ADF Session Replication on a Cluster for High Availability

Running an ADF application on a weblogic cluster is fairly simple. Just deploy the application to the cluster and your done. This gives you scalability as the load is distributed over multiple managed servers. However, it does not give you protection against machine failure out of the box. For that to work, you need to make sure all state in your ADF application is replicated across multiple nodes in the cluster. This post will explain the things you have to think about when setting up this session replication.

Most of this information is based on Configuring High Availability for Fusion Web Applications chapter of the Fusion Web Applications Developer's Guide but we've added some undocumented features and best practices based on our experience.

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• First thing you need to do is tell weblogic to replicate a http session to other nodes in the cluster. This is done by setting persistent-store-type to replicated_if_clustered in your weblogic.xml file:

  <weblogic-web-app>
    <session-descriptor>
      <persistent-store-type>
        replicated_if_clustered
      </persistent-store-type>
    </session-descriptor>
  </weblogic-web-app>
  

• During development on my local workstation I like to set the session-description slightly differently:

  <weblogic-web-app>
    <session-descriptor>
      <persistent-store-type>file</persistent-store-type>
      <persistent-store-dir>c:/temp/</persistent-store-type>
      <cache-size>0</cache-size>
    </session-descriptor>
  </weblogic-web-app>
  

  This tells weblogic to write the http session request to file after each request. Since the cache-size is set to 0 it will not keep any session in memory. This means each subsequent request from the same session has to restore the session from file first. Since this (de)serializes the http session on each request it behaves the same as a session failover on a cluster after each request. This is a great way to test if you application is really cluster-safe and if none of the managed beans or other objects will loose their state.
• In your web.xml you have to set CHECK_FILE_MODIFICATION to false:

  <web-app>
    ...
    <context-param>
      <param-name>
        org.apache.myfaces.trinidad.CHECK_FILE_MODIFICATION
      </param-name>
      <param-value>false</param-value>
    </context-param>
  <web-app>
  

  Having this on could lead to errors when a failover occurs. It is best practice to disable this for a production system anyhow. If you want this on during local development you should look into deployment plans to override such a setting per environment.