nameko-amqp-retry 0.3.2

Nameko extension allowing AMQP entrypoints to retry later

nameko-amqp-retry
=================

Extension for `nameko <http://nameko.readthedocs.org>`_ that allows the built-in AMQP entrypoints to schedule a later retry via redelivery of a message.

RabbitMQ 3.5.4 or later is required.


Installation
------------

Install the library from PyPI::

pip install nameko-amqp-retry


Usage
-----

This library subclasses nameko's built-in entrypoints. Use these subclasses in your service definition, and then raise :class:`nameko_amqp_retry.Backoff` inside an entrypoint you wish to retry later:

.. code-block:: python

from nameko_amqp_retry import Backoff
from nameko_amqp_retry.rpc import rpc

class Service:
name = "service"

@rpc
def calculate(self):
""" Calculate something, or schedule a retry if not ready yet.
"""
if not_ready_yet:
raise Backoff()

return 42

The caller will see the final result, or a :class:`Backoff.Expired` exception if more than the allowed number of retries were made:

.. code-block:: python

>>> n.rpc.service.calculate()
... # blocks for some time
>>> 42

.. code-block:: python

>>> n.rpc.service.calculate()
Traceback (most recent call last):
...
nameko.exceptions.RemoteError: Expired Backoff aborted after ...

The retry schedule is controlled by attributes on the `Backoff` class. You should override them in a subclass to control the behaviour. For example:

Fixed schedule:

.. code-block:: python

class RegularBackoff(Backoff):
""" Retries every 1000ms until limit
"""
schedule = (1000,) # ms

No limit:

.. code-block:: python

class InfiniteBackoff(Backoff):
""" Retries forever
"""
limit = 0


Custom schedule:

.. code-block:: python

class ImpatientBackoff(Backoff):
""" Retries after 100, then 200, then 500 milliseconds
"""
schedule = (100, 200, 500) # ms


Dynamic schedule:

.. code-block:: python

class DynamicBackoff(Backoff):
""" Calculates schedule dynamically
"""
@classmethod
def get_next_schedule_item(cls, index):
...

Alternatively, an entrypoint can be decorated with the `entrypoint_retry` decorator, to automatically retry the method if it raises certain exceptions:

.. code-block:: python

from nameko_amqp_retry import entrypoint_retry
from nameko_amqp_retry.rpc import rpc

class Service:
name = "service"

@rpc
@entrypoint_retry(retry_for=ValueError)
def calculate(self):
""" Calculate something, or schedule a retry if not ready yet.
"""
if not_ready_yet:
raise ValueError()

return 42

@rpc
@entrypoint_retry(
retry_for=(TypeError, ValueError),
limit=5,
schedule=(500, 600, 700, 800, 900),
)
def do_something(self):
""" Calculate something else, or schedule a retry if not ready yet.
"""
if type_not_ready_yet:
raise TypeError()

if value_not_ready_yet:
raise ValueError()

return 24


See docs/examples for more.