Confluence can be clustered.

If running as a single node, you may only have one instance of Confluence connecting to a single database.

There are a couple of reasons for this, but it all comes down to the fact that the Confluence application maintains a lot of state (caches, search indexes) separate to the database, and multiple front-ends will quickly see that state get out of sync, with disastrous effect.

Because of this, Confluence periodically checks to make sure it's the only application accessing the database, and if it finds a conflict it will shut down rather than risk corrupting your data.

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