JS is single threaded

By nature, JS as a high level language is single threaded. Though there is some internal multi threading, but as a user you can't write threads like Java. More on this later.

As in example Maximum call stack in v8 is 15000 frames and after that you will get maximum call stack error which you might have seen writing some recursive function.

There are a few ways to achieve(lets say emulate) multi threading, like different nodejs instances but there is no data sharing between them. We need to use message passing to share serialized objects between instances. SharedArrayBuffer can be shared between instances with worker_threads by passing message from one thread to another using postMessage(value) method

  • Cluster module - builtin cluster module for routing incoming requests to different instances
  • worker_threads module - run multiple js instances at once
  • child_process module - spawn and manage a full nodejs process

Nodejs internally is multi threaded. The libraries that run in the background do support multi threading. While application code, npm modules and nodejs core apis written in JS and are single threaded.

A quick overview of multiple layers of nodejs

Layer Thread
Application code Single
npm modules Single
core nodejs apis Single
Nodejs Bindings in C++ Multi
OpenSSL, V8, libuv Multi
Operating System Multi

For instance, libuv maintains a thread of I/O operations. From outside, nodejs may not be multi threaded but because of this, it can perform certain operations parallely.

The libuv thread pool size defaults to 4 and max is 1024. Can be overwritten by using env variable UV_THREADPOOL_SIZE=threads. Generally its fine to run with 4 threads. If the need comes to increase it, you need to do a perf test on productino like env first.

Nodejs process will exit if there are no more tasks on the queue. Threre are many nodejs api calls that result in creation of objects that keep the process alive. There are certain methods that can be used to force the object to no longer keep the process alive.

const t1 = setTimeout(() => {}, 1000000);
const t2 = setTimeout(() => {}, 2000000);

t1.unref(); // t1 timer is unreferenced. The callback can still run in 1000 seconds, but it won't keep the process alive.
clearTimeout(t2); // t2 time has been cleared and will never run. It no longer keeps the process alive

A point to note - setTimeout() returns an object in nodejs while in browsers it returns an integer and thats why we are able to call t1.unref() in nodejs.

Event loop

Though its similar in browsers and nodejs, it is optimized differently for the respective platforms.

Event loop phases

In nodejs there are different phases of event loop.

  • Poll - I/O related callbacks
  • Check - callbacks triggered via setImmediate()
  • Close - callbacks triggered via setEmitter close events. eg. Server TCP close
  • Timers - callbacks scheduled using setTimeout and setInterval
  • Pending - Special system events like "net.Socket TCP socket throws an ECONNREFUSED error"

Additionally there are two special micro task queues that can have callbacks added to them while a phase is running.

  • First microtask queue - handles callbacks registered using process.NextTick()
  • Second microtask queue - handles promises that reject or resolve

Callbacks in the microtask queues take priority over callbacks in the phases's normal queue. Callbacks in next tick microtask queue run before callbacks in the promise microtask queue

When event looop gets to a phase, it will run all the callbacks in that phases's queue. Once all the callbacks in a given phase are exhausted, the event loop then moves to next phase`.

Try coming up with the output before running the program and see if you understand the phases correctly.

const fs = require('fs');
setImmediate(() => console.log(1));
Promise.resolve().then(() => console.log(2));
process.nextTick(() => console.log(3));
fs.readFile(__filename, () => {
setTimeout(() => console.log(5));
setImmediate(() => console.log(6));
process.nextTick(() => console.log(7));

The outputs 8 3 2 1 4 7 6 5. Surprisingly I got the answer right.

Just a side note - tick is a complete pass, through the event loop. setImmediate() takes a tick to run, whereas process.nextTick() is more immediate - Confusing names!`

And of course async and await are just syntactic sugar for code that uses nested callbacks.

async and sync Callbacks

If you are passing a callback to a function, make sure that the callback either runs asynchrounously or synchronously and not both.

function updateSomeValue(count, callbackFn) {
    if (count <= 0) {
        return callbackFn("Not allowed"); // calling callback synchronously
    } else {
        someAsyncOperation(count, callbackFn); // calling callback asynchrounously

By doing this you are introducing a scenario where the callback can update a value present in parent scope which may or may not be updated depending on how you called that callback function which can lead to confusing bugs.

Creating application

Let's start by creating a basic structure of the application that will be used in later chapters. All the code that we will write will go inside these two directories. You can have a parent directory named learning nodejs so that the common files can go there.

