What exactly is a highly available system?
The SunPlex system 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 data service resources from a failing node to another operational node in the cluster.
There are two types of highly available data services, failover and scalable.
A failover data service runs an application 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.
For each application, one node hosts the physical interface to the cluster. This node is called a Global Interface (GIF) Node. There can be multiple GIF nodes in the cluster. Each GIF node hosts one or more logical interfaces that can be used by scalable services. These logical interfaces are called global interfaces. One GIF node hosts a global interface for all requests for a particular application and dispatches them to multiple nodes on which the application server is running. If the GIF node fails, 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.