You can use the load balancer and multiple clusters to upgrade components within the Enterprise Server without any loss of service. A component can, for example, be a JVM, the Enterprise Server, or a web application.
This approach is not possible if:
You change the schema of the high-availability database (HADB). For more information, see Chapter 3, Administering High Availability Database
The HADB software is supplied with the Enterprise Server standalone distribution of Sun GlassFish Enterprise Server. For information about available distributions of Sun GlassFish Enterprise Server, see Distribution Types and Their Components in Sun GlassFish Enterprise Server 2.1 Installation Guide. HADB features are available only in the enterprise profile. For information about profiles, see Usage Profiles in Sun GlassFish Enterprise Server 2.1 Administration Guide.
You perform an application upgrade that involves a change to the application database schema.
Upgrade all server instances in a cluster together. Otherwise, there is a risk of version mismatch caused by a session failing over from one instance to another where the instances have different versions of components running.
Stop one of the clusters using the Stop Cluster button on the General Information page for the cluster.
Upgrade the component in that cluster.
Start the cluster using the Start Cluster button on the General Information page for the cluster.
Repeat the process with the other clusters, one by one.
Because sessions in one cluster will never fail over to sessions in another cluster, there is no risk of version mismatch caused by a session’s failing over from a server instance that is running one version of the component to another server instance (in a different cluster) that is running a different version of the component. A cluster in this way acts as a safe boundary for session failover for the server instances within it.