: self.heartbeat.stop()
return access
# Is there a different owner for the tag?
owner = self.find_owner(name)
if owner is not None:
# Right now just drop the write on its face.
self.sim.logger.info("ownership conflict: dropped {} at {}".format(
access, self))
return access.drop()
# We're going to acquire the tag!
else:
# We're going to have some write latency, retry the write.
retry = Timer(self.env, self.heartbeat_interval,
lambda: self.write(access)).start()
# Request the ownership of the tag
self.acquire(access.name)
return access
class TagReplica(ConsensusReplica):
def __init__(self, simulation, **kwargs):
## Timers for work
self.session_timeout = kwargs.get('session_timeout', SESSION_TIMEOUT)
self.heartbeat_interval = kwargs.get('heartbeat_interval', HEARTBEAT_INTERVAL)
self.session = None
self.heartbeat = None
## Initialze the tag specific settings
self.epoch = 0
self.log = defaultdict(WriteLog)
self.view = defaultdict(set)
## Owner state
self.nextIndex = None
self.matchIndex = None
## Initialize the replica
super(TagReplica, self).__init__(simulation, **kwargs)
self.state = State.READY
######################################################################
## Core Methods (Replica API)
######################################################################
def read(self, name, **kwargs):
"""
When a tag replica performs a read it has to decide whether or not to
read locally or to make a remote read across the cluster.
Convert the read into an access, then check if we own the object.
If we do, then return the latest commit.
If we don't and no one else does either, attempt to acquire the tag.
If we don't and someone else does then either drop, wait, or remote.
Current implementation: #2, MR, no remote access.
If someone else owns tag, reads are dropped.
TODO: Remote vs Local Reads
"""
# Create the read event using super.
access = super(TagReplica, self).read(name, **kwargs)
# Record the number of attempts for the access
if access.is_local_to(self): access.attempts += 1
# Increase the session on access.
self.handle_session()
# Are we the owner of this tag?
if self.owns(access.name):
# TODO: Change to last commit!
version = self.log[access.name].lastVersion
# If the version is None, bail since we haven't read anything
if version is None: return access.drop(empty=True)
# Update the version, complete the read, and log the access
access.update(version, completed=True)
access.log(self)
# Return, we're done reading!
return access
# Is there a different owner for the tag?
owner = self.find_owner(access.name)
if owner is not None:
# Right now just drop the read on its face.
self.sim.logger.info(
"ownership conflict: dropped {} at {}".format(access, self)
)
return access.drop()
# We're going to acquire the tag!
else:
# Log the access from this particular replica.
access.log(self)
# We're going to have some read latency, retry the read.
retry = Timer(
self.env, self.heartbeat_interval, lambda: self.read(access)
).start()
if access.attempts <= 1 and self.state != State.TAGGING:
# Request the ownership of the tag
self.acquire(access.name)
return access
def write(self, name, **kwargs):
"""
When a replica performs a write it needs to decide if it can write to
the tag locally, can acquire a tag for this object, or if it has to do
something else like drop, wait, or remote write.
If the access is local:
- if the replica owns the tag, append and complete
- if someone else owns the tag then drop, wait, or remote
- if no one owns the tag, then attempt to acquire it
If access is remote:
- if we own the tag, then append but do not complete (at local)
- if someone else owns the tag, log and forward to owner
- if no one owns the tag then respond false
"""
# Create the read event using super.
access = super(TagReplica, self).write(name, **kwargs)
# Increase the session on access.
self.handle_session()
# Determine if the write is local or remote
if access.is_local_to(self):
# Record the number of attempts for the access
access.attempts += 1
# Fetch the latest version from the log.
latest = self.log[access.name].lastVersion
# Perform the write
if latest is None:
version = namespace(access.name)(self)
else:
version = latest.nextv(self)
# Update the access with the latest version
access.update(version)
else:
# If there is no version, raise an exception
if access.version is None:
raise AccessError(
"Attempting a remote write on {} without a version!".format(self)
)
# Save the version variable for use below.
version = access.version
# Log the access at this replica
access.log(self)
# Are we the owner of this tag?
if self.owns(access.name):
# Perform the append entries
self.log[name].append(version, self.epoch)
# Update the version to track visibility latency
version.update(self)
# Complete the access if it was local
if access.is_local_to(self): access.complete()
# Now do AppendEntries
# Also interrupt the heartbeat since we just sent AppendEntries
if not settings.simulation.aggregate_writes:
self.send_append_entries()
if self.heartbeat: self.heartbeat.stop()
