Threading model
===============

Eventlet
--------
Before the Flamingo release all OpenStack services used the *green thread*
model of threading, implemented through using the Python
`eventlet <http://eventlet.net/>`_ and
`greenlet <http://packages.python.org/greenlet/>`_ libraries.

Green threads use a cooperative model of threading: thread context
switches can only occur when specific eventlet or greenlet library calls are
made (e.g., sleep, certain I/O calls). From the operating system's point of
view, each OpenStack service runs in a single thread.

The use of green threads reduces the likelihood of race conditions, but does
not completely eliminate them. In some cases, you may need to use the
``@lockutils.synchronized(...)`` decorator to avoid races.

In addition, since there is only one operating system thread, a call that
blocks that main thread will block the entire process.

Yielding the thread in long-running tasks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If a code path takes a long time to execute and does not contain any methods
that trigger an eventlet context switch, the long-running thread will block
any pending threads.

This scenario can be avoided by adding calls to the eventlet sleep method
in the long-running code path. The sleep call will trigger a context switch
if there are pending threads, and using an argument of 0 will avoid introducing
delays in the case that there is only a single green thread::

    from eventlet import greenthread
    ...
    greenthread.sleep(0)

In current code, time.sleep(0) does the same thing as greenthread.sleep(0) if
time module is patched through eventlet.monkey_patch(). To be explicit, we recommend
contributors use ``greenthread.sleep()`` instead of ``time.sleep()``.

MySQL access and eventlet
~~~~~~~~~~~~~~~~~~~~~~~~~
There are some MySQL DB API drivers for oslo.db, like `PyMySQL`_, MySQL-python
etc. PyMySQL is the default MySQL DB API driver for oslo.db, and it works well with
eventlet. MySQL-python uses an external C library for accessing the MySQL database.
Since eventlet cannot use monkey-patching to intercept blocking calls in a C library,
so queries to the MySQL database will block the main thread of a service.

The Diablo release contained a thread-pooling implementation that did not
block, but this implementation resulted in a `bug`_ and was removed.

See this `mailing list thread`_ for a discussion of this issue, including
a discussion of the `impact on performance`_.

.. _bug: https://bugs.launchpad.net/nova/+bug/838581
.. _mailing list thread: https://lists.launchpad.net/openstack/msg08118.html
.. _impact on performance: https://lists.launchpad.net/openstack/msg08217.html
.. _PyMySQL: https://wiki.openstack.org/wiki/PyMySQL_evaluation

Native threading
----------------
Since the Flamingo release OpenStack started to transition away form
``eventlet``. During this transition Nova maintains support for running
services with ``eventlet`` while working to add support for running services
with ``native threading``.

To support both modes with the same codebase Nova started using the
`futurist`_ library. In native threading mode ``futurist.ThreadPoolsExecutors``
are used to run concurrent tasks and both the oslo.service and the
oslo.messaging libraries are configured to use native threads to execute tasks
like periodics and RPC message handlers.

.. _futurist: https://docs.openstack.org/futurist/latest/

To see how to configure and tune the native threading mode read the
:doc:`/admin/concurrency` guide.
