Multiple WebSphere Application Server cells sample topology

The multiple WebSphere Application Server cell topology involves setting up multiple WebSphere Application Server cells where a different physical machine hosts each cell. Applications are deployed onto multiple WebSphere Application Server administrative cells. This topology requires WebSphere Application Server Network Deployment.

This figure shows an example of a multiple cell topology:

This sample topology includes these features:

• Machines C and D host two separate WebSphere Application Server cells. Identical versions of an application are deployed in both cells.
• To ensure that identical versions of the application run in both cells, the application server cluster members in both cells are identical to each other.
• You administer each cell separately. Each cell has its own set of XML configuration files and its own Network Deployment manager.
• Both machines use logical partitioning (LPAR) for data and process isolation.
• Machine A hosts the primary Network Dispatcher node. The Network Dispatcher distributes incoming HTTP requests to the application servers that run in Cell 1 and Cell 2. The application is presented to clients as a single image. If one of the application server clusters is down, the Network Dispatcher routes all requests to the application servers in the other cell. A backup Network Dispatcher node (Machine B) provides failover support.
• To ensure that identical versions of the application run in each cluster member, install enterprise applications on the cluster in each deployment manager node. However, each cell is administered independently and has its own set of XML configuration files.
• This topology also shows the application database and persistent session database. Both cells in the topology share a common application database. The databases use data replication to enhance availability and avoid creating a single point of failure. For more information on high availability databases, see High availability database.

Note: In this sample topology, the application servers are vertically clustered. Use vertical scaling only if you need to. Clustering can increase the memory requirements of the application, and decrease performance. WebSphere Application Server for iSeries performs well with a single JVM. Vertical clustering is necessary only with some types of applications, or in the event of contention points in the JVM.

It also is possible to run a different version of the application in each cell. For example, you can create a test environment in one cell and a production environment in the other cell. Because the cells are isolated from one another, you can also run different versions of the WebSphere Application Server software in each cell.

Note: Running different versions of the application or WebSphere Application Server does not ensure high availability. To ensure availability in a multiple cell topology, configure at least two of the cells to run identical versions of your application.

Advantages and disadvantages of multiple WebSphere Application Server cells

These are some of the advantages of a multiple cell topology:

• Provides isolation of hardware failure. If one cell goes offline due to hardware problems, the remaining cells can still process client requests.

• You can perform hardware upgrades on a cell-by-cell basis during off-peak hours.

• Provides isolation of software failure. When you deploy an application in two or more cells, any problems that occur within one cell are isolated, and the other cells continue to handle client requests. Process isolation allows you to maintain high availability in any of these situations:

  • When you deploy a new application or a revision of an existing application. You can test the new application or revision in one cell while the other cells continue to process client requests.
  • When you deploy a new version of WebSphere Application Server for iSeries. You can test the new version into in a live environment without interrupting service.
  • When you apply fixes or patches to WebSphere Application Server for iSeries. You can perform hardware and software upgrades on a cell-by-cell basis during off-peak hours to avoid an interruption of service.

  If a problem occurs with the new software, you can accomplish a rollback to a previous software version more quickly.

• Improves workload balancing. In a combined topology you can use one or more of these workload management techniques:

  • Use the WebSphere Application Server Network Deployment workload management (WLM) facility to distribute work among application server cluster members.
  • Use Network Dispatcher to distribute client HTTP requests to each Web server.

  For example, an application can manage workloads at the Web server level with Network Dispatcher and at the application server level with WebSphere workload management. Using multiple workload management techniques in an application provides finer control of load balancing.

The primary disadvantages of a multiple cell topology are:

• Deployment is more complicated than for a single administrative cell.

• Multiple cells are more complicated to adminster because each cell is administered separately. To work around this problem, you can use wsadmin scripts to standardize and automate common administrative tasks.