Slow third-party APIs clogging Express server

Question

I am creating a question answering application using Node.js + Express for my back-end. Front-end sends the question data to the back-end, which in turn makes requests to multiple third-party APIs to get the answer data.

Problem is, some of those third-party APIs take too long to respond, since they have to do some intense processing and calculations. For that reason, i have already implemented a caching system that saves answer data for each different question. Nevertheless, that first request each time might take up to 5 minutes .

Since my back-end server waits and does not respond back to the front-end until data arrives (the connections are being kept open), it can only serve 6 requests concurrently (that's what I have found). This is unacceptable in terms of performance.

What would be a workaround to this problem? Is there a way to not "clog" the server, so it can serve more than 6 users?
Is there a design pattern, in which the servers gives an initial response, and then serves the full data?
Perhaps, something that sets the request to "sleep" and opens up space for new connections?

Answer 1

Your server can serve many thousands of simultaneous requests if things are coded properly and it's not CPU intensive, just waiting for network responses. This is something that node.js is particularly good at.

A single browser, however, will only send a few requests at a time (it varies by browser) to the same endpoint (queuing the others until the earlier ones finish). So, my guess is that you're trying to test this from a single browser. That's not going to test what you really want to test because the browser itself is limiting the number of simultaneous requests. node.js is particularly good at having lots of request in flight at the same time. It can easily do thousands.

But, if you really have an operation that takes up to 5 minutes, that probably won't even work for an http request from a browser because the browser will probably time out an inactive connection still waiting for a result.

I can think of a couple possible solutions:

First, you could make the first http request be to just start the process and have it return immediately with an ID. Then, the client can check every 30 seconds of so after that sending the ID in an http request and your server can respond whether it has the result yet or not for that ID. This would be a client-polling solution.

Second, you could establish a webSocket or socket.io connection from client to server. Then, send a message over that socket to start the request. Then, whenever the server finishes its work, it can just send the result directly to the client over the webSocket or socket.io connection. After receiving the response, the client can either keep the webSocket/socket.io connection open for use again in the future or it can close it.