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|Oracle GlassFish Server 3.1-3.1.1 High Availability Administration Guide|
Upgrading an application to a new version without loss of availability to users is called a rolling upgrade. Carefully managing the two versions of the application across the upgrade ensures that current users of the application complete their tasks without interruption, while new users transparently get the new version of the application. With a rolling upgrade, users are unaware that the upgrade occurs.
For more information about application versions and how they are identified, see Module and Application Versions in Oracle GlassFish Server 3.1 Application Deployment Guide.
In a clustered environment, a rolling upgrade redeploys an application with a minimal loss of service and sessions. A session is any artifact that can be replicated, for example:
Stateful session bean
A rolling upgrade can take place under light to moderate loads. The procedure requires about 10-15 minutes for each GlassFish Server instance.
Caution - To prevent the risk of version mismatch when a session fails over, upgrade all instances in a cluster at the same time. Otherwise a session might fail over to an instance where different versions of components are running.
Perform this task on each cluster separately. A cluster acts as a safe boundary for session failover for instances in the cluster. Sessions in one cluster can never fail over to sessions in another cluster. Therefore, the risk of version mismatch is avoided.