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.

Supports Python 3.7+ (1.1.0 was the last version to support 2.7)
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:

.. code :: python

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:

.. code :: python

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 <https://docs.python.org/3/library/asyncio.html>_'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.gevent_connection.GeventConnection - asynchronous adapter
for use with Gevent <http://www.gevent.org>_'s I/O loop.
pika.adapters.tornado_connection.TornadoConnection - asynchronous adapter
for use with Tornado <http://tornadoweb.org>_'s I/O loop.
pika.adapters.twisted_connection.TwistedProtocolConnection - asynchronous
adapter for use with Twisted <http://twistedmatrix.com>_'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.

.. code :: python

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:

.. code :: python

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:

.. code :: python
```
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 a 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:

.. code :: python

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 <https://github.com/invl/retry>_ can be used. Decorators make it
possible to configure some additional recovery behaviours, like delays between
retries and limiting the number of retries:

.. code :: python

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 <http://pypi.python.org/pypi/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,
pika.adapters.gevent_connection.GeventConnection 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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Dependencies of pika (3).
Dependency	Optional
gevent	true
tornado	true
twisted	true

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