ECONNRESET in Express.js (Node.js) with multiple requests

Question

Given a standard Express.js setup

const express = require('express');
const app = express();
const router = express.Router();

router.get('/test/:id', (req, res) => {
  return res.status(200).json({ hello: 'world' });
});

app.use('/api', router);

app.listen(3000, () => console.info('Up on port 3000));

I am making 1000 requests agains the endpoint, one after the other:

const fetch = require('node-fetch');
for (let i = 0; i < 1000; i++) {
  let id = Math.floor(Math.random() * 12) + 1;
  fetch(`http://localhost:3000/api/test/${id}`).then(res => res.json()).then(data => console.log(data)).catch(error => console.error(error));
}

I do see the data returned however, every now and then I see an ECONNRESET error. The amount of ECONNRESET error messages also vary: sometimes I get a few, sometimes a lot more. I do understand the message but I can't get my head around solving the issue behind it.

Here's a sample error:

{ FetchError: request to http://localhost:3000/api/test/8 failed, reason: connect ECONNRESET 127.0.0.1:3000
    at ClientRequest.<anonymous> (node_modules/node-fetch/lib/index.js:1345:11)
    at ClientRequest.emit (events.js:182:13)
    at Socket.socketErrorListener (_http_client.js:399:9)
    at Socket.emit (events.js:182:13)
    at emitErrorNT (internal/streams/destroy.js:82:8)
    at emitErrorAndCloseNT (internal/streams/destroy.js:50:3)
    at process.internalTickCallback (internal/process/next_tick.js:72:19)
  message:
   'request to http://localhost:3000/api/departments/8 failed, reason: connect ECONNRESET 127.0.0.1:3000',
  type: 'system',
  errno: 'ECONNRESET',
  code: 'ECONNRESET' }

Note that I have tried to make the request using axios, the built-in HTTP module all to avail. I'm sure the issue is with my Express app handling the request but not sure how to fix it exactly.

Update 1:

As per the suggestion in the comment, here's the async version:

async function f() {
  const array = Array.from(Array(1000).keys());
  for (const el of array) {
    try {
      let id = Math.floor(Math.random() * 12) + 1;
      const result = await fetch(`http://localhost:3000/api/test/${id}`).then(res => res.json());
      console.log(result);
      return result;
    } catch(e) {
      console.log(e);
    }
  }
}

f();

Now I am receiving occasional ECONNREFUSED messages.

Update 2:

Based on Mazki516's answer here's the solution that works:

// previous require statements
const cluster = require('cluster');
const os = require('os');

if (cluster.isMaster) {
  const cpuCount = os.cpus().length
  for (let i = 0; i < cpuCount; i++) {
      cluster.fork()
  }
} else {
  const app = express();
  // rest of the route definitions
  // also app.listen() etc...
}
cluster.on('exit', worker => {
  console.log(`${worker.id} removed`);
  cluster.fork();
});

Bluster

You are trying to use an aync action (fetch) in a sync code (for loop). See this, maybe it helps. – Cacoepy

updated the code, now I get ECONNREFUSED from fetch in some cases. – Bluster

Answer 1

One of the reasons you see this is because you make the calls in "parallel" . You do start the calls one after the other , but the loops will end probably before the first results returned from the server. The loop continues until the end , making the call stack filled with 1000 async requests to the server .

Your'e are hitting hardware/software limits and nothing is wrong with the code. if you did want to build a server which can handle 1k (and much more) requests concurrently I would take a look into the "cluster" module of node .

Please notice that when doing network job between server , it's acceptable to use a concurrency limit . (for example: up to 4 requests concurrently)

but you can always scale your server beyond one machine and handle much more traffic .

Peria

Is there a way to increase the limit? Can you also elaborate a bit more on the concurrent requests? – Bluster

You can always try tweaking file descriptors limits(os level) and timeouts (in express server) . but those are just "small" fixes and not the real solution. Please take a look into the cluster module so you learn quickly how to scale up your app beyond once process(multi-core server) . – Peria

Regard concurrency, Nodejs is based on event-driven architecture , where you put tasks into the queue and wait for them to finish . you first code , just created 1k requests , without waiting for some of them to finish before the others. This is a bad practice , especially working on IO (network , filesystem) Again , if you want "more power"/"juice" ;) , scale your app , and make sure you don't put to many tasks into the queue . – Peria

Would you know how to set the concurrency limit in an Express app? – Bluster

Express concurrency limit is bounded to the Nodejs process concurrency limits which is bounded to the performance of your cpu/memory/network . it's not a number you set or tweak , and it depends on the hardware performance . (better performance , more concurrency) – Peria

Answer 2

I faced the same issue as you, and I was able to solve with the @lependu comment (can't comment below there due to my reputation).

But basically I mapped my data array inside an await Promise.all and it worked perfectly.

In my case I switched from:

for (let i = 0; i < dataSet.length; i++) {
    const object = dataSet[i];
    const objectID = await post(url, object);
    ids.push(objectID);
}

To:

await Promise.all(dataSet.map(async (object) => {
    const objectID = await post(url, object);
    ids.push(objectID);
}));

Credits to @lependu - the answer in Using async/await with a forEach loop and the post How to use async/await inside loops in JavaScript

James