return access
# Is there a different owner for the tag?
owner = self.find_owner(name)
if owner is not None:
# Right now just drop the write on its face.
self.sim.logger.info(
"ownership conflict: dropped {} at {}".format(access, self)
)
return access.drop()
# We're going to acquire the tag!
else:
# We're going to have some write latency, retry the write.
retry = Timer(
self.env, self.heartbeat_interval, lambda: self.write(access)
).start()
# Request the ownership of the tag
self.acquire(access.name)
return access
def run(self):
"""
We have to check in at every heartbeat interval. If we own a tag then
send a heartbeat message, otherwise just keep quiescing.
"""
while True:
if self.state == State.OWNER:
self.heartbeat = Timer(
self.env, self.heartbeat_interval, self.on_heartbeat_timeout
)
yield self.heartbeat.start()
else:
yield self.env.timeout(self.heartbeat_interval)
######################################################################
## Helper Methods
######################################################################
def owns(self, name):
"""
Returns True if the name is in the current view for that owner.
"""
return name in self.view[self]
def find_owner(self, name):
"""
Looks up the owner of the name in the current view.
Returns None if there is no owner fo the tag.
"""
for owner, tag in self.view.items():
if name in tag:
return owner
return None
def acquire(self, tag):
"""
Sends out the acquire tag RPC
"""
# Construct the tag to send out
if not isinstance(tag, (set, frozenset)):
tag = frozenset([tag])
# Make sure to request the tag we already have
tag = frozenset(self.view[self] | tag)
# Request tag with all current tags
self.send_tag_request(tag)
# Log the tag acquisition
self.sim.logger.info(
"{} is atempting to acquire tag {}".format(self, self.tag)
)
def release(self, tag=None):
"""
Sends out the release tag RPC
"""
# Release all currently held tags
if tag is None: tag = self.view[self]
# Construct the tag to send out (if specified)
if not isinstance(tag, (set, frozenset)):
tag = frozenset([tag])
# Request the difference of the tags we already have
tag = frozenset(self.view[self] - tag)
# Request tag with all current tags
self.send_tag_request(tag)
# Log the tag release
self.sim.logger.info(
"{} is atempting to release tag {}".format(self, tag)
)
def handle_session(self):
"""
Starts a session timer if one isn't running, otherwise resets the
currently running session timer on an additional access.
"""
if not self.session:
self.session = Timer(
self.env, self.session_timeout,
partial(self.on_session_timeout, self.env.now)
)
else:
self.session = self.session.reset()
def get_log_state(self, tag=None):
"""
Constructs a log state object for append entries responses, either
for the current tag or simply the current view.
"""
if tag is None:
tag = [obj for view in self.view.values() for obj in view]
return {
obj: LogState(
self.log[obj].lastApplied,
self.log[obj].lastTerm,
self.log[obj].commitIndex
) for obj in tag
}
def send_tag_request(self, tag):
"""
Broadcasts a tag request for the passed in tag.
"""
# Change state to tagging and save tag locally
self.state = State.TAGGING
self.tag = tag
# Request the entire tag in your current view.
tagset = {
owner.id: tagset
for owner, tagset in self.view.items()
}
tagset[self.id] = self.tag
# Send the tag request RPC to each neighbor
rpc = RequestTag(self.epoch, tagset, self)
for neighbor in self.neighbors():
self.send(neighbor, rpc)
def send_append_entries(self, target=None):
"""
Helper function to send append entries to quorum or a specific node.
Note: fails silently if target is not in the neighbors list.
"""
# ownership check
if not self.state == State.OWNER:
return
# Go through follower list.
for node, objs in self.nextIndex.iteritems():
# Filter based on the target supplied.
if target is not None and node != target:
continue
# Construct the entries, or empty for heartbeat
# The tag contains the state of each item to be sent
entries = defaultdict(list)
tag = defaultdict(LogState)
for obj, nidx in objs.items():
# A rule directly from the Raft paper
if self.log[obj].lastApplied >= nidx:
entries[obj] = self.log[obj][nidx:]
# Compute the previous log index and term
prevLogIndex = nidx - 1
prevLogTerm = self.log[obj][prevLogIndex].term
commitIndex = self.log[obj].commitIndex
# Create the tag state
tag[obj] = LogState(prevLogIndex, prevLogTerm, commitIndex)
# Send the append entries message
self.send(
node, AppendEntries(
self.epoch, self.id, tag, entries
)
)
######################################################################
## Event Handlers
######################################################################
def on_state_change(self):
"""
Setting the state decides how the Tag node will interact.
