This document covers the following topics:
The functionality of ADABCK has been extended to enable the initial state processing. In order to keep the source and target file in sync, the ADABCK ET synchronization performed at the end of a backup is used. Normally this is used to guarantee a consistent state of the Adabas files on the backup: during the ET synchronization, only active transactions may be terminated, and any Adabas commands initiating a new transaction are put into a wait status until the ET synchronization has finished. At the end of the ET synchronization, when all open transactions are finished and the source data files are still locked, the replication status of the replications concerned is set to Recording in the replication metadata file in order to indicate that updates for the files to be replicated must be recorded from this point onwards. This guarantees that exactly those updates based on the saved state of the files are recorded for replication.
An important requirement for a replication solution is that data integrity must be maintained even if the source database or the target database crashes.
The source Adabas database is also used as intermediate storage of the update transaction; the replication data is stored in the same user transaction, that has to be replicated. This automatically guarantees that the storage of the replication data is also committed if an update transaction is committed, and that the storage of the replication data is also rolled back if an update transaction is rolled back. This guarantees that the replicator can process all committed transactions, and that no transactions are lost.
On the replicator side, it may occur that the source nucleus crashes during a transaction for the updates in the replication target file. Following the auto restart of the source database, a new OP command for the target database is performed by the replication database, which backs out the open transaction in the target database. This makes it possible to continue the Adabas session of the previous source nucleus session, and this means that an open transaction from the previous source nucleus session is rolled back.
If a target database crashes, it is possible that the replication database does not know if a transaction was committed in the target database or not. In order to guarantee the consistency of the target database, ET data are written by the replication database (please refer to section Adabas Basics, Database Design, Transaction Recovery for further details).
