Pika

Pika is a RabbitMQ (AMQP-0-9-1) client library for Python.

Introduction

Pika is a pure-Python implementation of the AMQP 0-9-1 protocol including RabbitMQ's extensions.

• Python 2.7 and 3.4+ are supported.
• Since threads aren't appropriate to every situation, it doesn't require threads. Pika core takes care not to forbid them, either. The same goes for greenlets, callbacks, continuations, and generators. An instance of Pika's built-in connection adapters is not thread-safe, however.
• People may be using direct sockets, plain old select(), or any of the wide variety of ways of getting network events to and from a python application. Pika tries to stay compatible with all of these, and to make adapting it to a new environment as simple as possible.

Documentation

Pika's documentation can be found at https://pika.readthedocs.io

Example

Here is the most simple example of use, sending a message with the BlockingConnection adapter:

import pika
connection = pika.BlockingConnection()
channel = connection.channel()
channel.basic_publish(exchange='example',
                      routing_key='test',
                      body=b'Test Message')
connection.close()

And an example of writing a blocking consumer:

import pika
connection = pika.BlockingConnection()
channel = connection.channel()

for method_frame, properties, body in channel.consume('test'):

    # Display the message parts and ack the message
    print(method_frame, properties, body)
    channel.basic_ack(method_frame.delivery_tag)

    # Escape out of the loop after 10 messages
    if method_frame.delivery_tag == 10:
        break

# Cancel the consumer and return any pending messages
requeued_messages = channel.cancel()
print('Requeued %i messages' % requeued_messages)
connection.close()

Pika provides the following adapters

• AsyncioConnection - adapter for the Python3 AsyncIO event loop
• BlockingConnection - enables blocking, synchronous operation on top of library for simple usage
• SelectConnection - fast asynchronous adapter without 3rd-party dependencies
• TornadoConnection - adapter for use with the Tornado IO Loop http://tornadoweb.org
• TwistedProtocolConnection - adapter for use with the Twisted asynchronous package http://twistedmatrix.com/

Multiple Connection Parameters

You can also pass multiple connection parameter instances for fault-tolerance as in the code snippet below (host names are just examples, of course). To enable retries, set connection_attempts and retry_delay as needed in the last pika.ConnectionParameters element of the sequence. Retries occur after connection attempts using all of the given connection parameters fail.

import pika
configs = (
    pika.ConnectionParameters(host='rabbitmq.zone1.yourdomain.com'),
    pika.ConnectionParameters(host='rabbitmq.zone2.yourdomain.com',
                              connection_attempts=5, retry_delay=1))
connection = pika.BlockingConnection(configs)

With non-blocking adapters, such as SelectConnection and AsyncioConnection, you can request a connection using multiple connection parameter instances via the connection adapter's create_connection() class method.

Requesting message ACKs 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 ioloop'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 ACK'ed directly from that thread, since all accesses to the connection adapter instance must be from a single thread - the thread that is running the adapter's ioloop. However, this may be accomplished by requesting a callback to be executed in the adapter's ioloop 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 ACKed was retrieved (AMQP protocol constraint).
    """
    if channel.is_open:
        channel.basic_ack(delivery_tag)
    else:
        # Channel is already closed, so we can't ACK 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 ioloop thread using an adapter-specific mechanism:

• :py:class:`pika.BlockingConnection` abstracts its ioloop from the application and thus exposes :py:meth:`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))

• When using a non-blocking connection adapter, such as :py:class:`pika.AsyncioConnection` or :py:class:`pika.SelectConnection`, you use the underlying asynchronous framework's native API for requesting an ioloop-bound callback from another thread. For example, SelectConnection's IOLoop provides add_callback_threadsafe(), Tornado's IOLoop has add_callback(), while asyncio's event loop exposes call_soon_threadsafe().

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

Some RabbitMQ clients (Bunny, Java, .NET, Objective-C/Swift) provide a way to automatically recover connection, its channels and topology (e.g. queues, bindings and consumers) after a network failure. Others require connection recovery to be performed by the application code and strive to make it a straightforward process. Pika falls into the second category.

Pika supports multiple connection adapters. They take different approaches to connection recovery.

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

import pika
while(True):
    try:
        connection = pika.BlockingConnection(parameters)
        channel = connection.channel()
        channel.basic_consume('queue-name', on_message_callback)
        channel.start_consuming()
    # Do not recover if connection was closed by broker
    except pika.exceptions.ConnectionClosedByBroker:
        break
    # Do not recover on channel errors
    except pika.exceptions.AMQPChannelError:
        break
    # Recover on all other connection errors
    except pika.exceptions.AMQPConnectionError:
        continue

This example can be found in examples/consume_recover.py.

Generic operation retry libraries such as retry can be used:

from retry import retry
@retry(pika.exceptions.AMQPConnectionError, delay=5, jitter=(1, 3))
def consume():
    connection = pika.BlockingConnection(parameters)
    channel = connection.channel()
    channel.basic_consume('queue-name', on_message_callback)
    try:
        channel.start_consuming()
    # Do not recover connections closed by server
    except pika.exceptions.ConnectionClosedByBroker:
        pass
consume()

Decorators make it possible to configure some additional recovery behaviours, like delays between retries and limiting the number of retries.

The above example can be found in examples/consume_recover_retry.py.

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

Contributing

To contribute to pika, please make sure that any new features or changes to existing functionality include test coverage.

Pull requests that add or change code without adequate test coverage will be rejected.

Additionally, please format your code using yapf with google style prior to issuing your pull request.

Extending to support additional I/O frameworks

New non-blocking adapters may be implemented in either of the following ways:

• By subclassing :py:class:`pika.adapters.base_connection.BaseConnection` and implementing its abstract method(s) and passing BaseConnection's constructor an implementation of :py.class:`pika.adapters.utils.nbio_interface.AbstractIOServices`. BaseConnection implements pika.connection.connection.Connection's pure virtual methods, including internally-initiated connection logic. For examples, refer to the implementations of :py:class:`pika.AsyncioConnection` and :py:class:`pika.TornadoConnection`.
• By subclassing :py:class:`pika.connection.connection.Connection` and implementing its abstract method(s). This approach facilitates implementation of of custom connection-establishment and transport mechanisms. For an example, refer to the implementation of :py:class:`pika.adapters.twisted_connection.TwistedProtocolConnection`.