class PgsqlQueueConsumer(AbstractConsumer):
"""Base class for consumers of events sent from PostgreSQL via rabbitmq.
This class is intended to be used in a completely "stand-alone" manner, such as
inside a script being run separately as a background job. As such, it also includes
connecting to rabbitmq, declaring the underlying queue and bindings, and
(optionally) connecting to the database to be able to fetch and modify data as
necessary. It relies on the environment variable INI_FILE being set.
Note that all messages received by these consumers are expected to be in JSON
format.
"""
PGSQL_EXCHANGE_NAME = "pgsql_events"
def __init__(self,
queue_name: str,
routing_keys: Sequence[str],
uses_db: bool = True):
"""Initialize a new queue, bindings, and consumer for it."""
self.connection = Connection()
self.channel = self.connection.channel()
self.channel.queue_declare(queue_name, durable=True, auto_delete=False)
for routing_key in routing_keys:
self.channel.queue_bind(queue_name,
exchange=self.PGSQL_EXCHANGE_NAME,
routing_key=routing_key)
if uses_db:
self.db_session = get_session_from_config(os.environ["INI_FILE"])
else:
self.db_session = None
super().__init__(self.channel, queue_name)
def consume_queue(self) -> None:
"""Declare the consumer and consume messages indefinitely."""
self.declare()
self.connection.loop()
@abstractmethod
def run(self, msg: Message) -> None:
"""Process a delivered message (subclasses must implement)."""
pass
def start(self, msg: Message) -> None:
"""Setup/teardown for message-processing (wraps run())."""
# decode the msg body from JSON
msg.body = json.loads(msg.body)
# process the message, will call run()
super().start(msg)
# after processing is finished, commit the transaction and ack the msg
if self.db_session:
self.db_session.commit()
msg.ack()
