Runtime considerations

The following information summarizes runtime behaviors between redundant engines.

Establishing RMC Communication

The active and standby engines communicate with each other during run-time and use the RMC to monitor each other's status. The redundant engines use the Remote Partner Address (RPA) attribute to locate each other and communicate. The RPA attribute contains the IP address or host name of the platform hosting the partner engine.

At startup, each redundant AppEngine establishes communication with its partner. When the failover service receives a connection across the RMC, it updates the RPA attribute of the receiving engine if it is different than the current configured value.

Note: The value of the RPA may be different if a partner engine has been relocated to a different platform.

Checkpointing

AppEngines store specific attributes in memory, then write them to disk in both single- or redundant engine configurations. The frequency of the write operation is determined by the Checkpoint period setting in the AppEngine editor.

Note: Checkpoint period configuration details are included in Tuning Redundant Engine attributes.

The checkpointed attribute types include:

• Scan rate

• Checkpoint directory location (default is blank but can be modified)

• StartUp attributes

• StartUp type (automatic, semi-automatic, manual)

• StartUp Reason

Redundant AppEngines maintain data synchronization through the RMC. Data is synchronized by reading checkpointed attribute values written to disk on each node, at each scan.

The checkpoint operations occurs at a pre-defined rate in the local node (Scheduler.ScanPeriod). The same operations (write to memory, then to disk) occur on the backup AppEngine at every scan (via the RMC).

When the Standby AppEngine becomes active, it reads the checkpointed values from the designated file in the local (backup) node. The system updates the Standby Engine with the values that were sent and written to disk in the last scan before the active engine failed.

The following attribute types are checkpointed:

• Attributes with Category User Writable or Object Writable that are not extended as Input/Output and Input extensions.

• Calculated Retentive category attributes are always checkpointed.

Complete a thorough evaluation and selection of checkpointed attributes. Unnecessary checkpointing may degrade the performance of the system by writing extra values to the local/backup disks, and increase data traffic over the RMC.