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Pika Python AMQP Client Library

Project description

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

Version Python versions Status Coverage License Documentation Status

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 isn’t 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 pika.BlockingConnection adapter:

import pika

connection = pika.BlockingConnection()
channel = connection.channel()
channel.basic_publish(exchange='test', 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 acknowledge 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

  • pika.adapters.asyncio_connection.AsyncioConnection - asynchronous adapter for Python 3 AsyncIO’s I/O loop.
  • pika.BlockingConnection - synchronous adapter on top of library for simple usage.
  • pika.SelectConnection - asynchronous adapter without third-party dependencies.
  • pika.adapters.tornado_connection.TornadoConnection - asynchronous adapter for use with Tornado’s I/O loop.
  • pika.adapters.twisted_connection.TwistedProtocolConnection - asynchronous adapter for use with Twisted’s I/O loop.

Multiple connection parameters

You can also pass multiple pika.ConnectionParameters 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

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

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

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))
    
  • When using a non-blocking connection adapter, such as pika.adapters.asyncio_connection.AsyncioConnection or pika.SelectConnection, you use the underlying asynchronous framework’s native API for requesting an I/O loop-bound callback from another thread. For example, pika.SelectConnection’s I/O loop provides add_callback_threadsafe(), pika.adapters.tornado_connection.TornadoConnection’s I/O loop has add_callback(), while pika.adapters.asyncio_connection.AsyncioConnection’s I/O 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 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

This example can be found in examples/consume_recover.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()

This 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. Note: only format those lines that you have changed in your pull request. If you format an entire file and change code outside of the scope of your PR, it will likely be rejected.

Extending to support additional I/O frameworks

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

  • By subclassing pika.BaseConnection, implementing its abstract method and passing its constructor an implementation of pika.adapters.utils.nbio_interface.AbstractIOServices. pika.BaseConnection implements pika.connection.Connection’s abstract methods, including internally-initiated connection logic. For examples, refer to the implementations of pika.adapters.asyncio_connection.AsyncioConnection and pika.adapters.tornado_connection.TornadoConnection.
  • By subclassing pika.connection.Connection and implementing its abstract methods. This approach facilitates implementation of custom connection-establishment and transport mechanisms. For an example, refer to the implementation of pika.adapters.twisted_connection.TwistedProtocolConnection.

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