Session replication / clustering / failover with Tomcat (Part 1 - Overview)

This is my first post of a series about Tomcat and clustering.
In this post i will explain some different replication mechanism.


NOTE: A common drawback of replication, like it's defined in the specs, is that only HttpSession#setAttribute(...) triggers replication.
This means that if you store a container object in the session and if you change a variable inside the object, you must HttpSession#setAttribute(...) again to replicate the modified object again.
Many frameworks implements a simple workaround for it - i will write about it another post.

All to all replication / DeltaManager 

With this mode, Tomcat just replicates the session to all registered nodes.

Pro:

• No single point of failure at software level
• Every node holds all sessions
• In theory, all other nodes can be turned off
• No extra software is needed
• Synchronous or Asynchronous replication
• Open Source
• Enterprise-Support available

Contra:

• High memory usage and network traffic because all nodes holds all sessions
• Objects which are stored in the session must be triggered manually to replicate again if a property has been changed
• Slower session serialization

Primary/Secondary Replication (Buddy Replication) / BackupManager

The sessions will be replicated to the next neighbor. If one Tomcat is not available anymore, the neighbor still holds the sessions.

 Pro:

• No single point of failure at software level
• Lower memory usage and network traffic because the session is not stored on all nodes
• No extra software is needed
• Synchronous or Asynchronous replication
• Open Source
• Enterprise-Support available

 Contra:

• Not quite as battle tested as the All-to-all replication (see Tomcat documentation)
• Objects which are stored in the session must be triggered manually to replicate again if a property has been change
• Slower session serialization

Memcached-session-manager

Each Tomcat replicates all sessions to N (multiple) Memcached nodes.
If a session is not available on the local Tomcat, the memcached-session-manager tries to lookup the session from the Memcached nodes.

 Pro:

• No single point of failure at software level
• More scalability
• Session will be replicated to N “Memcached” nodes – The sessions are still available if all webservers are down
• Automatically handles “Memcached” nodes failover
• Sticky mode: Lower memory usage because every node holds only the current used sessions
• Non-Sticky mode: Lowest memory usage because Tomcat does not hold the sessions anymore. Session will be received from memcached for each request
• MSM will automatically trigger replication if a property in a session stored object has been changed
• Faster session serialization – Changeable mechanism
• Synchronous or Asynchronous replication
• Open Source
• Enterprise-Support available for Tomcat

 Contra:

• Extra software needed (“Memcached” plugin and nodes)
• Cannot switch to another Java Webserver which does not automatically check changed values in the session - but only if the used framework has no implemented workaround
• AFAIK no Enterprise-Support available for the Memcached plugin but Martin Grotzke is very active on the mailing list

Enable failover with OpenWebBeans (> 1.2.0)

In my first post i already explained how it's possible to enable failover (aka clustering) with OpenWebBeans > 1.1.3: Enable failover with OpenWebBeans (> 1.1.3)

Because we moved the clustering logic to own module, we must include this module since OWB 1.2.0:

<dependency>
    <groupId>org.apache.openwebbeans</groupId>
    <artifactId>openwebbeans-clustering</artifactId>
</dependency>

You still need the org.apache.webbeans.web.failover.FailOverFilter but if you include this module, you must NOT add a custom openwebbeans.properties anymore. It will be registered automatically.

PrimeFaces Extensions 1.0

This week, Oleg released a RC of PrimeFaces Extensions as you can read here: PrimeFaces Extensions 1.0.0.RC1 released

To be honest, i had not much time to improve anything in PrimeFaces Extensions.
I'm PrimeFaces committer since January 2013 and it was more fun to implement new features, working on complex stuff together with Çağatay or improve performance directly in PrimeFaces Core.
I also will blog about some of this features the next weeks.

The biggest of my changes in PrimeFaces Extensions 1.0 are the PrimeFaces 4.0 compatibility and a new CKEditor version (4.2.1 now).

It's not sooo easy to marry CKEditor and the resource handling of JSF. I had to adjust some of there code and created some issues on their side.
If they will improve it and fix those issues, updating CKEditor will much easier in the future.
CKEditor removed and introduced new skins. Supported skins are "kama" and "moono" now.

We also added support for "my" new Search Expression Framework of PrimeFaces 4.0 in all our components.

We were also able to clean up some code because i could move some features or API changes directly to PrimeFaces Core.

Stay tuned for the final release!