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If your app is fully async, yes. Any blocking and you'll need another backend to service a simultaneous request.

So why are you even using Tornado if you fork a separate process for each simultaneous request? Doesn't that negate the benefits of an asynchronous http server?

Brett Taylor discusses this issue in his tech talk on Tornado (http://bret.appspot.com/entry/tornado-tech-talk). He uses a hybrid approach: Use synchronous programming when requests block for a short time (fetching data from local db or memcache) and async programming when something blocks for a long time (calling out to third party services, long polling requests).

He does not go all async because programming with callbacks or deferreds is more complicated than straight line synchronous programming.

Brett also mentions that the instances of the server are created once and reused. He does not fork an instance of the server per request.

Twisted has a nice asynchronous db api (http://twistedmatrix.com/documents/current/core/howto/rdbms....) that mitigates these concerns at the database level. More generally, I'd think the cleanest approach is to write async wrappers for your synchronous code, rather than forking an async server to service multiple requests. Otherwise you miss out on a lot of benefits of an async architecture, IMO.

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