A High Availability (HA) cluster is used in an environment that must always be available, such as a banking system to which clients must be able to connect, 365 days a year.
A HA cluster includes two or more machines that are configured so that if one machine (or application) is interrupted, the second machine assumes the workload of both machines. Each machine is one node of the High Availability cluster.
The machines are configured so that each machine can read the other machine's disks, although not at the same time. All of the disks that are failed-over should be shared disks.
For example, if Adaptive Server 1 is the primary companion server, and it crashes, Adaptive Server 2, as the secondary companion server, reads its disks (disks 1 - 4) and manages any databases on them until Adaptive Server 1 can be rebooted. Any clients connected to Adaptive Server 1 are automatically connected to Adaptive Server 2.
Failover allows Adaptive Server to work in a HA cluster in active-active or active-passive configuration.
During failover, clients connected to the primary companion using the failover property automatically reestablish their network connections to the secondary companion. Failover can be enabled by setting the connection property HASession to “1” (default value is “0”). If this property is not set, the session failover does not occur, even if the server is configured for failover. Also, you must set SecondaryServer (the IP address or the machine name of the secondary ASE server) and SecondaryPort (the port number of the secondary ASE server) properties.
When the OLE DB Provider driver detects a connection failure with the primary ASE server, it first tries to reconnect to the primary. If it cannot reconnect, it assumes that a failover has occurred. Then, it automatically tries to connect to the secondary ASE server using the connection properties set in SecondaryServer and SecondaryPort.
See the ASE book, Using Sybase Failover in a High Availability System, for information about configuring your system for HA.
