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Jobs can fail due to exceptions occurring. When your RQ workers run in the background, how do you get notified of these exceptions?

Default: the failed queue

The default safety net for RQ is the failed queue. Every job that fails execution is stored in here, along with its exception information (type, value, traceback). While this makes sure no failing jobs “get lost”, this is of no use to get notified pro-actively about job failure.

Custom exception handlers

Starting from version 0.3.1, RQ supports registering custom exception handlers. This makes it possible to replace the default behaviour (sending the job to the failed queue) altogether, or to take additional steps when an exception occurs.

This is how you register custom exception handler(s) to an RQ worker:

from rq.handlers import move_to_failed_queue  # RQ's default exception handler

w = Worker([q], exception_handlers=[my_handler, move_to_failed_queue])

The handler itself is a function that takes the following parameters: job, exc_type, exc_value and traceback:

def my_handler(job, exc_type, exc_value, traceback):
    # do custom things here
    # for example, write the exception info to a DB

You might also see the three exception arguments encoded as:

def my_handler(job, *exc_info):
    # do custom things here

Chaining exception handlers

The handler itself is responsible for deciding whether or not the exception handling is done, or should fall through to the next handler on the stack. The handler can indicate this by returning a boolean. False means stop processing exceptions, True means continue and fall through to the next exception handler on the stack.

It’s important to know for implementors that, by default, when the handler doesn’t have an explicit return value (thus None), this will be interpreted as True (i.e. continue with the next handler).

To replace the default behaviour (i.e. moving the job to the failed queue), use a custom exception handler that doesn’t fall through, for example:

def black_hole(job, *exc_info):
    return False