Overview

In this chapter we will be discussing the Cluster Application Availability (CAA) framework. CAA is used to make an application that would normally be restricted to running on only one member in a cluster capable of automatically relocating to another cluster member in the event that the member where it is running were to fail or is shutdown for maintenance. In other words, CAA can make an application highly available.

Many applications can only run on one member in a cluster at any given time, due to the way they're written. An application that writes to a file, without first locking it, is just one example. In fact, many applications (known as single-instance applications) are written without any thought to concurrency. These are candidates for CAA.

Another example is an application that performs its synchronization using interprocess communication (IPC). This type of application can run multiple processes (or instances) that coordinate access to shared resources. However, since the coordination is done in memory, all instances are restricted to running on a single member and are therefore candidates for CAA if high availability is a requirement.

23.1 Single-Instance Applications

A single-instance application is an application that is/was not designed to run more than one copy (or instance) on any cluster member at a time. This is due to lack of synchronization built into the application. As stated above in the introduction, an application that writes to a file without...