Availability is the description of the system’s ability to respond to requests no matter the circumstances. Availability requires that the topology provide some degree of process redundancy in order to eliminate single points of failure. While vertical scalability can provide this by creating multiple processes, the physical machine then becomes a single point of failure. For this reason, a high availability topology typically involves horizontal scaling across multiple machines.
Using a WebSphere Application Server multiple machine configuration eliminates a given application server process as a single point of failure. In WebSphere Application Server V5.0 and higher, the removal of the application dependencies on the administrative server process for security, naming and transactions further reduces the potential that a single process failure can disrupt processing on a given node. In fact, the only single point of failure in a WebSphere cell is the Deployment Manager, where all central administration is performed. However, a failure at the Deployment Manager only impacts the ability to change the cell configuration and to run the Tivoli Performance Viewer which is now included in the Administrative Console; application servers are more self-sufficient in WebSphere Application Server V5.0 and higher compared to WebSphere Application Server V4.x.