crontabber 0.18.1
A cron job runner with self-healing and job dependencies.

crontabber
==========

A cron job runner with self-healing and job dependencies.

License: `MPL 2 <http://www.mozilla.org/MPL/2.0/>`__

|Coverage Status|

|Build Status|

How to run tests
----------------

First you need to create a dedicated test database. We recommend you
call it ``test_crontabber``. Then you need the necessary credentials for
it.

Before running the tests you need to install some extras to be able to
run tests at all:

::

    pip install -r test-requirements.txt

Next, in the root directory of the project create a file called
``test-crontabber.ini`` and it should look something like this:

::

    [crontabber]
    user=myusername
    password=mypassword
    dbname=test_crontabber

To start all the tests run:

::

    PYTHONPATH=. nosetests

If you want to run a specific test in a specific file in a specific
class you can define it per the ``nosetests`` standard like this for
example:

::

    PYTHONPATH=. nosetests tests crontabber/tests/test_crontabber.py:TestCrontabber.test_basic_run_job

If you want the tests to stop as soon as the first test fails add ``-x``
to that same command above.

Also, if you want ``nosetests`` to *not* capture ``stdout`` add ``-s``
to that same command as above.

How to do code coverage analysis
--------------------------------

First you need to install the
`coverage <http://nedbatchelder.com/code/coverage/>`__ module. Then,
with ``nosetests``, you can run this:

::

    PYTHONPATH=. nosetests --with-coverage --cover-erase --cover-html --cover-package=crontabber

After it has run, you can open the file ``cover/index.html`` in browser.

How to run the exampleapp
-------------------------

The example app helps you set up a playground to play around with and
test crontabber to gain a better understanding of how it works.

The best place to start with is to read the ``exampleapp/README.md``
file and go through its steps. Once you get the basics to work you can
start experimenting with adding your job classes.

How locking works
-----------------

crontabber supports locking. It basically means if you start a second
instance of crontabber whilst it's already ongoing in another
terminal/server the second one will exist early. This is only applicable
if there is an actual job ongoing.

There are two kinds of locking.

1. **General locking.** The first thing crontabber does before it starts
   an app is to ask the state (stored in PostgreSQL) if it's ongoing and
   if it is, it exists with an error code of ``3``.

2. **Sub-second locking.** If the general locking (see point above) says
   "No, the job is not ongoing", it's going to proceed to update the
   state with a `row-level locking transaction in
   PostgreSQL <https://www.postgresql.org/docs/9.5/static/explicit-locking.html#LOCKING-ROWS>`__.
   That basically means PostgreSQL only allows one single ``UPDATE``
   from the process that gets there first. The second crontabber process
   will will exit early with an error code of ``2`` if the first
   crontabber process managed to run the ``UPDATE`` first.

Imagine two separate terminals starting crontabber at the almost same
time:

::

    # Terminal 1
    $ python crontabber.py --admin.conf=crontabber.ini
    $ echo $?
    0

::

    # Terminal 2 (started almost simultaneously)
    $ python crontabber.py --admin.conf=crontabber.ini
    $ echo $?
    3

**Note!** If a job has been ongoing to a maximum period of time, the
locking is ignored. This is controlled by the config option
``crontabber.max_ongoing_age_hours`` which defaults to **12 hours**.
This is applicable if crontabber, updates the state that it's starting a
job, then when it tries to update the state that it finished
(successfully or not) and that write fails, if for example it's unable
to make a connection to PostgreSQL. If this happens crontabber will just
ignore the lock and run it anyway.

.. |Coverage Status| image:: https://coveralls.io/repos/mozilla/crontabber/badge.png
   :target: https://coveralls.io/r/mozilla/crontabber
.. |Build Status| image:: https://travis-ci.org/mozilla/crontabber.svg?branch=master
   :target: https://travis-ci.org/mozilla/crontabber