Why clusters don't work when requesting the same route at the same time in Express Node JS?

Question

I wrote a simple express application example handling 2 GET routes. The first route contains a while loop which represent a blocking operation in 5 seconds. The second route is simply return a Hello world text.

Also I set up a cluster following the simple guide on Node JS documentation.

Result of what I've tried:

1. Make 2 requests to 2 different routes at the same time => They work dependently as expected. Route / took 5 seconds and route /hello took several ms.

2. Make 2 requests to the same route / at the same time => They work synchronously, one responds after 5 seconds and the other after 10 seconds.

const cluster = require("cluster");
const express = require("express");
const app = express();

if (cluster.isMaster) {
  cluster.fork();
  cluster.fork();
} else {
  function doWork(duration) {
    const start = Date.now();

    while (Date.now() - start < duration) {}
  }

  app.get("/", (req, res) => {
    doWork(5000);
    res.send("Done");
  });

  app.get("/hello", (req, res) => {
    res.send("Hello world");
  });

  app.listen(3000);
}

I expect it would handle 2 requests of the same route in parallel. Can anyone explain what is going on?

Answer 1

It happens due to the cleverness of the modern browsers. If you make the same request in two different tab at the same time, the browser notice that and it wait to finish it and use the cache data of the first request to response the second request. No matter you use the clusters or how many fork().

To get rid out of this, simply disable cache in the network tab just shown as below:

Disable Cache

Answer 2

I expect it would handle 2 requests of the same route in parallel. Can anyone explain what is going on?

This is not the case as you have created two instances of server (two event loops, using cluster.fork()) ,so each of this request gets executed in different event loops (Server instance ) and the /hello will give you prompt request, whereas / request still wait for 5 seconds to send response.

Now if you haven't created cluster ,then the / request would have blocked the event loop and until it gets executed (Sends the response to browser ) /hello wouldn't have executed.

/ will take 5 seconds time to execute because you are blocking the event loop it executes in ,so whether you create single event loop or two event loops (using fork()) it will execute after 5 seconds

I tried your scenario in two different browsers and both request took 5.05 seconds(Both executed by different worker threads at same time)

const cluster = require("cluster");
const express = require("express");
const app = express();

if (cluster.isMaster) {
  cluster.fork();
  cluster.fork();
} else {
  function doWork(duration) {
    const start = Date.now();

    while (Date.now() - start < duration) {}
  }

  app.get("/", (req, res) => {
    console.log("Cluster ID",cluster.worker.id); // publish the workerid

    doWork(5000);
    res.send("Done");
  });




  app.listen(3000);
}

But with same browser ,the request always went to one worker thread, which executes the second request only after it has executed first ,So I guess its all about how the requests are distributed among worker threads created by cluster.fork()

As quoted from node docs

The cluster module supports two methods of distributing incoming connections.

The first one (and the default one on all platforms except Windows), is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

The second approach is where the master process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly. Node.js does not provide routing logic. It is, therefore important to design an application such that it does not rely too heavily on in-memory data objects for things like sessions and login.

Answer 3

I ran your code, first response came after 5 seconds and the other after 8 seconds, so clusters are working. Find out the number of cores of your machine using the below code. If it ones, then there is only one main thread.

const cpuCount = require('os').cpus().length;