How Communications ServerProvides High Availability

Communications Server provides high availability through the following subcomponents and features:

• Storage for Session State Data

• In-Memory Replication on Other Servers in the Cluster

• Highly Available Clusters

Storage for Session State Data

Storing session state data enables the session state to be recovered after the failover of a server instance in a cluster. Recovering the session state enables the session to continue without loss of information. Communications Server provides the following types of high availability storage for HTTP/SIP session and stateful session bean data:

• In-memory replication on other servers in the cluster

In-Memory Replication on Other Servers in the Cluster

In-memory replication on other servers provides lightweight storage of session state data without the need to obtain a separate database. This type of replication uses memory on other servers for high availability storage of HTTP/SIP session and stateful session bean data. Clustered server instances replicate session state in a ring topology. Each backup instance stores the replicated data in memory. Replication of session state data in memory on other servers enables sessions to be distributed.

The use of in-memory replication requires the Group Management Service (GMS) to be enabled. For more information about GMS, see Group Management Service.

If server instances in a cluster are located on different machines, ensure that the following prerequisites are met:

• To ensure that GMS and in-memory replication function correctly, the machines must be on the same subnet.

• To ensure that in-memory replication functions correctly, the system clocks on all machines in the cluster must be synchronized as closely as possible.

Highly Available Clusters

A cluster is a collection of instances that work together as one logical entity. A cluster provides a runtime environment for one or more Java EE applications. A highly available cluster integrates a state replication service with clusters and load balancer.

Using clusters provides the following advantages:

• High availability, by allowing for failover protection for the server instances in a cluster. If one server instance goes down, other server instances take over the requests that the unavailable server instance was serving.

• Scalability, by allowing for the addition of server instances to a cluster, thus increasing the capacity of the system. The load balancer plug-in distributes requests to the available server instances within the cluster. No disruption in service is required as an administrator adds more server instances to a cluster.

All instances in a cluster:

• Reference the same configuration.

• Have the same set of deployed applications (for example, a Java EE application EAR file, a web module WAR or SAR file, or an EJB JAR file).

• Have the same set of resources, resulting in the same JNDI namespace.

Every cluster in the domain has a unique name; furthermore, this name must be unique across all node agent names, server instance names, cluster names, and configuration names. The name must not be domain. You perform the same operations on a cluster (for example, deploying applications and creating resources) that you perform on an unclustered server instance.

Clusters and Configurations

A cluster's settings are derived from a named configuration, which can potentially be shared with other clusters. A cluster whose configuration is not shared by other server instances or clusters is said to have a stand-alone configuration . By default, the name of this configuration is cluster_name -config, where cluster_name is the name of the cluster.

A cluster that shares its configuration with other clusters or instances is said to have a shared configuration.

Clusters, Instances, Sessions, and Load Balancing

Clusters, server instances, load balancers, and sessions are related as follows:

• A server instance is not required to 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 one or multiple machines. You can group server instances across different machines into a cluster.

• Each session is tied to a particular cluster. Therefore, although you can deploy an application on multiple clusters, session failover will occur only within a single cluster.

The cluster thus acts as a safe boundary for session failover for the server instances within the cluster.