Connection Adapters

Pika uses connection adapters to provide a flexible method for adapting pika’s core communication to different IOLoop implementations. In addition to asynchronous adapters, there is the BlockingConnection adapter that provides a more idiomatic procedural approach to using Pika.

Adapters

• Interacting with Pika from another thread
• Class Reference
• Select Connection Adapter
• Asyncio Connection Adapter
• Tornado Connection Adapter
• Twisted Connection Adapter
• Gevent Connection Adapter

Requesting message acknowledgements from another thread

The single-threaded usage constraint of an individual Pika connection adapter instance may result in a dropped AMQP/stream connection due to AMQP heartbeat timeout in consumers that take a long time to process an incoming message. A common solution is to delegate processing of the incoming messages to another thread, while the connection adapter’s thread continues to service its I/O loop’s message pump, permitting AMQP heartbeats and other I/O to be serviced in a timely fashion.

Messages processed in another thread may not be acknowledged directly from that thread, since all accesses to the connection adapter instance must be from a single thread, which is the thread running the adapter’s I/O loop. This is accomplished by requesting a callback to be executed in the adapter’s I/O loop thread. For example, the callback function’s implementation might look like this:

def ack_message(channel, delivery_tag):
    """Note that `channel` must be the same Pika channel instance via which
    the message being acknowledged was retrieved (AMQP protocol constraint).
    """
    if channel.is_open:
        channel.basic_ack(delivery_tag)
    else:
        # Channel is already closed, so we can't acknowledge this message;
        # log and/or do something that makes sense for your app in this case.
        pass

The code running in the other thread may request the ack_message() function to be executed in the connection adapter’s I/O loop thread using an adapter-specific mechanism:

• pika.BlockingConnection abstracts its I/O loop from the application and thus exposes pika.BlockingConnection.add_callback_threadsafe(). Refer to this method’s docstring for additional information. For example:

  connection.add_callback_threadsafe(functools.partial(ack_message, channel, delivery_tag))
  

Please see the documentation of other adapters for their specific methods.

This threadsafe callback request mechanism may also be used to delegate publishing of messages, etc., from a background thread to the connection adapter’s thread.

Connection recovery

Pika requires connection recovery to be performed by the application code and strives to make it a straightforward process.

Different connection adapters take different approaches to connection recovery.

For pika.BlockingConnection adapter exception handling can be used to check for connection errors. Here is a very basic example:

import pika

while True:
    try:
        connection = pika.BlockingConnection()
        channel = connection.channel()
        channel.basic_consume('test', on_message_callback)
        channel.start_consuming()
    # Don't recover if connection was closed by broker
    except pika.exceptions.ConnectionClosedByBroker:
        break
    # Don't recover on channel errors
    except pika.exceptions.AMQPChannelError:
        break
    # Recover on all other connection errors
    except pika.exceptions.AMQPConnectionError:
        continue

A similar example can be found in examples/blocking_consume_recover_multiple_hosts.py.

Generic operation retry libraries such as retry can be used. Decorators make it possible to configure some additional recovery behaviours, like delays between retries and limiting the number of retries:

from retry import retry


@retry(pika.exceptions.AMQPConnectionError, delay=5, jitter=(1, 3))
def consume():
    connection = pika.BlockingConnection()
    channel = connection.channel()
    channel.basic_consume('test', on_message_callback)

    try:
        channel.start_consuming()
    # Don't recover connections closed by server
    except pika.exceptions.ConnectionClosedByBroker:
        pass


consume()

For asynchronous adapters, use on_close_callback to react to connection failure events. This callback can be used to clean up and recover the connection.

An example of recovery using on_close_callback can be found in examples/asynchronous_consumer_example.py.