Firebird clustering - the bad news

The bad news is that it involves coding in firebird core (at least in the lock manager code)

I'm afraid it won't work. The Classic processes should be able to
synchronize the logical I/O operations (page reads/writes) in order to
perform properly. It's done by the integrated lock manager, but it
cannot work between cluster nodes. As soon as the lock manager is
enhanced (or better say fixed) to delegate the calls to the external
distributed lock manager (e.g. RedHat DLM), this would become a workable
solution.

http://tech.groups.yahoo.com/group/firebird-support/message/88130

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