[Koha-devel] RFC: Koha::Persistent - for plack and general performance

[Frédéric Demians]

Food for though...

With Plack, we are in a persistent environment. It means we can keep
Koha configuration information in each thread. Config info are all data
that doesn't change that much and are not too large (like biblio
catalog).

In each thread, at startup, loading configuration data is required. In
Koha: frameworks, authorized_values, item types, branches, etc. It
implies a lot of SQL requests. If we have a application-level cache,
rather than loading info from DB or config files (koha-conf.xml XML
parsing), we can get them from the global cache (memcached, redis, MySQL
based, in-memory, etc.). A key/value cache doesn't handle Perl native
data type. So it means that data has to be deserialized. It consumes CPU,
less than getting data from DB/config files, but still. So in a
persistent environment, we should try to avoid that, and cache locally,
in each thread, configuration data.

We have something like in this schema:

   thread-1       thread-2       thread-n
      |              |              |
    local          local          local
    cache          cache          cache
      (              |              )
       \             |             /
         \           |           /
           +--- application----+
                   cache
                     |
             DB or config files

So if thread-2 modify something in its local cache, it can cascading
update application cache and DB / config files.  The difficulty is to
notify other threads to update their local cache.

The question is: is there such a mechanism in Plack? I don't think so.
Another solution could be to define a configurable timeout for each
task. Every n seconds, the thread is restarted. Or we can have directly
in Koha a timer which force reloading local cache. In both cases, the
timeout can be adjusted.

About the application cache, we may take a look at CHI module which seem
to be standard, and allow to switch easily cache repository and cache
serializer.

-- 
Frédéric DEMIANS
http://www.tamil.fr/u/fdemians.html