Clustering
==========

Clustering an OMERO instance consists of starting multiple
:doc:`/developers/server-blitz` servers with each allocating user
sessions based on some criteria. There are at least two reasons you may
want to cluster the OMERO server: availability and throughput.

Availability
------------

Having the ability to have two servers up at the same time implies that
even if you have to restart one of the servers, there should be no
down-time. Currently, |OmeroSessions| are
sticky to a cluster node and so it is not possible shutdown a node at any
time. All new sessions can be redirected to the server which is to
be left turned on however, then when all active sessions have completed, the
chosen server can be shutdown.

Throughput
----------

The other main reason to have other servers running is to service more
user sessions simultaneously. Out of the box, each
:doc:`/developers/server-blitz` process is configured for 400MB of
memory. When dealing with memory intensive operations like rendering,
each added server can make a positive difference. This is only a part of
the story, since much of the bottleneck is not the server itself but
other shared resources, like the database or the filesystem, and so to
further extend throughput, you will need to parallelize these.

Installation
------------

If you are using the default |OmeroGrid|
:source:`application descriptor <etc/templates/grid/default.xml>`
quickly enabling clustering is as simple as executing:

::

     bin/omero config set omero.cluster.redirector configRedirector
     bin/omero node backup start

This starts a second node, named "backup", which contains a second
:doc:`/developers/server-blitz` server, "Blitz-1". By default, this
newly created server will not be used until sessions are manually
redirected to it.

.. seealso:: :doc:`/developers/Server/ScalingOmero`