title: Wait for several db connections before starting express server?
body: <p>I'm working on an Express app. At startup it connects to a Redis server and to a PostgreSQL sever. I'd like to wait for both connections to succeed before starting the express server. Now, if I were waiting on just <em>one</em> callback, I could initiate the Express server within that callback. But how to best do this when waiting on more than one async operation? I'm new to this. Is there a good idiomatic pattern for doing this sort of thing? Maybe grouping promises together or..?</p>