Redundancy overview

A failover is the condition during which run-time operations are moved from one critical component to another. Failover can occur due to failure conditions or it can be forced manually, called a forced failover.

Application Server provides redundancy in two critical areas:

• AppEngine redundancy: You must configure two AppEngine objects, one as primary and one as backup, and two WinPlatform objects to host them.

  Note: When you enable redundancy for the initial (primary) AppEngine, the backup AppEngine is automatically created.

• Data acquisition redundancy: You must configure two DIObjects (data sources) and a RedundantDIObject.

The Primary/Backup AppEngines form a redundant pair. The Application Server infrastructure will generate the backup AppEngine automatically when redundancy is enabled for an AppEngine. Hierarchy of ApplicationObjects can only be assigned to the primary AppEngine. The primary and backup engines need to be assigned to redundancy enabled platforms, and they can be deployed separately.

For data acquisition, the Primary/Backup DIObjects (the data sources) must be separately created, configured, and deployed. Also, you must create, configure, and deploy a RedundantDIObject to control failovers between the two data source objects.

In a redundant system, install and configure the primary OPC server on the backup engine node.

When you configure redundancy, you configure the Primary object and the Backup object.

• Primary object: The main or central object that provides the functionality during run time. For AppEngines, this is the object you enable for redundancy. For data acquisition, this is the DIObject you intend to use first as your data source in run time.

• Backup object: The object that provides the functionality of the Primary object when the Primary object fails. For AppEngines, this is the object created by the Application Server infrastructure when the Primary object is enabled for redundancy. For data acquisition, this is the DIObject you do not intend to use first as your data source in the run-time.