  • web-api - server from browser pov
  • recipe-api - server from web-api pov


Cluster module

Cluster module allows running multiple copies of a Node.js application on the same machine, dispatching incoming network messages to the copies. Its similar to child_process module which provides a fork() method for spawning Node.js sub process. But cluster modules also provides a mechanism for routing incoming requests.

Creating cluster

#!/usr/bin/env node
const cluster = require('cluster');
console.log(`master pid=${process.pid}`);
    exec: __dirname+'/producer-http-basic.js'
cluster.on("disconnect", worker => {
    console.log("disconnect", workder.id);
}).on("exit", (worker,code, signal) => {
    console.log("exit", worker.id, code, signal);
}).on("listening", (worker, {address, port}) => {
    console.log("listening", worker.id, `${address}:${port}`);
master pid=30839
worker pid=30840
(node:30840) [FSTDEP011] FastifyDeprecation: Variadic listen method is deprecated. Please use ".listen(optionsObject)" instead. The variadic signature will be removed in `fastify@5`.
(Use `node --trace-warnings ...` to show where the warning was created)
Producer running at
listening 1
worker pid=30841
(node:30841) [FSTDEP011] FastifyDeprecation: Variadic listen method is deprecated. Please use ".listen(optionsObject)" instead. The variadic signature will be removed in `fastify@5`.
(Use `node --trace-warnings ...` to show where the warning was created)
Producer running at
listening 2

Now if you run ps command you will see 3 processes. One for master.js and two for basic.js.

Request dispatching

On macOS and Linux the requests will be dispatched round-robin to the workers by default. On windows the request will be dispatched depending on which worker is least busy.

Now try and hit the service

curl http://localhost:4000/recipes/42

In the logs you should see that the workers are hit in round robin

worker request pid=30840
worker request pid=30841
worker request pid=30840
worker request pid=30841

You can now kill one of the worker nodes and the master should still be running with the remaining workers and serve the request from it. If you kill all child workers, the master also exits.

Cons of cluster module

  • Kind of an anti pattern as scalability is better done by an external tool which gives more visiblity while running instances
  • This works at the later 4 TCP/UDP and not layer 7.
  • Performance is not good on single cpu machine as the cpu is shared and for a high load, the worker threads will have to wait for cpu cycle
  • Lastly cluster module only dispatches incoming requests to processes running on same machine.

Reverse Proxy

Sits between the backend service and the clients. It will redirect the incoming requests to running services and will also direct the response from the service to the client back.

There are multiple ways reverse proxies can redirect requests

  • Round robin is default
  • Based on the least busy service
  • random dispatch
  • based on content of the initial request such as session Id or cookie
  • proxy can poll the backend and redirect to the ones which are healthy

One of the popular proxies is HAProxy.

Installing HAProxy.

For Linux you can use distro's in-built repo to install or compile from source.

For macbook you can install using brew

brew install haproxy@2.1.8

Run below to check if HAProxy was installed correctly

$ haproxy -v
HAProxy version 2.8.2-61a0f57 2023/08/09 - https://haproxy.org/
Status: long-term supported branch - will stop receiving fixes around Q2 2028.
Known bugs: http://www.haproxy.org/bugs/bugs-2.8.2.html
Running on: Darwin 22.6.0 Darwin Kernel Version 22.6.0: Wed Jul  5 22:22:05 PDT 2023; root:xnu-8796.141.3~6/RELEASE_ARM64_T6000 arm64

Start HAProxy

Create a config file haproxy/stats.cfg and add below content to it. Make sure to add a new line at the end.

frontend inbound
    mode http
    bind localhost:8000
    stats enable
    stats uri /admin?stats

Now start HAProxy

$haproxy -f haproxy/stats.cfg

You should see below logs

14:41:40 distributed-nodejs $ haproxy -f haproxy/stats.cfg
[NOTICE]   (36068) : haproxy version is 2.8.2-61a0f57
[WARNING]  (36068) : config : missing timeouts for frontend 'inbound'.
   | While not properly invalid, you will certainly encounter various problems
   | with such a configuration. To fix this, please ensure that all following
   | timeouts are set to a non-zero value: 'client', 'connect', 'server'.

Now if you hit the below URL, you should see a dashboard


If you setup HAProxy with multiple routes, you will see a dashboard like below

HAProxy alternatives

Nginx, ELB, Traefik, Kong Gateway.

One point to note about Nginx. It can also act as a web server while HAProxy can't. Nginx can map requests to files on disk.

