Switching Over and Switching Back › How Switchover and Switchback Work

How Switchover and Switchback Work

After the HA scenario starts running and the synchronization process is completed, the Replica checks the Master on a regular basis, by default every 30 seconds, to see if it is alive. There are three types of monitoring checks:

• Ping -- a request sent to the Master to verify that the Master is up and responding
• Database check -- a request that verifies the Oracle Server is running
• User-defined check -- a custom request you can tailor to monitor specific applications

If an error occurs with any part of the set, the entire check is considered to have failed. If all checks fail throughout a configured timeout period (by default, 5 minutes), the Master server is considered to be down. Then, depending on the HA scenario configuration, Arcserve HA sends you an alert or automatically initiates a switchover.

When you created an HA scenario, you defined how you want the switchover to be initiated.

• If you selected the Initiate Switchover manually option from the Switchover and Reverse Replication Initiation page, perform a manual switchover. For more information, refer to the topic, Initiate Switchover.
• If you selected the Initiate Switchover automatically option, you can still perform a manual switchover, even if the Master is alive. You can initiate switchover when you want to test your system, or you want to use the Replica server to continue the application service while some form of maintenance is performed on the Master server.

Triggered (automatic) switchover is in all ways identical to manual switchover performed by the administrator. Automatic switchover is triggered by a resource failure on the master server rather than by an administrator manually initiating the switchover by clicking the Perform Switchover button. Server ping response, application service status, and database connectivity are monitored. The timeout parameters are configurable and are more extensively covered in the Arcserve HA Administration Guide

When you created an HA scenario, you defined how you want the reverse scenario to be initiated.

• If you selected the Initiate Reverse Replication automatically option from the Switchover and Reverse Replication Initiation page, replication in the reverse direction (from Replica to Master) automatically begins after a switchover, if the original Master server is online.
• If you selected the Initiate Reverse Replication manually option, you need to perform switchback manually. If you performed switchover with the Master server offline, or if you stop a backward scenario and restart it again, in both cases, data from the Replica to the Master must be resynchronized before you can switch back to the Master.

When the Reverse Replication feature is off, to start reverse replication after a switchover has occurred, click the Run button. The benefit to this feature is, if both the master and replica servers were online and connected during switchover, resynchronization in the reverse direction is not required. Resynchronization involves comparing the data on the master and replica servers to determine which changes to transfer before real-time replication starts; this can take some time. If automatic reverse replication is turned on, and both servers were online during switchover, replication is reversed without the need for resynchronization. This is the one situation in which resynchronization is not required.