> I did witness it brutally murder a MySQL cluster because it couldn't serve it fast enough. That was both fun and terrifying to watch on the dashboards.

Haha yep. In my experience, everyone running CGI/process-per-request application servers is bullish on switching to a concurrent or cooperative runtime...until they realize they just removed the primary ratelimiter on downstream DB/service accesses.

The converse war stories are also amusing: people rewrite their whole app in a concurrent/asynchronous framework and nothing changes, because the DB driver is still farming out all queries to a tiny fixed-size threadpool of connections that was the bottleneck all along.