"""
# Do state specific tag modifications
if self.state == State.READY:
self.votes = None
self.tag = None
# Remove owner state
self.nextIndex = None
self.matchIndex = None
# Also interrupt the heartbeat
if self.heartbeat: self.heartbeat.stop()
elif self.state == State.TAGGING:
# Convert to tag acquisition/release
self.epoch += 1
# Create election and vote for self
self.votes = Election([node.id for node in self.quorum()])
self.votes.vote(self.id)
# Also interrupt the heartbeat
if self.heartbeat: self.heartbeat.stop()
elif self.state == State.OWNER:
# Create the next index and match index
self.nextIndex = {
node: {
obj: self.log[obj].lastApplied + 1
for obj in self.view[self]
} for node in self.neighbors()
}
self.matchIndex = {
node: {
obj: 0 for obj in self.view[self]
} for node in self.neighbors()
}
else:
raise SimulationException(
"Unknown Tag Replica State: {!r} set on {}".format(state, self)
)
def on_heartbeat_timeout(self):
"""
Time to send a heartbeat message to all tags.
"""
if not self.state == State.OWNER:
return
# Send heartbeat or aggregated writes
self.send_append_entries()
def on_session_timeout(self, started):
"""
If the session times out then go ahead and release the tag.
"""
duration = self.env.now - started
self.sim.logger.info(
"session on {} terminated at {} ({} ms)".format(
self.id, self.env.now, duration
)
)
self.sim.results.update(
'session length', (self.id, duration)
)
self.session = None
self.release()
def on_request_tag_rpc(self, msg):
"""
Respond to a request for a tag acquisition from a server.
"""
rpc = msg.value
accept = True
# The requested epoch must be less than or greater than local.
if rpc.epoch < self.epoch: accept = False
# Ensure that no one else owns the tag in your current view.
for candidate, tagset in rpc.tag.items():
# Short circuit
if not accept: break
for tag in tagset:
owner = self.find_owner(tag)
if owner is not None and owner.id != candidate:
accept = False
break
# Log the vote decision
amsg = "accepted" if accept else "did not accept"
lmsg = "{} {} tag [{}] for {}".format(
self, amsg, ",".join(rpc.tag[rpc.candidate.id]), rpc.candidate.id
)
self.sim.logger.info(lmsg)
# Send the vote response back to the tag requester
return self.send(
msg.source, TagResponse(self.epoch, accept)
)
def on_tag_response_rpc(self, msg):
"""
Handle the votes from tag requests to other nodes.
"""
rpc = msg.value
if self.state == State.TAGGING:
# If the epoch is greater than the current epoch
if rpc.epoch > self.epoch:
# Retry the tag request
self.epoch = rpc.epoch
self.send_tag_request(self.tag)
self.sim.logger.info(
"{} retrying tag request for {}".format(self, self.tag)
)
# Exit: no more work required!
return
# Update the current election
self.votes.vote(msg.source.id, rpc.accept)
if self.votes.has_passed():
# Update our local tag and become owner.
if self.tag:
self.state = State.OWNER
self.view[self] = set(self.tag)
else:
self.state = State.READY
# Send out the ownership change append entries
self.send_append_entries()
# Log the new tag owner
self.sim.logger.info(
"{} tag goes to: {}".format(self, self.view[self])
)
# Record tag length over time
self.sim.results.update(
'tag size', (self.id, self.env.now, len(self.view[self]))
)
elif self.state in (State.READY, State.OWNER):
# Ignore vote responses if we've changed our state
return
else:
raise TagRPCException(
"Tag request response in unknown state: '{}'".format(self.state)
)
def on_append_entries_rpc(self, msg):
rpc = msg.value
# reply false if the epoch < current epoch
if rpc.epoch < self.epoch:
self.sim.logger.info(
"{} doesn't accept append entries in epoch {} for epoch {}".format(
self, self.epoch, rpc.epoch
)
)
# Send back the request that you made originally.
return self.send(
msg.source, AEResponse(
self.epoch,
{obj: False for obj in rpc.tag.keys()},
rpc.tag, Reason.EPOCH
)
)
# Update the view to match the view of the append entries
# Update the epoch to match the rpc of the append entries
self.view[msg.source] = set(rpc.tag.keys())
if self.epoch < rpc.epoch:
