What behaviour for creator when no connection can be created?

[telukwg]

Hi, I am using DBUtils (perhaps incorrectly) to handle client-managed server failover for a database cluster (ie. the client must select a new DB host when the current host has become unresponsive/failed-in-some-way.)

I have a scenario where there are potentially, no working hosts left in the cluster, and I want to code my creator function correctly so that DBUtils responds most correctly.

So - when I have run out of working hosts - should I have my creator function raise an Exception in this case? Should it be any particular Exception? (ie. one listed in the "failures" pool argument, one specifically NOT in the "failures".) Should I just return a "None"? (*I'm guessing not - my test code resulted in connection object after calling "pool.connection()".

I am using v3.11 of DBUtils, and the PooledDB type.

Thank-you and regards, Bill.

[Cito]

If you want to signal an actual database error, use an exception that is not in failures, because otherwise DBUtils will try to reconnect automatically.

However, to me it sounds you would be better off to interconnect some kind of load balancer between DBUtils and your database cluster. DBUtils would then only connect to the load balancer address. DBUtils just automatically tries to reconnect if a database connection in the pool has been broken (raises exceptions from "failures"), that's all it does.

[telukwg]

Thanks @Cito - yeah the infrastructure is outside of my control - so stuck with having to select hosts using the client. Planning to use Pooled Connection with mostly non-transaction interactions - one-offs writes/reads, then closing the connection. Using the Creator function to select a different host each time. Then relying on the automatic ping check facility to verify the connection and force a new connection request if the connection failed (ping=7). It's nice that the DBUtils cursor, can also force a connection change if something is wrong with it (if I understand correctly.) Using shortened time-outs to enable quickly detected host problems. So it can't track a broken host, but worst case impact is just the timeout delay. If there's later a way to access the connection log - that might enable me to detect a broken host and excluded it for some time period (I don't think I can do that in the Creator function - unless I health test it with a ping - but that only then lets me know when new connections go bad - and not existing ones, which the tough cursor might detect..

[Cito]

Yes, a custom creator function will not help. DBUtils was not made with that scenario in mind.

If you really want to go that route, you probably need to create a subclass of PooledDB, overwriting the connection() method. In your own method, call super() - if that fails (due to the ping), change the host in the creator function, close() the pool. and try to call super() again. Something along these lines.