Chapter 2: What is High Availability?

As more businesses depend on computer systems they also expect these systems to be available at all times. High availability means that a system is setup so that if a computer system or network has a hardware or software failure, the system fails over into a backup system. Business can then go on as usual. Once the problem is resolved, the system fails back to the primary system.

Sybase^® High Availability Failover enables Adaptive Server Enterprise to work in a cluster of servers in a network in a specific configuration such as active-active or active-passive. Such a system allows for failover and then the failback on servers. This manual includes information about how to set up and run the Adaptive Server high availability system.

A high availability cluster includes two or more machines that are configured so that, 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.

When the primary companion or machine fails, the databases, metadata, and user connections are moved to a secondary server so that users can still access data. This is known as failover.

When the primary companion or machine is prepared to resume operation, the user with the ha_role performs a failback, which returns the servers to normal companion mode.

An active-active setup is a two-node configuration where both nodes in the cluster include Adaptive Servers managing independent workloads, 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 fail over. After the primary companion is ready to resume its workload, it is moved back to its original node. This movement is called a failback.

When a system fails over, clients that are connected to the primary companion and use the failover property automatically reestablish their network connections to the secondary companion.

You must tune your operating system to successfully manage both Adaptive Servers during fail over. See your operating system documentation for information about configuring your system for high availability.

An active-passive configuration is a multi-node setup that involves a single Adaptive Server, a primary node on which the Adaptive Server runs, and a set of secondary nodes which can host the Adaptive Server and its resources, if necessary.

When the Adaptive Server cannot run on the primary node, it fails over, and the Adaptive Server is relocated and restarted on a secondary node. The Adaptive Server can be moved back to the primary node after it recovers and when it can successfully host the Adaptive Server and any associated resources.

In the case of fail over or fail back, clients connected to the Adaptive Server reestablish their network connections and resubmit any uncommitted transactions when the Adaptive Server is restarted on the secondary node. Client connections using the failover property automatically reestablish their connections.