Can two http request come together? If it can, how

Question

Yesterday I was giving some presentation on nodeJS. Some one asked me following question:

As we know nodeJS is a single threaded server, several request is coming to the server and it pushes all request to event loop. What if two request is coming to server at exact same time, how server will handle this situation?

I guessed a thought and replied back following:

I guess No two http request can come to the server at exact same time, all request come through a single socket so they will be in queue. Http request has following format:

timestamp of request is contained in it's header and they may be pushed to the event loop depending upon their timestamp in header.

but I'm not sure whether I gave him right or wrong answer.

Answer 1

I guess No two http request can come to the server at exact same time, all request come through pipe so they will be in queue.

This part is basically correct. Incoming connections go into the event queue and one of them has to be placed in the queue first.

What if two request is coming two server at exact same time, how server will handle this situation?

Since the server is listening for incoming TCP connections on a single socket in a single process, there cannot be two incoming connections at exactly the same time. One will be processed by the underlying operating system slightly before the other one. Think of it this way. An incoming connection is a set of packets over a network connection. One of the incoming connections will have its packets before the other one.

Even if you had multiple network cards and multiple network links so two incoming connections could literally arrive at the server at the exact same moment, the node.js queue will be guarded for concurrency by something like a mutex and one of the incoming connections will grab the mutex before the other and get put in the event queue before the other.

The one that is processed by the OS first will be put into the node.js event queue before the other one. When node.js is available to process the next item in the event queue, then whichever incoming request was first in the event queue will start processing first.

Because node.js JS execution is single threaded, the code processing that request will run its synchronous code to completion. If it has an async operation, then it will start that async operation and return. That will then allow the next item in the event queue to be processed and the code for the second request will start running. It will run its synchronous to completion. Like with the first request, if it has an async operation, then it will start that async operation and return.

At that point after both requests have started their async operations and then returned, then its just up to the event queue. When one of those async operations finishes, it will post another event to the event queue and when the single thread of node.js is free, it will again process the next item in the event queue. If both requests have lots of async operations, their progress could interleave and both could be "in-flight" at the same as they fire an async operation and then return until that async operation completes where their processing picks up again when node.js is free to process that next event.

timestamp of request is contained in it's header and they may be pushed to the event loop depending upon their timestamp in header.

This part is not really right. Incoming events of the same type are added to the queue as they arrive. The first ones to arrive go into the queue first - there's isn't any step that examines some timestamp.

Answer 2

Multiple concurrent HTTP requests are not a problem. Node.js is asynchronous, and will handle multiple requests in the event-loop without having to wait for each one to finish.

For reference, example: https://stackoverflow.com/a/34857298/1157037