.. _`distribute tests across machines`:

``py.test`` can ad-hoc distribute test runs to multiple CPUs or remote
machines.  This allows to speed up development or to use special resources
of remote machines.  Before running tests remotely, ``py.test`` efficiently 
synchronizes your program source code to the remote place.  All test results 
are reported back and displayed to your local test session.  You may 
specify different Python versions and interpreters.

Speed up test runs by sending tests to multiple CPUs
----------------------------------------------------------

To send tests to multiple CPUs, type::

    py.test -n NUM

Especially for longer running tests or tests requiring 
a lot of IO this can lead to considerable speed ups. 

Test on a different python version
----------------------------------------------------------

To send tests to a python2.4 interpreter process, you may type::

    py.test --tx popen//python=python2.4

This will start a subprocess which is run with the "python2.4"
Python interpreter, found in your system binary lookup path. 

If you prefix the --tx option like this::

    py.test --tx 3*popen//python=python2.4

then three subprocesses would be created. 


Sending tests to remote SSH accounts
------------------------------------

Suppose you have a package ``mypkg`` which contains some 
tests that you can successfully run locally. And you
have a ssh-reachable machine ``myhost``.  Then    
you can ad-hoc distribute your tests by typing::

    py.test -d --tx ssh=myhostpopen --rsyncdir mypkg mypkg

This will synchronize your ``mypkg`` package directory 
to an remote ssh account and then locally collect tests 
and send them to remote places for execution.  

You can specify multiple ``--rsyncdir`` directories 
to be sent to the remote side. 

Sending tests to remote Socket Servers
----------------------------------------

Download the single-module `socketserver.py`_ Python program 
and run it on the specified hosts like this::

    python socketserver.py

It will tell you that it starts listening.  You can now
on your home machine specify this new socket host
with something like this::

    py.test -d --tx socket=192.168.1.102:8888 --rsyncdir mypkg mypkg


no remote installation requirements 
++++++++++++++++++++++++++++++++++++++++++++

Synchronisation and running of tests only requires
a bare Python installation on the remote side.   No
special software is installed - this is realized through the
*zero installation* principle that the underlying 
`py.execnet`_ mechanisms implements. 


.. _`socketserver.py`: ../execnet/script/socketserver.py
.. _`py.execnet`: execnet.html

Differences from local tests
----------------------------

* Test order is rather random (instead of in file order). 
* the test process may hang due to network problems 
* you may not reference files outside of rsynced directory structures

Specifying test exec environments in a conftest.py
-------------------------------------------------------------

Instead of specifying command line options, you can 
put options values in a ``conftest.py`` file like this::

    pytest_option_tx = ['ssh=myhost//python=python2.5', 'popen//python=python2.5']
    pytest_option_dist = True

Any commandline ``--tx`` specifictions  will add to the list of available execution
environments. 

Specifying "rsync" dirs in a conftest.py
-------------------------------------------------------------

In your ``mypkg/conftest.py`` you may specify directories to synchronise
or to exclude::

    rsyncdirs = ['.', '../plugins']
    rsyncignore = ['_cache']

These directory specifications are relative to the directory
where the ``conftest.py`` is found.