def push(engine: Engine, id: str) -> Tuple[int, int]:
"""Returns (status, position) tuple.
next_position should be a guess; always guess low, or misordering can occur.
Position will be the reserved queue position, even if the user already existed."""
# -1. Non-authoritative check for existing user.
# This doesn't guarantee the user doesn't exist, it just saves a lot of empty queue positions
# when a malicious actor tries to enqueue themselves many times.
# (Mitigation factor depends on )
try:
existing = engine.query(Person, key=Person.id == id).first()
# If we don't hit an exception, person already exists
return 409, existing.position
except ConstraintViolation:
# Probably doesn't exist, keep going
pass
# 0. Reserve a row in QueueEntry with the next queue position.
# Don't insert the id until we confirm that this user
# hasn't already been enqueued.
entry = QueueEntry(position=0, enqueued_at=pendulum.now(), served_at=None)
while True:
try:
engine.save(entry, condition=QueueEntry.NOT_EXIST)
except ConstraintViolation:
# TODO | can be much faster:
# TODO | - use a binary walk from 0 until we hit a missing,
# TODO | - then binary walk back to an existing,
# TODO | - then use try save with increment of 1
entry.position += 1
else:
break
# 1. Now that we've reserved a row, try to build the id -> position mapping.
# Note that a failure here means the user is already queued, so we abort.
# Can't roll back the QueueEntry though, since another caller may have already
# advanced past us.
person = Person(id=id, position=entry.position)
try:
engine.save(person, condition=Person.NOT_ENQUEUED)
except ConstraintViolation:
return 409, entry.position
# 2. Success! Finally, push the id into the QueueEntry.
# It's ok if this fails, because we can always rebuild the id from the Person table.
entry.id = id
try:
engine.save(entry)
except BloopException:
# Can't be sure the entry is saved, but it's fine because we'll
# be able to rebuild from the Person
return 404, entry.position
return 200, entry.position
def serve(engine: Engine, id: str) -> int:
"""200 success, 404 missing row, 409 already served"""
# 0. Find the QueueEntry by Person
person = Person(id=id)
try:
engine.load(person)
except MissingObjects:
return 404
entry = QueueEntry(position=person.position)
try:
engine.load(entry)
except MissingObjects:
return 404
# 1. Update the time the entry was served at.
# If the entry was already served, don't change the time.
entry.served_at = pendulum.now()
try:
engine.save(entry, condition=QueueEntry.served_at.is_(None))
except ConstraintViolation:
# Already served, no change
return 409
else:
return 200
class Item(BaseModel):
id = Column(UUID, hash_key=True)
data = Column(Product)
engine = Engine()
engine.bind(BaseModel)
# ================================================
# Usage
# ================================================
item = Item(id=uuid.uuid4())
item.data = {
'Name': 'item-name',
'Rating': decimal.Decimal(str(random.random())),
'Updated': datetime.now(timezone.utc),
'Description': {
'Title': 'item-title',
'Body': 'item-body',
},
'Sellers': set()
}
for i in range(4):
seller_id = 'seller-{}'.format(i)
item.data['Sellers'].add(seller_id)
engine.save(item)
class Tweet(engine.model):
class Meta:
write_units = 10
account = Column(UUID, hash_key=True)
id = Column(String, range_key=True)
content = Column(String)
date = Column(DateTime(timezone='EU/Paris'))
favorites = Column(Integer)
by_date = GlobalSecondaryIndex(
hash_key='date', projection='keys_only')
engine.bind()
# ================================================
# Usage
# ================================================
account = Account(
id=uuid.uuid4(), name='@garybernhardt',
email='REDACTED')
tweet = Tweet(
account=account.id, id='616102582239399936',
content='today, I wrote a type validator in Python, as you do',
favorites=9,
date=arrow.now())
engine.save([account, tweet])
class Tweet(BaseModel):
class Meta:
write_units = 10
account = Column(UUID, hash_key=True)
id = Column(String, range_key=True)
content = Column(String)
date = Column(DateTime)
favorites = Column(Integer)
by_date = GlobalSecondaryIndex(hash_key='date', projection='keys')
engine = Engine()
engine.bind(BaseModel)
# ================================================
# Usage
# ================================================
account = Account(id=uuid.uuid4(), name='@garybernhardt', email='REDACTED')
tweet = Tweet(account=account.id,
id='616102582239399936',
content='today, I wrote a type validator in Python, as you do',
favorites=9,
date=datetime.now(timezone.utc))
engine.save(account, tweet)
})
class Item(engine.model):
id = Column(UUID, hash_key=True)
data = Column(Product)
engine.bind()
# ================================================
# Usage
# ================================================
item = Item(id=uuid.uuid4())
item.data = {
'Name': 'item-name',
'Rating': decimal.Decimal(str(random.random())),
'Updated': arrow.now(),
'Description': {
'Title': 'item-title',
'Body': 'item-body',
},
'Sellers': {}
}
for i in range(4):
seller_name = 'seller-{}'.format(i)
item.data['Sellers'][seller_name] = random.randint(0, 100)
engine.save(item)
