How can a connection be represented only by a small space in a HTTP server running on node.js?

I have read that a HTTP server created in node.js does not create new threads for each incoming c开发者_如何转开发onnection(request). Instead it executes a function that has been registered as a callback corresponding to the event of receiving a request.

It is said that each connection is represented by some small space in the heap. I cannot figure this out. Are connections not represented by sockets ? Should sockets not be opened for every connection made to the node.js server and this would mean each connection cannot be represented by just a space allocation in the javascript heap ?

It is described on the nodejs.org website that instead of spawning threads (2mb overhead per thread!) per connection, the server uses select(), epoll, kqueue or /dev/poll to wait until a socket is ready to read / write. It is this method that allows node to avoid thread spawning per connection, and the overhead is that associated with the heap allocation of the socket descriptor for the connection. This implementation detail is largely hidden from developers, and the net.socket API exposed by the runtime provides everything you need to take advantage of that feature without even thinking about it.

Node also exposes its own event API through events.EventEmitter. Many node objects implement events to provide asynchronous (non-blocking) event notification, which is perfect for I/O operations, which in other languages - such as PHP - are synchronous (blocking) by default. In the case of the node net.socket API, events are triggered for several API methods dealing with socket I/O, and the callbacks that are passed by parameter to these methods are triggered when an event occurs. Events can have callback functions bound to them in a variety of different ways, accepting a callback function as a parameter is only a convenience for the developer.

Finally, do not confuse OS events with nodejs events. In the case of the net API, OS events are passed to the nodejs runtime, but nodejs events are javascript.

I hope this helps.