The high availability disaster recovery (HADR) feature provides a high availability solution for both partial and complete site failures. HADR protects against data loss by replicating data changes from the primary database (source), to one or more standby databases (targets).
A partial site failure can be caused by a hardware, network, or software failure. Without HADR, a partial site failure requires restarting the database management system (DBMS) server that contains the database. The length of time that it takes to restart the database and the server where it is located is unpredictable. It may take several minutes before the database is brought back to a consistent state and made available. With HADR, a standby database will assume control in seconds. In addition, the system can redirect the clients that used the original primary database to the new primary database by using automatic client reroute or retry logic in the application.
A complete site failure will occur when a disaster, such as fire, causes the entire site to be destroyed. For example, the primary database may be located at your primary data center in one city, and a standby database might be located at your second data center in another city. If a disaster occurs at the primary site, data availability is maintained by having the remote standby database take over as the primary database with full DB2® functionality. After a takeover operation occurs, one can bring the original primary database back up and return it to its primary database status; this is known as failback. One can initiate a failback if one can make the old primary database consistent with the new primary database. After reintegration of the old primary database into the HADR setup as a standby database, one can switch the roles of the databases to enable the original primary database to once again be the primary database.
With HADR, the level of protection from potential loss of data is based on configuration and topology choices. Some of the key choices will now be detailed:
What level of synchronization will one use?
Standby databases are synchronized with the primary database through log data that is generated on the primary and shipped to the standbys. The standbys constantly roll forward through the logs. One can choose from four different synchronization modes. In order of most to least protection, these modes are SYNC, NEARSYNC, ASYNC, and SUPERASYNC.
Will one use a peer window?
The peer window feature specifies that the primary and standby databases are to behave as though they are still in peer state for a configured amount of time if the primary loses the HADR connection in peer state. If primary fails in peer or this "disconnected peer" state, the failover to standby will have zero data loss. This feature provides the greatest protection.
How many standbys will one deploy?
With HADR, one can use either single standby mode or multiple standby mode. With multiple standbys, a user can achieve both high availability and disaster recovery objectives with a single technology