Synchronization points

Strictly speaking, all you need to be able to restart a system after a failure is the log: as long as you start out correctly and log everything you do thereafter, you can always recover. Restart in this case would require the system to read every log record ever written for the system.

The more time that has passed since the first time you started the system, the more log records will have been written, and the more time-consuming the restart will be.

Definition: A synchronization point, or sync point, is a known point of data or system integrity. It is a point in time at which all of the changes made to the database are complete. If a failure makes restart necessary, you can begin reprocessing from the most recent sync point.

The value of having a sync point is that when IMS restarts, it can ignore all logs created before the sync point was taken. If your sync points are infrequent, they become less valuable because the amount of log data IMS has to read during a restart increases. Thus, you should establish periodic sync points. Then, if a problem arises, you do not need to examine as much of the log, because you established the last sync point relatively recently.

IMS uses two types of sync points: those taken by IMS itself (called system checkpoints), and those taken by individual application programs running under IMS (called application program sync points or application program commit points).