### To Upgrade Components Without Loss of Service

You can use the load balancer and multiple clusters to upgrade components within the Application Server without any loss of service. A component can, for example, be a JVM, the Application Server, or a web application.

This approach is not possible if:

- You change the schema of the high-availability database (HADB).

- You perform an application upgrade that involves a change to the application database schema.

Caution - 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.

1. Stop one of the clusters using the Stop Cluster button on the General Information page for the cluster.

2. Upgrade the component in that cluster.

3. Start the cluster using the Start Cluster button on the General Information page for the cluster.

4. 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.

##### See Also

- To Create a Cluster

- To Configure a Cluster

- To Create Server Instances for a Cluster

- To Configure Applications for a Cluster

- To Configure Resources for a Cluster

- To Delete a Cluster