The ORM connection pooler is client-side - so each instance of your application using SQLAlchemy will have its own connection pool. This may become unwieldy as you scale up the number of app servers/containers/processes.

In contrast, connection poolers like Odyssey or pgbouncer live external to your applications, meaning that:

1. You can connect to them from any application written in any language (not just Python) 2. They provide a global pool for all of the applications connecting and help to not overload Postgres.