self.epoch = rpc.epoch
# Now for each object in the RPC, perform Raft-like append entries.
# The success tracking is a complete tracking for all objects, will
# return false even if we need to update the log for only one thing.
# We will reply back with a state object that has per-object details.
success = defaultdict(bool)
state = defaultdict(LogState)
for obj, prev in rpc.tag.items():
entries = rpc.entries[obj]
objlog = self.log[obj]
# If log doesn't contain an entry at prev index matching epoch.
if objlog.lastApplied < prev.index or objlog[prev.index].term != prev.epoch:
# Perform the logging of this state failure
if objlog.lastApplied < prev.index:
self.sim.logger.info(
"{} doesn't accept append to {} index {} where last applied is {}".format(
self, obj, prev.index, objlog.lastApplied
)
)
else:
self.sim.logger.info(
"{} doesn't accept append to {} due to epoch mismatch: {} vs {}".format(
self, obj, prev.epoch, objlog[prev.index].term
)
)
# Mark that there is a problem and continue
success[obj] = False
state[obj] = LogState(objlog.lastApplied, objlog.lastTerm, objlog.lastCommit)
continue
# At this point the entries are accepted because of continue statements
if entries:
if objlog.lastApplied >= prev.index:
# If existing entry conflicts with a new one (same index, different epochs)
# Delete the existing entry and all that follow it.
if objlog[prev.index].term != prev.epoch:
objlog.truncate(prev.index)
if objlog.lastApplied > prev.index:
# Better look into what's happening here!
raise TagRPCException(
"{} is possibly receiving duplicate append entries".format(self)
)
# Append any new entries not already in the log.
for entry in entries:
# Add the entry/epoch to the log
objlog.append(*entry)
# Update the versions to compute visibilities
entry[0].update(self)
# Log the last write from the append entries
self.sim.logger.debug(
"appending {} entries to {} log on {} (term {}, commit {})".format(
len(entries), obj, self, objlog.lastTerm, objlog.commitIndex
)
)
# Update the commit index and save the state of the object.
if prev.commit > objlog.commitIndex:
objlog.commitIndex = min(prev.commit, objlog.lastApplied)
success[obj] = True
state[obj] = LogState(objlog.lastApplied, objlog.lastTerm, objlog.lastCommit)
# Return the response back to the owner
reason = Reason.OK if all(success.values()) else Reason.LOG
return self.send(
msg.source, AEResponse(self.epoch, success, state, reason)
)
def on_ae_response_rpc(self, msg):
"""
Handles acknowledgment of append entries messages.
"""
rpc = msg.value
retry = False
if self.state == State.OWNER:
# Update state of followers in the tag group
for obj, success in rpc.success.items():
if success:
self.nextIndex[msg.source][obj] = rpc.tag[obj].index + 1
self.matchIndex[msg.source][obj] = rpc.tag[obj].index
else:
# If the epoch is not the same, update accordingly.
if rpc.epoch > self.epoch:
self.epoch = rpc.epoch
# If the failure was because of the epoch, simply retry.
if rpc.reason == Reason.EPOCH:
retry = True
# Otherwise decrement the next index and to retry
elif rpc.reason == Reason.LOG:
self.nextIndex[msg.source][obj] -= 1
retry = True
else:
raise TagRPCException(
"Unknown append entries failure reason: {}".format(rpc.reason)
)
# Determine if we can commit the entry
for obj, state in rpc.tag.items():
log = self.log[obj]
for n in xrange(log.lastApplied, log.commitIndex, -1):
commit = Election(self.matchIndex.keys())
for node, objs in self.matchIndex.items():
match = objs[obj]
commit.vote(node, match >= n)
if commit.has_passed() and log[n].term == self.epoch:
# Commit all versions from the last log to now.
for idx in xrange(log.commitIndex, n+1):
if not log[idx].version: continue
log[idx].version.update(self, commit=True)
# Set the commit index and break
log.commitIndex = n
break
# If retry, send append entries back to the source.
if retry: self.send_append_entries(msg.source)
elif self.state == State.TAGGING:
# Determine if we need to retry the tagging again.
if rpc.epoch > self.epoch:
# Retry the tag request
self.epoch = rpc.epoch
self.send_tag_request(self.tag)
self.sim.logger.info(
"{} retrying tag request for {}".format(self, self.tag)
)
return
elif self.state == State.READY:
# Ignore AE messages if we're not an owner anymore.
return
else:
raise TagRPCException(
"Response in unknown state: '{}'".format(self.state)
)
def on_remote_access(self, msg):
"""
Handles remote writes to and from the replicas.
