Clusters, Instances, Load Balancers, and Sessions
Clusters, server instances, load balancers, and sessions are related as follows in the Application Server:
- It is not mandatory that a server instance be part of a cluster. However, an instance that is not part of a cluster cannot take advantage of high availability through transfer of session state from one instance to other instances.
- The server instances within a cluster can be hosted on different machines or on the same machine. That is, you can group server instances across different machines into a cluster.
- A particular load balancer can forward requests to server instances on multiple clusters. You can use this ability of the load balancer to perform an online upgrade without loss of service. For more information, see "Using Multiple Clusters for Online Upgrades Without Loss of Service".
- A single cluster can receive requests from multiple load balancers. If a cluster is served by more than one load balancer, you must configure the cluster in exactly the same way on each load balancer.
- Each session is tied to a particular cluster. What this implies is that though an application can be deployed on multiple clusters, sessions will only be failed over to server instances within the same cluster.
- The Application Server supports failover for HTTP sessions and stateful session bean (SFSB) sessions. Failover of certain J2EE object references that are stored within HTTP sessions is also supported.
The cluster thus acts as a safe boundary for session failover for the server instances within the cluster. It is possible to use the load balancer and upgrade components within the Application Server without loss of service. For more information, see "Using Multiple Clusters for Online Upgrades Without Loss of Service".
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