They serve somewhat different purposes. For example, application-internal connection pools won't shield the database server from a rapid scale-out of applicati

They serve somewhat different purposes. For example, application-internal connection pools won't shield the database server from a rapid scale-out of application instances: n * the instance count will still result in n * connections. I've always considered the #1 purpose of internal pools to be minimising latency, by moving the database connection handshakes outwith the critical path.