.. _performance:

================================

Optimizing Kallithea performance

================================

When serving a large amount of big repositories, Kallithea can start performing

slower than expected. Because of the demanding nature of handling large amounts

of data from version control systems, here are some tips on how to get the best

performance.

Fast storage

------------

Kallithea is often I/O bound, and hence a fast disk (SSD/SAN) and plenty of RAM

is usually more important than a fast CPU.

Caching

-------

Tweak beaker cache settings in the ini file. The actual effect of that is

questionable.

Database

--------

SQLite is a good option when having a small load on the system. But due to

locking issues with SQLite, it is not recommended to use it for larger

deployments.

Switching to MySQL or PostgreSQL will result in an immediate performance

increase. A tool like SQLAlchemyGrate_ can be used for migrating to another

database platform.

Horizontal scaling

------------------

Scaling horizontally means running several Kallithea instances and let them

share the load. That can give huge performance benefits when dealing with large

amounts of traffic (many users, CI servers, etc.). Kallithea can be scaled

horizontally on one (recommended) or multiple machines.

It is generally possible to run WSGI applications multithreaded, so that

several HTTP requests are served from the same Python process at once. That can

in principle give better utilization of internal caches and less process

overhead.

One danger of running multithreaded is that program execution becomes much more

complex; programs must be written to consider all combinations of events and

problems might depend on timing and be impossible to reproduce.

Kallithea can't promise to be thread-safe, just like the embedded Mercurial

backend doesn't make any strong promises when used as Kallithea uses it.

Instead, we recommend scaling by using multiple server processes.

Web servers with multiple worker processes (such as ``mod_wsgi`` with the

``WSGIDaemonProcess`` ``processes`` parameter) will work out of the box.

In order to scale horizontally on multiple machines, you need to do the

following:

    - Each instance's ``data`` storage needs to be configured to be stored on a

      shared disk storage, preferably together with repositories. This ``data``

      dir contains template caches, sessions, whoosh index and is used for

      task locking (so it is safe across multiple instances). Set the

      ``cache_dir``, ``index_dir``, ``beaker.cache.data_dir``, ``beaker.cache.lock_dir``

      variables in each .ini file to a shared location across Kallithea instances

    - If using several Celery instances,

      the message broker should be common to all of them (e.g.,  one

      shared RabbitMQ server)

    - Load balance using round robin or IP hash, recommended is writing LB rules

      that will separate regular user traffic from automated processes like CI

      servers or build bots.

Serve static files directly from the web server

-----------------------------------------------

With the default ``static_files`` ini setting, the Kallithea WSGI application

will take care of serving the static files from ``kallithea/public/`` at the

root of the application URL.

The actual serving of the static files is very fast and unlikely to be a

problem in a Kallithea setup - the responses generated by Kallithea from

database and repository content will take significantly more time and

resources.

To serve static files from the web server, use something like this Apache config

snippet::

        Alias /images/ /srv/kallithea/kallithea/kallithea/public/images/

        Alias /css/ /srv/kallithea/kallithea/kallithea/public/css/

        Alias /js/ /srv/kallithea/kallithea/kallithea/public/js/

        Alias /codemirror/ /srv/kallithea/kallithea/kallithea/public/codemirror/

        Alias /fontello/ /srv/kallithea/kallithea/kallithea/public/fontello/

Then disable serving of static files in the ``.ini`` ``app:main`` section::

        static_files = false

If using Kallithea installed as a package, you should be able to find the files

under ``site-packages/kallithea``, either in your Python installation or in your

virtualenv. When upgrading, make sure to update the web server configuration

too if necessary.

It might also be possible to improve performance by configuring the web server

to compress responses (served from static files or generated by Kallithea) when

serving them. That might also imply buffering of responses - that is more

likely to be a problem; large responses (clones or pulls) will have to be fully

processed and spooled to disk or memory before the client will see any

response. See the documentation for your web server.

.. _SQLAlchemyGrate: https://github.com/shazow/sqlalchemygrate