Applications on clusters can be divided into three basic types:
A single-instance application runs on only one cluster member at a time. To make this type of application highly available, the cluster must provide a mechanism for starting the application on another cluster member in the event that the current member can no longer run the application. The TruCluster Server high availability mechanism for single-instance applications is the cluster application availability (CAA) subsystem; see Chapter 5 for a description of CAA.
The Cluster Highly Available Applications manual provides detailed information about moving applications from the TruCluster Software Version 1.x series of products to TruCluster Server Version 5.1A.
A multi-instance application can run on multiple cluster members at the same time. A multi-instance application by definition is highly available because the failure of one cluster member does not affect the instances of the application running on other members. See Chapter 6 for a discussion of how cluster aliases provide transparent client access to applications.
A distributed application is specifically designed to run on a cluster, using different members for specific purposes. These applications use the Memory Channel, distributed lock manager (DLM), and cluster alias application programming interfaces (APIs) to integrate applications with cluster resources.
TruCluster Server lets you run components of distributed applications in parallel, providing high availability while taking advantage of cluster-specific synchronization mechanisms and performance optimizations.
See Chapter 6, Section 7.2, Chapter 8, and the Cluster Highly Available Applications manual for more information on the subsystems and interfaces that you can use to create distributed applications.
Note
Section 6.7 discusses the
in_singleand thein_multiservice attributes, which determine whether the cluster alias subsystem arbitrarily selects one member to receive all packets for a service that is reached through an alias or distributes packets among all eligible members of an alias. Although these terms are similar in form to single-instance and multi-instance, thein_singleandin_multiattributes affect how the cluster alias subsystem routes packets and connection requests that are addressed to a service's port. Do not confuse these routing attributes with the generic terms that describe how many instances of an application can run in the cluster. The Cluster Administration manual has a section in its Cluster Alias Subsystem chapter that describes the differences between the cluster alias subsystem and the cluster application availability subsystem.