Server cluster

This page is going to describe some basic information related to server clustering...

Cluster installation

Requirements

database server - ACID compliant, for example PostgreSQL and MariaDB
main server that is able to share dataroot - locking support recommended, for example NFS
load balancer - for example Nginx
cluster nodes - web servers
Memcached server for shared caches

Note: this guide is not intended for Windows OS or any other Microsoft technologies.

Initial installation

Perform standard CLI installation on the main server using shared database and dataroot directory.
Setup web servers on cluster nodes - use local dirroot and shared database and dataroot.
Configure load balancing.

Related config.php settings

$CFG->wwwroot

It must be the same on all nodes, it must be the public facing URL. It cannot be dynamic.

$CFG->sslproxy

Enable if you have https:// wwwroot but the SSL is doen by load-balancer instead of web server.

Enable Secure cookies only to make your site really secure, without it cookie stealing is still trivial.

$CFG->reverseproxy

Enable if your nodes are accessed via different URL. Please note that it is not compatible with $CFG->loginhttps.

$CFG->dirroot

It is strongly recommended that $CFG->dirroot (which is automatically set via realpath(config.php)) contains the same path on all nodes. It does not need to point to the same shared directory though. The reason is that some some low level code may use the dirroot value for cache invalidation.

The simplest solution is to have the same directory structure on each cluster node and synchronise these during each upgrade.

The dirroot should be always read only for apache process because otherwise built in add-on installation and plugin uninstallation would get the nodes out of sync.

$CFG->dataroot

This MUST be a shared directory where each cluster node is accessing the files directly. It must be very reliable, administrators cannot manipulate files directly there.

Locking support is not required, if any code tries to use file locks in dataroot outside of cachedir or muc directory it is a bug.

$CFG->tempdir

It is recommended to use separate ram disks on each node. Scripts may use this directory during one request only. The contents of this directory may be deleted if there is no pending HTTP request, otherwise delete only files with older timestamps.

Always purge this directory when starting cluster node, you may get some notices or temporary errors if you purge this directory while other requests are active.

If any script tries to use temp files that were not created during current request it is a bug that needs to be fixed.

$CFG->cachedir

This directory MUST be shared by all cluster nodes. Locking is required.

$CFG->localcachedir

The difference from normal $CFG->cachedir is that the directory does not have to be shared by all cluster nodes, the file contents do not change.

Performance improvements

TODO

Upgrade procedure