Clusters

Clusters are sets of servers that are managed together and participate in workload management. The servers that are members of a cluster can be on different host machines, as opposed to the servers that are part of the same node and must be located on the same host machine.

A cell can have no clusters, one cluster, or multiple clusters.

Servers that belong to a cluster are members of that cluster set and must all have identical application components deployed on them. Other than the applications configured to run on them, cluster members do not have to share any other configuration data. One cluster member might be running on a huge multi-processor enterprise server system while another member of that same cluster might be running on a small laptop. The server configuration settings for each of these two cluster members are very different, except in the area of application components assigned to them. In that area of configuration, they are identical.

A vertical cluster has cluster members on the same node. A horizontal cluster has cluster members on multiple nodes.

A network dispatcher routes application access among cluster members by server-weighting, to provide better distribution control.

WAS can respond to increased use of an enterprise application by automatically replicating the application to additional cluster members as needed. This lets you deploy an application on a cluster instead of on a single node, without considering workload.

To run two or more clusters in one instance of WebSphere:

1. Edit the HTTP Transports for each server and make sure the port numbers are unique for each cluster.

   Each server within a cluster should have the same HTTP transport port and each cluster should have different port numbers. For example, assume servers A and B on cluster TEST1, and servers D and C on cluster TEST2. Servers A and B could both use port 9080, and servers D and C could both use port 9081.

2. Go to...

   /opt/WebSphere/AppServer/config/cells/cellname

   ...and insert your new port numbers into virtualhosts.xml

 

Multiple Clusters

To install two clusters in the same WAS instance, you'll need different HTTP transports for each cluster, and you'll also have to tweak the virtualhosts.xml. For example, if you have two clusters, one with servers running on HTTP Transport 9080, and one with servers running on HTTP Transport 9081, to get the latter to function:

1. Stop all WAS processes
2. Go to /opt/WebSphere/DeploymentManager/config/cells/cellname/...
3. Edit virtualhosts.xml
4. Search for 9080
5. Copy line with port 9080 to new line.
6. Change port number of new line to 9081
7. Change virtualhost id of new line to something unique.
8. Start up all your WAS processes.
9. If you still are unable to pull up the app, getting a blank screen, make sure that the build level of the database is in sync with the build level of the application.