Writing a highly available cluster application

A highly available application is one that can be resilient to a system outage in a clustered environment.

1. If an application error occurs, the application restarts itself on the same node and corrects any potential cause for error (such as corrupt control data). You can view the application as though it had started for the first time.
2. The application performs some amount of checkpoint-restart processing. You can view the application as if it were close to the point of failure.
3. If a system outage occurs, the application is restarted on a backup server. You can view the application as though it had started for the first time.
4. If a system outage occurs, the application is restarted on a backup server and performs some amount of checkpoint-restart processing across the servers. You can view the application as if it were close to the point of failure.
5. If a system outage occurs, a coordinated failover of both the application and its associated data to another node or nodes in the cluster occurs. You can view the application as though it had started for the first time.
6. If a system outage occurs, a coordinated failover of both the application and its associated data to another node or nodes in the cluster occurs. The application performs some amount of checkpoint-restart processing across the servers. You can view the application as if it were close to the point of failure.
   Note: In cases 1 through 4 above, you are responsible for recovering the data.

• Make application programs resilient
  Learn how to make application programs resilient.
• Restart highly available cluster applications
  To restart an application, the application needs to know its state at the time of the failover or switchover.
• Calling a cluster resource group exit program
  The cluster resource group exit program is called during different phases of a cluster environment.