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Oracle database - copying objects to remote database and modifying so they use the remote database only if the local database is offline

If database A is offline, a database link from database A to database B will not do you any good, as it will not be accessible. The same is the case with PL/SQL, PL/SQL on database A will not be available if database A is down. This is not a problem you will solve with PL/SQL and database links. Transparent Application Failover (http://docs.oracle.com/cd/B19306_01/java.102/b14355/ocitaf.htm) could solve part of the problem, allowing a connection to failover to database B if database A is down. However, it will not automatically switch back to database A when the database comes back up, unless you try a connection to database A again. You will need to have your PL/SQL code copied on both databases.

There are already robust failover mechanisms in Oracle, some free and required some hacking, such as redo log application, some costly but well supported, such as Oracle Data Guard. I would suggest you review your design and what you are trying to accomplish. If I had to absolutely do what you are trying to do, I would throw a web service in front of each database, use a load balancer in front of the web services that always connected to service A, unless it was down, in which case it would connect to service B.





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