"""
access = msg.value.access
# Ensure that we own the object
if not self.owns(access.name):
return self.send(
msg.source, AccessResponse(self.epoch, False, access)
)
# If we do own the object, then respond:
method = {
'read': self.read,
'write': self.write,
}[access.type]
# Call the remote method with the access.
method(access)
return self.send(
msg.source, AccessResponse(self.epoch, True, access)
)
def on_access_response_rpc(self, msg):
"""
Handles responses to remote accesses.
"""
rpc = msg.value
if rpc.success:
rpc.access.complete()
class RaftReplica(ConsensusReplica):
def __init__(self, simulation, **kwargs):
## Initialize the replica
super(RaftReplica, self).__init__(simulation, **kwargs)
## Initialize Raft Specific settings
self.state = State.FOLLOWER
self.currentTerm = 0
self.votedFor = None
self.log = MultiObjectWriteLog()
self.cache = {}
## Policies
self.read_policy = ReadPolicy.get(kwargs.get('read_policy', READ_POLICY))
self.aggregate_writes = kwargs.get('aggregate_writes', AGGREGATE_WRITES)
## Timers for work
eto = kwargs.get('election_timeout', ELECTION_TIMEOUT)
hbt = kwargs.get('heartbeat_interval', HEARTBEAT_INTERVAL)
self.timeout = ElectionTimer.fromReplica(self, eto)
self.heartbeat = Timer(self.env, hbt, self.on_heartbeat_timeout)
## Leader state
self.nextIndex = None
self.matchIndex = None
######################################################################
## Core Methods (Replica API)
######################################################################
def recv(self, event):
"""
Before dispatching the message to an RPC specific handler, there are
some message-wide checks that need to occur. In this case the term
must be inspected and if the replica is behind, become follower.
"""
message = event.value
rpc = message.value
# If RPC request or response contains term > currentTerm
# Set currentTerm to term and convert to follower.
if rpc.term > self.currentTerm:
self.state = State.FOLLOWER
self.currentTerm = rpc.term
# Record the received message and dispatch to event handler
return super(RaftReplica, self).recv(event)
def read(self, name, **kwargs):
"""
Raft nodes perform a local read of the most recent commited version
for the name passed in. Because the committed version could be stale
(a new version is still waiting for 2 phase commit) a fork is possible
but the Raft group will maintain full linearizability.
"""
# Create the read event using super.
access = super(RaftReplica, self).read(name, **kwargs)
# Record the number of attempts for the access
if access.is_local_to(self): access.attempts += 1
# NOTE: Formerly, this was ALWAYS read commit not read latest, now
# it is set by the read policy on the replica. We previously noted that
# read committed was one of the key differences from eventual.
version = self.read_via_policy(access.name)
# If the version is None, that we haven't read anything!
if version is None: return access.drop(empty=True)
# Because this is a local read committed, complete the read.
access.update(version, completed=True)
# Log the access from this particular replica.
access.log(self)
return access
def write(self, name, **kwargs):
"""
The write can be initiated on any replica server, including followers.
Step one is to create the access event using super, which will give us
the ability to detect local vs. remote writes.
If the write is local:
- create a new version from the latest write.
- if follower: send a RemoteWrite with new version to the leader (write latency)
store a cache copy so that followers can read their own writes.
cached copy of the write goes away on AppendEntries.
- if leader: append to log and complete (no leader latency)
If the write is remote:
- if follower: log warning and forward to leader
- if leader: append to log but do not complete (complete at local)
Check the committed vs. latest new versions.
