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•WebLogic Server provides clustering services for:
–>EJB and RMI applications
•Essential services of clustering:
–>Heartbeats and JNDI updates – Multicast (UDP)
–>Session replication –IP sockets(TCP)
–>JNDI state dump – http url connection
•J2EE spec does not mention about clustering.
So this is a value add on feature that most application server vendors provide.Since the spec does not talk about it, each vendor would implement this in their own way. However the ultimate objectives are “loadbalancing and failover”.So all the cluster related information would be defined in weblogic.xml (This is vendor specific descriptor file)
WebLogic Server provides clustering support for servlets and JSPs by replicating the HTTP session state of clients that access clustered servlets and JSPs. WebLogic Server can maintain HTTP session states in memory, a filesystem, or a database.
Replication involves, copying all the session information on to another server (called secondary server). In case of weblogic this is a RMI (t3) call.
WebLogic has five different implementations of session persistence: (set in weblogic.xml)
•Memory (default – session info stored in memory)
•File system persistence (session info stored in text file)
•JDBC persistence (session info stored in database)
•Cookie-based session persistence (session info stored in client browser)
•In-memory replication (Session info stored both in primary and secondary servers memory. Most commonly used in cluster setup)
Communication between Apache and WebLogic Cluster
A typical journey of request (Shared platform),Request comes through hardware loadbalancer and gets forwarded to one of the Apache server.
WebLogic Apache plugin gets invoked, delegates the http request to one of the servers in cluster. (It gets the information of weblogic instances from httpd.conf file).
WebLogic server would create a session cookie and send back the session cookie as http response header.As long as browser is alive, the session cookie is passed (as http header) and Apache plugin would parse the session cookie and route the request to same server.