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Debugging Tasks Remotely (using pdb)

Basics

celery.contrib.rdb is an extended version of pdb that enables remote debugging of processes that does not have terminal access.

Example usage:

from celery import task
from celery.contrib import rdb

@task()
def add(x, y):
    result = x + y
    rdb.set_trace()  # <- set breakpoint
    return result

set_trace() sets a breakpoint at the current location and creates a socket you can telnet into to remotely debug your task.

The debugger may be started by multiple processes at the same time, so rather than using a fixed port the debugger will search for an available port, starting from the base port (6900 by default). The base port can be changed using the environment variable CELERY_RDB_PORT.

By default the debugger will only be available from the local host, to enable access from the outside you have to set the environment variable CELERY_RDB_HOST.

When the worker encounters your breakpoint it will log the following information:

[INFO/MainProcess] Received task:
    tasks.add[d7261c71-4962-47e5-b342-2448bedd20e8]
[WARNING/PoolWorker-1] Remote Debugger:6900:
    Please telnet 127.0.0.1 6900.  Type `exit` in session to continue.
[2011-01-18 14:25:44,119: WARNING/PoolWorker-1] Remote Debugger:6900:
    Waiting for client...

If you telnet the port specified you will be presented with a pdb shell:

$ telnet localhost 6900
Connected to localhost.
Escape character is '^]'.
> /opt/devel/demoapp/tasks.py(128)add()
-> return result
(Pdb)

Enter help to get a list of available commands, It may be a good idea to read the Python Debugger Manual if you have never used pdb before.

To demonstrate, we will read the value of the result variable, change it and continue execution of the task:

(Pdb) result
4
(Pdb) result = 'hello from rdb'
(Pdb) continue
Connection closed by foreign host.

The result of our vandalism can be seen in the worker logs:

[2011-01-18 14:35:36,599: INFO/MainProcess] Task
    tasks.add[d7261c71-4962-47e5-b342-2448bedd20e8] succeeded
    in 61.481s: 'hello from rdb'

Tips

Enabling the breakpoint signal

If the environment variable CELERY_RDBSIG is set, the worker will open up an rdb instance whenever the SIGUSR2 signal is sent. This is the case for both main and worker processes.

For example starting the worker with:

CELERY_RDBSIG=1 celery worker -l info

You can start an rdb session for any of the worker processes by executing:

kill -USR2 <pid>