Share connection to postgres db across processes in Python

Question

I have a Python script running as a daemon. At startup, it spawns 5 processes, each of which connects to a Postgres database. Now, in order to reduce the number of DB connections (which will eventually become really large), I am trying to find a way of sharing a single connection across multiple processes. And for this purpose I am looking at the multiprocessing.sharedctypes.Value API. However, I am not sure how I can pass a psycopg2.connection object using this API across processes. Can anyone tell me how it might be done?

I'm also open to other ideas in order to solve this issue.

The reason why I did not consider passing the connection as part of the constructor to the 5 processes is mutual exclusion handling. I am not sure how I can prevent more than one process from accessing the connection if I follow this approach. Can someone tell me if this is the right thing to do?

Answer 1

You can't sanely share a DB connection across processes like that. You can sort-of share a connection between threads, but only if you make sure the connection is only used by one thread at a time. That won't work between processes because there's client-side state for the connection stored in the client's address space.

If you need large numbers of concurrent workers, but they're not using the DB all the time, you should have a group of database worker processes that handle all database access and exchange data with your other worker processes. Each database worker process has a DB connection. The other processes only talk to the database via your database workers.

Python's multiprocessing queues, fifos, etc offer appropriate messaging features for that.