Application resilience
Application resilience can be classified
by the effect to the user. Under an IBM i clustering
infrastructure, application resiliency is controlled with an application
Cluster Resource Group object (CRG). This CRG provides the mechanism,
using an exit program, to control start, stop, restart, and switch
of the application to back up systems. The entire application environment,
including data replication and switchable devices can be controlled
through the clustering infrastructure as a single entity.
Application resilience is classified into the following categories.
- No application recovery
- After an outage, users must manually restart their applications. Based on the state of the data, users determine where to restart processing within the application.
- Automatic application restart and manual repositioning within applications
- Applications that were active at the time of the outage are automatically restarted through the CRG exit program. The user must still determine where to resume within the application, based on the state of the data.
- Automatic application restart and semi-automatic recovery
- In addition to the applications automatically restarting, the users are returned to some predetermined “restart point” within the application. The restart point may be, for example, a primary menu within the application. This is normally consistent with the state of the resilient application data, but the user might need to advance within the application to actually match the state of the data. Application changes are needed to save user state data. At sign on, the application detects the state of each user and determines if it needs to recover the application from the last saved state.
- Automatic application restart and automatic recovery to last transaction boundary
- The user is repositioned within the application to the processing point that is consistent with the last committed transaction. The application data and the application restart point match exactly. This category requires code changes in the application to save user states at the end of each commit cycle so the application knows where each user is in the application in case of a failure.
- Full application resilience with automatic restart and transparent failover
- In addition to being repositioned to the last committed transaction, the user continues to see exactly the same window with the same data as when the outage occurred. There is no data loss, signon is not required, and there is no perception of loss of server resources. The user perceives only a delay in response time. This category can only be obtained in an application with a client/server relationship.