Controlling the Speed of Application Failover

Insulate Users from Outages

Wherever possible, insulate your end users from outages. Issues include the following:

•Do not require user intervention to reconnect when a connection is lost due to a failed server.

•Where possible, warn users of slight delays due to a failover in progress.

•Minimize the reentry of data.

•Engineer the system for reserve capacity to minimize the performance degradation experienced by users.

Define Application Startup and Shutdown

Applications must be re-startable without manual intervention. If the application requires a switch to be flipped on a piece of hardware, then automated restart is impossible. Procedures for application startup, shutdown and monitoring must be created so that the HA software can perform these functions automatically.

To ensure automated response, there should be defined procedures for starting up the application and stopping the application. In Serviceguard these procedures are placed in the package control script. These procedures must check for errors and return status to the HA control software. The startup and shutdown should be command-line driven and not interactive unless all of the answers can be predetermined and scripted.

In an HA failover environment, HA software restarts the application on a surviving system in the cluster that has the necessary resources, like access to the necessary disk drives. The application must be re-startable in two aspects:

•It must be able to restart and recover on the backup system (or on the same system if the application restart option is chosen).

•It must be able to restart if it fails during the startup and the cause of the failure is resolved.

Application administrators need to learn to startup and shutdown applications using the appropriate HA commands. Inadvertently shutting down the application directly will initiate an unwanted failover. Application administrators also need to be careful that they don't accidently shut down a production instance of an application rather than a test instance in a development environment.

A mechanism to monitor whether the application is active is necessary so that the HA software knows when the application has failed. This may be as simple as a script that issues the command ps -ef grep xxx for all the processes belonging to the application.

To reduce the impact on users, the application should not simply abort in case of error, since aborting would cause an unneeded failover to a backup system. Applications should determine the exact error and take specific action to recover from the error rather than, for example, aborting upon receipt of any error.