Occasionally, a gap can occur between the capturing of the changed data for a source table and the replication of the changed data to the target table. When this happens, a user with sufficient privileges might need to reset the control table information before executing the definition again. However, to preserve data integrity, check the control tables and take any necessary action.
For example, if you shut down the Capture program and then cold start it, it deletes all rows from the CD table. (See the Capture and Apply chapter for your platform in this book to learn more about cold start.) Between the time you shut down the Capture program and cold started it, updates might have been made that the Capture program did not capture. Additionally, any updates that were in the CD table were deleted at the cold start before the Apply program could copy them. A gap now exists between the target table and the CD table.
When a gap is present, the Apply program attempts to refresh complete copies unless the target table is not complete (COMPLETE=N). If the target table is not complete and the Apply program cannot perform a refresh, data integrity could be lost. In this case, before you reset the copy definition and resume copying, you need to check:
The Capture and Apply programs use the synchpoint to coordinate their work. The synchpoint is the log sequence number and indicates the progress of the replication subscription through a subscription cycle. The Capture and Apply programs maintain and use this value to prevent the pruning of data that the Apply program did not copy. The synchpoint value is maintained in the SYNCHPOINT column of the subscription set control table.
To verify whether a gap exists:
Compare the SYNCHPOINT column in the subscription set control table with the CD_OLD_SYNCHPOINT column in the register control table. If SYNCHPOINT is lower than CD_OLD_SYNCHPOINT, a gap exists.
Determine whether continued performance or data integrity is more important.