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.