After local vs. remote do the following:
1. update the version for visibility latency
2. if leader send append entries
"""
access = super(RaftReplica, self).write(name, **kwargs)
# Determine if the write is local or remote
if access.is_local_to(self):
# Record the number of attempts for the access
access.attempts += 1
# Write a new version to the latest read by policy
version = self.write_via_policy(access.name)
# Update the access with the latest version
access.update(version)
# Log the access from this particular replica.
access.log(self)
if self.state == State.LEADER:
# Append to log and complete if leader and local
self.append_via_policy(access, complete=True)
else:
# Store the version in the cache and send remote write.
self.cache[access.name] = version
return self.send_remote_write(access)
else:
# Log the access from this particular replica.
access.log(self)
# If there is no version, raise an exception
if access.version is None:
raise AccessError(
"Attempting a remote write on {} without a version!".format(self)
)
# Save the version variable for use below.
version = access.version
if self.state == State.LEADER:
# Append to log but do not complete since its remote
self.append_via_policy(access, complete=False)
else:
# Remote write occurred from client to a follower
self.sim.logger.info(
"remote write on follower node: {}".format(self)
)
# Store the version in the cache and send remote write.
self.cache[access.name] = version
return self.send_remote_write(access)
# At this point we've dealt with local vs. remote, we should be the leader
assert self.state == State.LEADER
# Update the version to track visibility latency
forte = True if settings.simulation.forte_on_append else False
version.update(self, forte=forte)
# Now do AppendEntries
# Also interrupt the heartbeat since we just sent AppendEntries
if not self.aggregate_writes:
self.send_append_entries()
self.heartbeat.stop()
return access
def run(self):
"""
Implements the Raft consensus protocol and elections.
"""
while True:
if self.state in {State.FOLLOWER, State.CANDIDATE}:
yield self.timeout.start()
elif self.state == State.LEADER:
yield self.heartbeat.start()
else:
raise SimulationException(
"Unknown Raft State: {!r} on {}".format(self.state, self)
)
######################################################################
## Helper Methods
######################################################################
def send_append_entries(self, target=None):
"""
Helper function to send append entries to quorum or a specific node.
Note: fails silently if target is not in the neighbors list.
"""
# Leader check
if not self.state == State.LEADER:
return
# Go through follower list.
for node, nidx in self.nextIndex.iteritems():
# Filter based on the target supplied.
if target is not None and node != target:
continue
# Construct the entries, or empty for heartbeat
entries = []
if self.log.lastApplied >= nidx:
entries = self.log[nidx:]
# Compute the previous log index and term
prevLogIndex = nidx - 1
prevLogTerm = self.log[prevLogIndex].term
# Send the heartbeat message
self.send(
node, AppendEntries(
self.currentTerm, self.id, prevLogIndex,
prevLogTerm, entries, self.log.commitIndex
)
)
def send_remote_write(self, access):
"""
Helper function to send a remote write from a follower to leader.
"""
# Find the leader to perform the remote write.
leader = self.get_leader_node()
# If not leader, then drop the write
if not leader:
self.sim.logger.info(
"no leader: dropped write at {}".format(self)
)
return access.drop()
# Send the remote write to the leader
self.send(
leader, RemoteWrite(self.currentTerm, access)
)
return access
def get_leader_node(self):
"""
Searches for the leader amongst the neighbors. Raises an exception if
there are multiple leaders, which is an extreme edge case.
"""
leaders = [
node for node in self.quorum() if node.state == State.LEADER
]
if len(leaders) > 1:
raise SimulationException("MutipleLeaders?!")
elif len(leaders) < 1:
return None
else:
return leaders[0]
def read_via_policy(self, name):
"""
This method returns a version from either the log or the cache
according to the read policy set on the replica server as follows:
- COMMIT: return the latest commited version (ignoring cache)
- LATEST: return latest version in log or in cache
This method raises an exception on bad read policies.
"""
# If the policy is read committed, return the latest committed version
if self.read_policy == ReadPolicy.COMMIT:
return self.log.get_latest_commit(name)
# If the policy is latest, read the latest and compare to cache.
if self.read_policy == ReadPolicy.LATEST:
# Get the latest version from the log (committed or not)
version = self.log.get_latest_version(name)
# If name in the cache and the cache version is greater, return it.
if name in self.cache and version is not None:
if self.cache[name] > version:
return self.cache[name]
# Return the latest version
return version
# If we've reached this point, we don't know what to do!
raise SimulationException("Unknown read policy!")
def write_via_policy(self, name):
"""
This method returns a new version incremented from either from the
log or from the cache according to the read policy. It also handles
any "new" writes, e.g. to objects that haven't been written yet.
"""
# Fetch the version from the log or the cache according to the
# read policy. This implements READ COMMITTED/READ LATEST
latest = self.read_via_policy(name)
# Perform the write
if latest is None:
return namespace(name)(self)
return latest.nextv(self)
def append_via_policy(self, access, complete=False):
"""
This method is the gatekeeper for the log and can implement policies
like "don't admit forks". It must drop the access if it doesn't meet
the policy, and complete it if specified.
