A high availability cluster includes two machines that are configured so if one machine (or application) is brought down, the second machine assumes the workload of both machines. Each of these machines is called one node of the high availability cluster. A high availability cluster is typically used in an environment that must always be available, for example, a banking system to which clients must connect continuously, 365 days a year.
Sybase Failover enables Adaptive Server to work in a high availability cluster in an active-active configuration. That is, both nodes in the cluster include Adaptive Servers managing independent workloads, and are capable of taking over each other’s workload in the event of a failure. The Adaptive Server that takes over the workload is called a secondary companion, and the Adaptive Server that fails is called the primary companion. Together they are companion servers. This movement from one node to another is called failover. After the primary companion is ready to resume its workload, it is moved back to its original node. This movement is called failback. Clients connected to the failed Adaptive Server automatically reestablish their network connections via the second machine.
You can configure Sybase Failover for either an asymmetric or a symmetric setup. In an asymmetric setup, the primary Adaptive Server performs the work during day-to-day operations, while the secondary Adaptive Server is prepared to take over the work during system failure or scheduled maintenance. The secondary companion is an independent Adaptive Server, and can have its own applications running. Figure 6-1 describes an asymmetric setup.
Figure 6-1: Asymmetric configuration in a high availability system
A symmetric setup consists of two fully functional Adaptive Servers running on separate machines, with their own system devices, system databases, user databases, and user logins. However, in the case of failover, either of the Adaptive Servers acts as a primary or secondary companion for the other Adaptive Server. Figure 6-2 describes a symmetric setup.
Figure 6-2: Symmetric configuration in a high availability system
For information about running Sybase Failover in a high availability system, see Using Sybase Failover in a High Availability System.
