A high availability system includes both hardware and software. Sybase Failover is software that allows a companion server to withstand a single point of failure in the cluster.
A system that uses Sybase Failover includes two machines. Each machine is one node of the high availability cluster. Each Adaptive Server is either a primary companion or secondary companion. Each companion performs work during operations; the secondary companion takes over the workload when the primary companion fails or is brought down. The primary companion can be brought down for any number of reasons: scheduled maintenance, system failure, power outage, and so on. When the second server assumes another server’s workload, it is called fail over. Moving the workload back to the original server once it is up and running again is called a failback.
Figure 2-1 describes a typical configuration consisting of two Adaptive Servers.
Included with the operating system is a high availability system (for example, Sun Cluster for Sun) that detects and broadcasts to the cluster that part of the system is failing or is being shut down for maintenance. When Adaptive Server goes down, the high availability system tells the second machine to take over the workload. Any clients connected to the Adaptive Server that is failing are automatically reconnected to the second Adaptive Server.
Figure 2-1: High availability system using Sybase Failover
The machines in Figure 2-1 are configured so that each machine can read the other machine’s disks, although not at the same time (all the disks that are failed-over should be shared disks).
For example, if Adaptive Server1 is the primary companion and it fails, Adaptive Server2, as the secondary companion, reads its disks (1 – 4) and manages any databases on them until Adaptive Server1 can be brought back online. Any clients that are connected to Adaptive Server1 and are using the failover property are connected automatically to Adaptive Server2.