NOTE: This is a leader-only method (followers have entries appended
to their logs via AppendEntries) and will raise an exception if the
node is not the leader.
"""
if self.state != State.LEADER:
raise RaftRPCException(
"Append via policies called on a follower replica!"
)
# The default policy is just append anything
# NOTE: subclasses (as in Federated) can modify this
self.log.append(access.version, self.currentTerm)
# Complete the access if specified by the caller.
if complete:
access.complete()
# Indicate that we've successfully appended to the log
return True
######################################################################
## Event Handlers
######################################################################
def on_state_change(self):
"""
When the state on a replica changes the internal state of the replica
must also change, particularly the properties that define how the node
interacts with RPC messages and client reads/writes.
"""
if self.state in (State.FOLLOWER, State.CANDIDATE):
self.votedFor = None
self.nextIndex = None
self.matchIndex = None
elif self.state == State.CANDIDATE:
pass
elif self.state == State.LEADER:
self.nextIndex = {node: self.log.lastApplied + 1 for node in self.quorum() if node != self}
self.matchIndex = {node: 0 for node in self.quorum() if node != self}
elif self.state == State.READY:
# This happens on the call to super, just ignore for now.
pass
else:
raise SimulationException(
"Unknown Raft State: {!r} set on {}".format(self.state, self)
)
def on_heartbeat_timeout(self):
"""
Callback for when a heartbeat timeout occurs, for AppendEntries RPC.
"""
if not self.state == State.LEADER:
return
# Send heartbeat or aggregated writes
self.send_append_entries()
def on_election_timeout(self):
"""
Callback for when an election timeout occurs, e.g. become candidate.
"""
# Set state to candidate
self.state = State.CANDIDATE
# Create Election and vote for self
self.currentTerm += 1
self.votes = Election([node.id for node in self.quorum()])
self.votes.vote(self.id)
self.votedFor = self.id
# Inform the rest of the quorum you'd like their vote.
rpc = RequestVote(
self.currentTerm, self.id, self.log.lastApplied, self.log.lastTerm
)
for follower in self.quorum():
if follower == self: continue
self.send(
follower, rpc
)
# Log the newly formed candidacy
self.sim.logger.info(
"{} is now a leader candidate".format(self)
)
def on_request_vote_rpc(self, msg):
"""
Callback for RequestVote RPC call.
"""
rpc = msg.value
if rpc.term >= self.currentTerm:
if self.votedFor is None or self.votedFor == rpc.candidateId:
if self.log.as_up_to_date(rpc.lastLogTerm, rpc.lastLogIndex):
self.sim.logger.info(
"{} voting for {}".format(self, rpc.candidateId)
)
self.timeout.stop()
self.votedFor = rpc.candidateId
return self.send(
msg.source, VoteResponse(self.currentTerm, True)
)
return self.send(
msg.source, VoteResponse(self.currentTerm, False)
)
def on_vote_response_rpc(self, msg):
"""
Callback for AppendEntries and RequestVote RPC response.
"""
rpc = msg.value
if self.state == State.CANDIDATE:
# Update the current election
self.votes.vote(msg.source.id, rpc.voteGranted)
if self.votes.has_passed():
## Become the leader
self.state = State.LEADER
self.timeout.stop()
## Send the leadership change append entries
self.send_append_entries()
## Log the new leader
self.sim.logger.info(
"{} has become raft leader".format(self)
)
return
elif self.state in (State.FOLLOWER, State.LEADER):
# Ignore vote responses if we've already been elected.
return
else:
raise RaftRPCException(
"Vote response in unknown state: '{}'".format(self.state)
)
def on_append_entries_rpc(self, msg):
"""
Callback for the AppendEntries RPC call.
