After-imaging provides an extra layer of protection around the database. Every high-availability system should implement after-imaging. It is essential that you have a good backup and recovery plan prior to implementing after-imaging.
Once started, the OpenEdge after-image feature keeps a log of all transactions that can be rolled forward into a backup copy of the database to bring it up-to-date.
The primary reason to enable after-imaging is to protect you from media loss. Media loss can be the loss of a database, a primary recovery area disk, or a backup tape. In the case of a lost backup, you can go to the previous backup and roll-forward to bring the system up-to-date. This assumes that your after-image backup was not stored on the same piece of media as the database backup that you were unable to recover. This is the main reason for doing a backup of your data on one tape or tape set and your after-image files to a second tape.
After-imaging can also be used to keep a warm standby copy of your database. This standby database can be stored on the same system as the primary copy. However, for maximum protection, you should store it on a different system. When you use after-imaging to implement a custom warm standby replication solution, you periodically update your standby database by transferring after-image files from the primary database and applying or rolling-forward those files to the standby database. In the case of a system failure, you apply the last after-image file to the standby database, and start using the standby database. If it is not possible to apply the last after-image file to the database, you only lose the data entered since the last application of the after-image file. In the event of a system failure, having implemented a warm standby database typically results in significantly less downtime for the users than if the database needs to be restored from a backup.
