Tornado and asynchronous requests handling

Question

My question is two-part:

1. What exactly does it mean by an 'asynchronous server', which is usually what people call Tornado? Can someone please provide an concrete example to illustrate the concept/definition?

2. In the case of Tornado, what exactly does it mean by 'non-blocking'? is this related to the asynchronous nature above? In addition, I read it somewhere it always uses a single thread for handling all the requests, does this mean that requests are handled sequentially one by one or in parallel? If the latter case, how does Tornado do it?

Answer 1

1. Tornado uses asynchronous, non-blocking I/O to solve the C10K problem. That means all I/O operations are event driven, that is, they use callbacks and event notification rather than waiting for the operation to return. Node.js and Nginx use a similar model. The exception is tornado.database, which is blocking. The Tornado IOLoop source is well documented if you want to look at in detail. For a concrete example, see below.

2. Non-blocking and asynchronous are used interchangeably in Tornado, although in other cases there are differences; this answer gives an excellent overview. Tornado uses one thread and handles requests sequentially, albeit very very quickly as there is no waiting for IO. In production you'd typically run multiple Tornado processes.

As far as a concrete example, say you have a HTTP request which Tornado must fetch some data (asynchronously) and respond to, here's (very roughly) what happens:

1. Tornado receives the request and calls the appropriate handler method in your application
2. Your handler method makes an asynchronous database call, with a callback
3. Database call returns, callback is called, and response is sent.

What's different about Tornado (versus for example Django) is that between step 2 and 3 the process can continue handling other requests. The Tornado IOLoop simply holds the connection open and continues processing its callback queue, whereas with Django (and any synchronous web framework) the thread will hang, waiting for the database to return.