11.1 Using a Backup Replica

To back up a VOB consistently, the ClearCase administrator must lock the VOB. However, many sites cannot find convenient times to lock the VOB so that the lock does not interfere with development work. One solution is to use MultiSite to create a replica of a VOB in the same local area network as the original. Updates from the original VOB to the backup replica are scheduled to match the recovery characteristics desired, that is, how much development work your site can afford to lose. At backup time, the backup replica is locked and backed up, thereby not interfering with development work at the original VOB.

Handling Objects That Are Not Replicated

The most important thing to note is that a MultiSite replica is not a complete copy of a VOB; the following objects are not replicated, and therefore are not restored from backup:

• Derived objects

After a recovery from backup, developers must rebuild derived objects associated with the VOB. Checked-in derived objects are replicated, so they are backed up.

• Triggers

To make sure you can re-create triggers after a restoration from backup, you must record information about all triggers in a VOB replica. For example, use the command lstype -kind trtype to list all triggers in a VOB.

• Nonobsolete locks

As with triggers, you must record information about nonobsolete locks. You can write shell scripts that capture and re-create the trigger and lock information.

Also, pool assignments are specific to a replica, so re-creating the replica from a backup replica can undo changes made to them. If you make major changes to a VOB's pool structure, use the chpool command to duplicate these changes at the backup replica. (At replica creation, you can also use the -pooltalk option with mkreplica -import to make pool assignments.)

Designing Synchronization Strategy

You must determine the frequency and direction of synchronization. Typically, synchronization occurs in one direction only; that is, the backup replica never sends packets to the development replica, except during restoration.

Frequency of synchronization depends on your development environment. Some sites synchronize every 24 hours, but sites with rapid development may synchronize every 15 minutes.