About Database Connection Pool Sizing
This article would be better if it was called "connection pool sizing for MySQL". If you had a better database, you would have to worry way less about configuring pool size on the clients.The proper way to do this would be for the server (i.e. database) itself to have maximum active transaction limits and ways to setup quotas for different use cases, especially as your company gets large, and you have many different use cases mixed in the same database. Basically the queue should exist mostly on the server, and clients shouldn't have to worry about overwhelming the server. If server queue gets large, server should start rejecting requests faster, and clients would do a backoff and retry with a delay based on that instead. This also makes sure server can't be easily overworked if you have one misconfigured and misbehaving client.Lot of issues with many idle connections in MySQL are specific to MySQL itself and its implementation. In MySQL the perf drops not only when you have many active transaction, but even when you have many just many connected idle sessions. This is why there are tons of different random "MySQL Connection Proxy" projects that exist in the open source.