Celery: abort task on connection error - rabbitmq

Question

I have to implement a Task subclass that gracefully fails if the broker is not running - currently I'm using RabbitMQ.
I could probably just use a try statement to catch the exception:
try:
Mytask.delay(arg1, arg2)
except socket.error:
# Send an notice to an admin
pass
but I'd like to create a subclass of Task that can handle that.
I've tried something like that:
class MyTask(Task):
ignore_result = True
def __call__(self, *args, **kwargs):
try:
return self.run(*args, **kwargs)
except socket.error:
# Send an notice to an admin
return None
but the workflow is clearly wrong. I think I need to inject maybe a backend subclass or a failure policy somehow.
Do you have any suggestion?

Answer 1

A possible solution I came up with:
import socket
from celery.decorators import task
from celery.task import Task
from celery.backends.base import BaseBackend
UNDELIVERED = 'UNDELIVERED'
class DummyBackend(BaseBackend):
"""
Dummy queue backend for undelivered messages (due to the broker being down).
"""
def store_result(self, *args, **kwargs):
pass
def get_status(self, *args, **kwargs):
return UNDELIVERED
def _dummy(self, *args, **kwargs):
return None
wait_for = get_result = get_traceback = _dummy
class SafeTask(Task):
"""
A task not raising socket errors if the broker is down.
"""
abstract = True
on_broker_error = None
errbackend = DummyBackend
#classmethod
def apply_async(cls, *args, **kwargs):
try:
return super(SafeTask, cls).apply_async(*args, **kwargs)
except socket.error, err:
if cls.on_broker_error is not None:
cls.on_broker_error(err, cls, *args, **kwargs)
return cls.app.AsyncResult(None, backend=cls.errbackend(),
task_name=cls.name)
def safetask(*args, **kwargs):
"""
Task factory returning safe tasks handling socket errors.
When a socket error occurs, the given callable *on_broker_error*
is called passing the exception object, the class of the task
and the original args and kwargs.
"""
if 'base' not in kwargs:
on_broker_error = kwargs.pop('on_broker_error', SafeTask.on_broker_error)
errbackend = kwargs.pop('errbackend', SafeTask.errbackend)
kwargs['base'] = type('SafeTask', (SafeTask,), {
'on_broker_error': staticmethod(on_broker_error),
'errbackend': errbackend,
'abstract': True,
})
return task(*args, **kwargs)
You can both subclass SafeTask or use the decorator #safetask.
If you can think of an improvement, don't hesitate to contribute.