During rollforward, all committed changes to the database must be reapplied to the database image. Only aborted transactions or transactions which are neither committed or aborted are ignored.
Ordering is essential, and must be in the same order as the changes occurred when the log was formed.
The changes are not applied to the current image, as this could contain changes from transactions which have not completed. Thus first all changes made since the last checkpoint are removed.
Only when the rollforward has completed will the database image be in a consistent state, so only then can new transactions be allowed to run.
- Transactions which are in the log but not yet committed are restarted where they left off.
- Committed transactions in the log are reapplied.
- Committed transactions are reapplied in the same order as they appear in the log.
- Transactions which are reapplied are reapplied to the image as it was since the last checkpoint.
- Only once the rollforward has been completed can new transactions be performed.
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