MySQL comes to mind. Unlike Postgres where connections are expensive MySQL encourages loading up the server with hundreds of simultaneous connections per server.

Like anything, it’s possible to split this work out to a separate process but the IPC overhead is a lot.

Most databases (unfortunately), and since a ton of web apps use databases for state..

That said, many such databases are (or already have) rolled out connection pool proxies for reasons like this, so meh.

Web front end backed by data base with expensive login, so you cache connections. Two tier architecture it would be called.