Is Tornado really non-blocking?

Tornado is non-blocking if you write non-blocking code on the top if it, eg. using asyncmongo and @tornado.web.asynchronous decorator. Tornado as a framework provides tools for that.

Bret Taylor, one of the original authors, writes:

We experimented with different async DB approaches, but settled on synchronous at FriendFeed because generally if our DB queries were backlogging our requests, our backends couldn't scale to the load anyway. Things that were slow enough were abstracted to separate backend services which we fetched asynchronously via the async HTTP module.

It's true that Tornado doesn't include a non-blocking database layer; in fact the database layer is not integral part of the Tornado framework at all, as opposed to e.g. Django's ORM. Yes, Tornado ships with blocking MySQL wrapper because that's what FriendFeed happened to use, but it's more an external library than core functionality. I'm pretty sure most people are using something else for database access.

Yes, absent other measures, the server will wait for the query to finish executing. That does not mean Tornado is not a non-blocking web server.

A "non-blocking web server" doesn't block on network I/O (and may have some provision for disk I/O if it does static file serving). That does not mean you get instant, causality-violating instruction execution in your application.

Doing a database call takes time, just like reading files, formatting strings, processing templates, etc. takes time. Doing any of these things in the same thread as the server's main event loop will prevent the loop from moving on until you're done.

Tornado is nonblocking,but just limited to some IO operation like read or write to a socket file.

if you want everything in your code nonblocking,you have to design by yourself.but if your code is computation intensively, then nonblocking is meaningless for you. this situation you might use multi-process.

remember one thing nonblocking just mean the server send/receive date won’t blocking. If you can’t make your code nonblocking,then your whole application is someway blocking.