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RE: Replication question

From: Bill Pass <wbpass_at_yahoo.com>
Date: Thu, 20 Jun 2002 09:49:06 -0800
Message-ID: <F001.00482FF3.20020620094906@fatcity.com>


Conflict resolution also has to be considered for log based replication as well. Same problems exist. If you think about it Oracles AQ approach is pretty much a log.

Multi-master replication is still trigger/AQ based in 9i. It's working well for us.

Some of the things you can do to reduce (but not elleminate) conflict resolution issues are: 1). Stagger sequence generated primary keys by site 2). Put triggers in place to prevent updating of primary keys (avoids some uniqueness conflicts) 3). Add a last_updated_dt to each table and use that as your primary method of update conflict resolution 4). Add a "primary" site capability for tie breakers. Oracle has a built in method, but it requires you add a site column to each table. If it is not data dependent, I prefer a custom stored procedure method where you choose a "primary" site that is not data dependent.

Deletes are still our biggest problem. Don't really understand why they can't be applied in the same order at the remote site (perhaps because we am using parallel propagation?). But for some reason if we do massive deletes across multiple related tables in the same TX, we always end up having to manually reconcile.

An approach recommended by oracle for this is to add a deleted_dt column to every table and do deferred batch deletions. We tried this in conjunction with views and instead of triggers to hide it from the application. A couple of problems with this approach is that if you delete, then try to re-insert with the same alternate key before the deferred purge process runs, you will get ORA-00001 errors. If you run the deferred deletion process too often you defeat the purpose of it and overload the system.


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Author: Bill Pass
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Received on Thu Jun 20 2002 - 12:49:06 CDT

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