"""
rpc = msg.value
# Stop the election timeout
self.timeout.stop()
# Reply false if term < current term
if rpc.term < self.currentTerm:
self.sim.logger.info("{} doesn't accept write on term {}".format(self, self.currentTerm))
return self.send(
msg.source, AEResponse(self.currentTerm, False, self.log.lastApplied, self.log.lastCommit)
)
# Reply false if log doesn't contain an entry at prevLogIndex whose
# term matches previous log term.
if self.log.lastApplied < rpc.prevLogIndex or self.log[rpc.prevLogIndex][1] != rpc.prevLogTerm:
if self.log.lastApplied < rpc.prevLogIndex:
self.sim.logger.info(
"{} doesn't accept write on index {} where last applied is {}".format(
self, rpc.prevLogIndex, self.log.lastApplied
)
)
else:
self.sim.logger.info(
"{} doesn't accept write for term mismatch {} vs {}".format(
self, rpc.prevLogTerm, self.log[rpc.prevLogIndex][1]
)
)
return self.send(
msg.source, AEResponse(self.currentTerm, False, self.log.lastApplied, self.log.lastCommit)
)
# At this point AppendEntries RPC is accepted
if rpc.entries:
if self.log.lastApplied >= rpc.prevLogIndex:
# If existing entry conflicts with new one (same index, different terms)
# Delete the existing entry and all that follow it.
if self.log[rpc.prevLogIndex][1] != rpc.prevLogTerm:
self.log.truncate(rpc.prevLogIndex)
if self.log.lastApplied > rpc.prevLogIndex:
# Otherwise this could be a message that is sent again
# raise RaftRPCException(
# "{} is possibly receiving a duplicate append entries!".format(self)
# )
self.sim.logger.warn(
"{} is possibly receiving a duplicate append entries!".format(self)
)
return self.send(msg.source, AEResponse(self.currentTerm, True, self.log.lastApplied, self.log.lastCommit))
# Append any new entries not already in the log.
for entry in rpc.entries:
# Add the entry/term to the log
self.log.append(*entry)
self.sim.logger.debug(
"appending {} to {} on {}".format(entry[0], entry[1], self)
)
# Update the versions to compute visibilities
entry[0].update(self)
# Log the last write from the append entries.
self.sim.logger.debug(
"{} writes {} at idx {} (term {}, commit {})".format(
self, self.log.lastVersion, self.log.lastApplied, self.log.lastTerm, self.log.commitIndex
))
# If leaderCommit > commitIndex, update commit Index
if rpc.leaderCommit > self.log.commitIndex:
self.log.commitIndex = min(rpc.leaderCommit, self.log.lastApplied)
# Return success response.
return self.send(msg.source, AEResponse(self.currentTerm, True, self.log.lastApplied, self.log.lastCommit))
def on_ae_response_rpc(self, msg):
"""
Handles acknowledgment of append entries message.
"""
rpc = msg.value
if self.state == State.LEADER:
if rpc.success:
self.nextIndex[msg.source] = rpc.lastLogIndex + 1
self.matchIndex[msg.source] = rpc.lastLogIndex
else:
# Decrement next index and retry append entries
# Ensure to floor the nextIndex to 1 (the start of the log).
nidx = self.nextIndex[msg.source] - 1
self.nextIndex[msg.source] = max(nidx, 1)
self.send_append_entries(msg.source)
# Decide if we can commit the entry
for n in xrange(self.log.lastApplied, self.log.commitIndex, -1):
commit = Election(self.matchIndex.keys())
for k, v in self.matchIndex.iteritems():
commit.vote(k, v >= n)
if commit.has_passed() and self.log[n][1] == self.currentTerm:
# Commit all versions from the last log entry to now.
for idx in xrange(self.log.commitIndex, n+1):
if self.log[idx][0] is None: continue
forte = True if settings.simulation.forte_on_commit else False
self.log[idx][0].update(self, commit=True, forte=forte)
# Set the commit index and break
self.log.commitIndex = n
break
elif self.state == State.CANDIDATE:
# Decide whether or not to step down.
if rpc.term >= self.currentTerm:
## Become a follower
self.state = State.FOLLOWER
## Log the failed election
self.sim.logger.info(
"{} has stepped down as candidate".format(self)
)
return
elif self.state == State.FOLLOWER:
# Ignore AE messages if we are the follower.
return
else:
raise RaftRPCException(
"Append entries response in unknown state: '{}'".format(self.state)
)
def on_remote_write_rpc(self, message):
"""
Unpacks the version from the remote write and initiates a local write.
"""
# Write the access from the remote replica
access = message.value.version
self.write(access)
# Check if the access was dropped (e.g. the write failed)
success = not access.is_dropped()
# Send the write response
self.send(message.source, WriteResponse(self.currentTerm, success, access))
def on_write_response_rpc(self, message):
"""
Completes the write if the remote write was successful.
"""
rpc = message.value
if rpc.success:
rpc.access.complete()