= Memcached Infrastructure SOP

Our memcached setup is currently only used for wiki sessions. With
mediawiki, sessions stored in files over NFS or in the DB are very slow.
Memcached is a non-blocking solution for our session storage.

== Contents

* <<_contact_information>>
* <<_checking_status>>
* <<_flushing_memcached>>
* <<_restarting_memcached>>
* <<_configuring_memcached>>

== Contact Information

Owner::
  Fedora Infrastructure Team
Contact::
  #fedora-admin, sysadmin-main, sysadmin-web groups
Servers::
  memcached03, memcached04
Purpose::
  Provide caching for Fedora web applications.

== Checking Status

Our memcached instances are currently firewalled to only allow access
from wiki application servers. To check the status of an instance, use:

....
echo stats | nc memcached0{3,4} 11211
....

from an allowed host.

== Flushing Memcached

Sometimes, wrong contents get cached, and the cache should be flushed.
To do this, use:

....
echo flush_all | nc memcached0{3,4} 11211
....

from an allowed host.

== Restarting Memcached

Note that restarting an memcached instance will drop all sessions stored
on that instance. As mediawiki uses hashing to distribute sessions
across multiple instances, restarting one out of two instances will
result in about half of the total sessions being dropped.

To restart memcached:

....
sudo /etc/init.d/memcached restart
....

== Configuring Memcached

Memcached is currently setup as a
https://pagure.io/fedora-infra/ansible/blob/main/f/roles/memcached[role in the ansible git repo].
The main two tunables are the MAXCONN (the maximum number of concurrent
connections) and CACHESIZE (the amount memory to use for storage). These
variables can be set through $memcached_maxconn and $memcached_cachesize
in ansible. Additionally, other options (as described in the memcached
manpage) can be set via `$memcached_options`.
