'Why does an await within a server endpoint blocks other incoming requests?

I have two different versions of an endpoint for testing. The first one is this:

app.get('/order', async (req, res) => {
    console.log("OK")
    getPrintFile()
})

And the second one is this:

app.get('/order', async (req, res) => {
    console.log("OK")
    await getPrintFile()
})

the getPrintFile is an async function that returns a promise when every operation is done. Withing the function I upload an image to a server, I download a new image, and re upload that new image to another server.

I noticed that in the first example, without the await, if I send a lot of requests to the "order" endpoint, I get the "OK" instantly for each request, which is what I want because that "OK" will get replaced by a res.status("200"). I need to send a status 200 immediatly after getting the endpoint hit for various reasons. Then I don't care how long it takes for the server to do all the processing of the images/uploading, as long as the res.send(200) is executed instantly when there is a new incoming request.

However, when I use the await, even if new requests are coming in, it takes a lot to display the next "OK" if a previous request is still processing. Usually it displays the OK only when the code within the getPrintFile function is done executing (that is, images are uploaded and everything is done)

It's like the event loop is blocked but I don't understand why.

What is happening here?

Edit: So it is clearer, I tested it. If I send 5 requests to the "order" endpoint, the "OK" is displayed in the console immediately for all of them, and then the images are processed and uploaded at their own speed for each request. In the second example, if I send 5 requests, the first OK is displayed, and then the remaining 4 are displayed one at a time when the previous request is done executing, or if not exactly in that order, they get logged with tremendous delay, and not consecutively

node.jsexpressasync-await


Solution 1:[1]

I'll try to answer based on my understanding of your problem. The first thing missing is the res.sendStatus(200) in your examples to make them actually work. But then, indeed it happens as you describe it: the /order endpoint actually is "blocking" you from making another request as the await is blocking you from reaching the final statement (res.send). Here a full example

const express = require("express");

const app = express();

async function takeTime() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log("resolved");
      resolve(1);
    }, 2000);
  });
}

app.get("/order", (req, res) => {
  console.log("ok"); // Happens immediately
  await takeTime();
  return res.sendStatus(200);
});

app.get("/", async (req, res) => {
  console.log("ok"); // Happens immediately
  takeTime();
  return res.sendStatus(200);
});

app.listen(3000, () => {
  console.log("server running on port 3000");
});

When testing the API (i.e. from Postman), you won't be able to run immediately another request on the /order endpoint, because you will be locked in waiting for the answer of the request just sent.

In the / endpoint, on the other hand, you will receive immediately the HTTP 200 response (as there is no await) and the code for the takeTime function will keep running asynchronously until it's done.

Here you can find more information, I hope it's useful.

Edit 1

here I add the .html page I'm using to test the await loop requests

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body></body>

  <script>
    for (let i = 0; i < 10; i++) {
      fetch("http://localhost:3000/order");
    }
  </script>
</html>

The fetch is not working for me in the browser, but having a web page making those request works

Sources

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Source: Stack Overflow

Solution Source
Solution 1

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