High Availability FAQ

• What exactly is a highly available system?

  Sun Cluster defines high availability (HA) as the ability of a cluster to keep an application up and running, even though a failure has occurred that would normally make a server system unavailable.

• What is the process by which the cluster provides high availability?

  Through a process known as failover, the cluster framework provides a highly available environment. Failover is a series of steps performed by the cluster to migrate an application from a failing node to another operational node in the cluster.

• What is the difference between an HA and scalable service?

  An HA service means that an application runs on only one primary node in the cluster at a time. Other nodes might run other applications, but each application runs on only a single node. If a primary node fails, the applications running on the failed node fail over to another node and continue running.

  A scalable service spreads an application across multiple nodes to create a single, logical service. Scalable services leverage the number of nodes and processors in the entire cluster on which they run. One node receives all application requests and dispatches them to multiple nodes on which the application server is running. If this node fails (it is called the Global Interface Node or GIF), the global interface fails over to a surviving node. If any of the nodes on which the application is running fails, the application continues to run on the other nodes with some performance degradation until the failed node returns to the cluster.