Question

In the context of Tomcat, can session replication takes place without enabling sticky session?

I understand the purpose of sticky session is to have the client ‘sticks’ to 1 server throughout the session. With session replication, the client’s interaction with the server is replicated throughout the cluster (many web servers).

In the above case, can session replication takes place? i.e. client’s session is spread though out the web servers and each interaction with any one web server is replicated across, thus, allowing seamless interaction.

Answer 1

AFAIK, tomcat clustering does not support non-sticky sessions. From tomcat docs:

Make sure that your loadbalancer is configured for sticky session
mode.

But there’s a solution (I created, so you know I’m biased :-)) called memcached-session-manager (msm), that also supports non-sticky sessions. msm uses memcached (or any backend speaking the memcached protocol) as backend for session backup/storage.

In non-sticky mode sessions are only stored in memcached and no longer in tomcat, as with non-sticky sessions the session-store must be external (to avoid stale data).

It also supports session locking: with non-sticky sessions multiple, parallel requests may hit different tomcats and could modify the session in parallel, so that some of the session changes might get overwritten by others. Session locking allows synchronization of parallel requests going to different tomcats.

The msm home page mainly describes the sticky session approach (as it started with this only), for details regarding non-sticky sessions you might search or ask on the mailing list.

Details and examples regarding the configuration can be found in the msm wiki (SetupAndConfiguration).

Just to give you an idea regarding the complexity: what you need is one or more memcached servers running (or s.th. similar speaking memcached) and an updated tomcat context.xml like this:

<Context>
  ...
  <Manager className="de.javakaffee.web.msm.MemcachedBackupSessionManager"
    memcachedNodes="n1:host1.domain.com:11211,n2:host2.domain.com:11211"
    sticky="false"
    sessionBackupAsync="false"
    lockingMode="auto"
    requestUriIgnorePattern=".*\.(ico|png|gif|jpg|css|js)$"
    />
</Context>

Your load-balancer does not need special configuration, so once you have these things in place you can start testing your application.