.. _tut-remote_debug:

======================================
 Debugging Tasks Remotely (using pdb)
======================================

Basics
======

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

Example usage:

.. code-block:: python

    from celery import task
    from celery.contrib import rdb

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


:func:`~celery.contrib.rdb.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
:envvar:`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 :envvar:`CELERY_RDB_HOST`.

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

    [INFO/MainProcess] Got task from broker:
        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:

.. code-block:: bash

    $ 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'

.. _`Python Debugger Manual`: http://docs.python.org/library/pdb.html


Tips
====


Enabling the breakpoint signal
------------------------------

If the environment variable :envvar:`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>
