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Oracle Streams conflict resolution & data integrity

Oracle Tips by John Garmany


Is it possible to have a data anomaly when the same row is updated at the same time on two Streams instances? I like the idea of using n-way Streams replication instead of RAC but I'm concerned about managing data integrity.
For example, if I changes JONES salary to $50k on one instance while someone on the other instances changes JONES to $70k, won't these changes cross-replicate causing data overlay, or does Oracle Streams have a conflict resolution mechanism?


Both advanced replication and streams send both the before image and the changed image.  If the before image does not match, either conflict resolution must be used to resolve the conflict or the transaction will go in the error queue.  So if Jones's salary is changed on both computers at the same time, both changes will end up in the replication error queue because the before images will not match.

T.J. Ruggiero, an Oracle support engineer notes: When a transaction is committed at the source, an LCR (logical change record) is generated and enqueued via Oracle AQ.  This LCR contains both old values and new values (assuming that the proper supplemental logging has been set up to put this information in the redo streams) just like in advanced replication. 

The LCR is propagated to the target if a propagation is involved, and the apply process there attempts to apply it.  If it is found that that the old values in the LCR don't match the current values in the row at the target, an error is signaled and the LCR is moved to the error queue.  This is called 'Conflict Detection' and is implemented by default.

'Conflict Resolution' is where a method is called to settle the conflict once it is detected before the LCR is placed in the error queue.  This must be set up by the DBA.

Conflict is expected when two sites can update the same row at the same time.  If this type of functionality is desired, then conflict resolution should be implemented.  Conflict can be avoided if sites practice data ownership, i.e. each site only hits certain data.

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