What you need for sharded queries is concurrency, not necessarily asynchronous requests. Async callbacks (ala Node.js, Twisted Python, Event Machine, etc.) give

What you need for sharded queries is concurrency, not necessarily asynchronous requests. Async callbacks (ala Node.js, Twisted Python, Event Machine, etc.) give you a kind of cooperative multitasking, which is one way to have concurrent I/O-bound tasks going; multithreaded programs are another. (Ruby and Python threads are kind of in-between, due to their respective GIL limitations.) That being said, above a certain scale and complexity level, you probably want the topology of your persistent data store hidden from your web request handlers anyway. For one thing, making requests to N backend shards from M frontend web workers starts to get bad when N and M are both large; for another, introducing really complex scatter-gather query logic into your request-handling pipeline can be a maintenance and debugging nightmare. Introducing a proxy or data-abstraction service in between cuts down on the number of open connections and lets you change the data storage topology without updating frontend code.