How does main stream web server implement this feature?

Question

This means, for example, a module can start compressing the response from a backend server and stream it to the client before the module has received the entire response from the backend. Nice!

I know it's some kind of asynchronous IO but simple like that isn't enough.

Anyone knows?

Answer 1

Without looking at the source code of an actual implementation, I'm speculating here:

It's most likely some kind of stream (abstract buffered IO) that is passed from one module to the other ("chaining"). One module (maybe a servlet container) writes to a stream that is read by another module (the compression module in your example), which then writes its output to another stream. The contents of that stream may then be processed further or transmitted to the client.