Drawback of read-write replica mode

[aafemt]

Will putting a database into read-write replication mode (without actually participating in replication) have any negative consequences for its normal operation?

[mrotteveel]

I can't speak for the server-side of this, but client-side, if you have a driver or application that checks if it is connected to the primary or to a replica, it could have negative repercussions.

For example, I have plans to add support in Jaybird to accept multiple or alternative connections string (not sure exactly how yet), where the driver will check which one is the primary, and then use the connection to the primary. I you have a standalone database marked as a replica, that could then result in a connection not working (though it would depend on the exact implementation and logic of such a check, obviously).

[aafemt]

It would be a very unwise behavior because even in normal HA standby scenario temporary absence/unavailability of the master node should not interrupt user operations (let alone bidirectional replication setup where every node is in rw replica mode).

[mrotteveel]

That really depends on your replication setup and needs. If you have failover in a normal and simple replication setup, the new master should be marked as no longer being a replica.

[mrotteveel]

This is also one of the factors that the PostgreSQL JDBC driver uses to decide which connection to use, using its targetServerType connection option (which has values like any, primary, master, slave, secondary, preferSlave, preferSecondary and preferPrimary) (though there it considers a primary a connection that is writable).

[mrotteveel]

In any case, you asked for possible consequences; a possible consequence is something or someone incorrectly assuming something is a replica, while it is not, and doing something wrong or unexpected.

[dyemanov]

Will putting a database into read-write replication mode (without actually participating in replication) have any negative consequences for its normal operation?

It should not have any consequences.