Configuring HAProxy with multiple backends. Create a config file like below.

    mode http
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 550000ms

frontend inbound
    bind localhost:3000
    default_backend web-api
    stats enable
    stats uri /admin?stats

backend web-api
    option httpchk GET /health
    server web-api-1 localhost:3001 check
    server web-api-2 localhost:3002 check

If we compare this to node's cluster module, we don't need a third nodejs process running. Instead the routing will be handled by HAProxy process. Moreover, now using HAProxy we can route requests to outside the machine which is what generally a use case for all scalable solutions in the cloud.

Now to test the setup perform below opeerations

node recipe-api/producer-http-basic.js                      // start nodejs backend
PORT=3001 node web-api/consumer-http-healthendpoint.js      // start api-1
PORT=3002 node web-api/consumer-http-healthendpoint.js      // start api-2
haproxy -f ./haproxy/load-balance.cfg                       // start HAProxy

Run below command multiple times to see that response is coming from api-1 and api-2 in round-robin method.


If you close one of the processes, HAProxy will redirect the requests to remaining processes. Same can be observed on the HAProxy Dashboard page.

And if all api endpoints go down, you will endup in error which can be seen in HAPRoxy logs as well.

15:29:59 distributed-nodejs $ curl http://localhost:3000

<html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.


HAProxy can do compression as well. You can configure HAProxy to compress the response before sending to the client. There are multiple config options.

Configure the compression by adding below lines to HAProxy config

backend web-api
    compression offload
    compression algo gzip
    compression type application/json text/plain
    server web-api-1 localhost:3001

And you can test by running below commands

$ node recipe-api/producer-http-basic.js
$ PORT=3001 node web-api/consumer-http-basic.js
$ haproxy -f haproxy/compression.cfg
$ curl http://localhost:3000/
$ curl -H 'Accept-Encoding: gzip' http://localhost:3000/ | gunzip

TLS Termination

HAProxy can do encryption as well. You need to generate a .pem certificate first.

Generate certificate and private key. Combine both to one file combined.pem.

openssl req -nodes -new -x509 -keyout haproxy/private.key -out haproxy/certificate.cert
cat haproxy/certificate.cert haproxy/private.key > haproxy/combined.pem

Configure HAProxy like below to use the .pem file for SSL

    tune.ssl.default-dh-param 2048

frontend inbound
    bind localhost:3000 ssl crt haproxy/combined.pem
    default_backend web-api

And test it out by using a curl command. You will need to use --insecure because the certificate is still self signed. If you use a certificate from a known CA, you do not need this flag. One of my earlier posts around related topic - thisisunsafe

15:49:57 distributed-nodejs $ curl --insecure https://localhost:3000/
{"consumer_pid":42250,"producer_data":{"producer_pid":42054,"recipe":{"id":42,"name":"Chicken Tikka Masala","steps":"Throw it in a pot...","ingredients":[{"id":1,"name":"Chicken","quantity":"1 lb"},{"id":2,"name":"Sauce","quantity":"2 cups"}]}}}%

Rate limiting

If you need to limit the server to handle only a max number of connections at a given time, use below to achieve it

One way is to do this with the nodejs http server.

const http = require('http');

const server = http.createServer((req, res) => {
    console.log("current conn", server._connections);
    setTimeout(() => res.end("OK"), 10_000);

server.maxConnections = 2; // rate limiting the server itself
server.listen(3020, 'localhost');

But this would mean a hard limit to all the servers that you start.

Here HAProxy can help. It can queue the requests for the application as well. So the server not necessarily needs to be limited to a fixed number. This approach makes it easy to configure the scaling when you are using external tools for service orchestration like k8s etc.

    maxconn 8
    mode http

frontend inbound
    bind localhost:3010
    default_backend web_api

backend web_api
    option httpclose
    server web-api-1 localhost:3020 maxconn 2

global maxconn includes incoming and outgoing backend connections

Now start HAProxy with new config and run some curl commands parallely. You will see that the server is able to serve more than 2 requests. The requests holds with HAProxy and only goto nodejs when the count comes below the threshold.

SLA and Load Testing

To start load testing we need to install autocannon. There are many alternatives to autocannon like Apache Bench(ab), wrk, and Siege.

npm install -g autocannon

Create new directory benchmark/ and create a file native-http.js within it with below contents

#!/usr/bin/env node

const HOST = process.env.HOST || '127.0.0l1';
const PORT = process.env.PORT || 4000;

require("http").createServer((req, res) => {
}).listen(PORT, () => {
    console.log(`Producer running at http://${HOST}:${PORT}`)

Now first start the service and then run autocannon to start a loastest.

node benchmark/native-http.js
autocannon -d 60 -c 10 -l http://localhost:4000/

-d: set the overall duration of the test
-c: set the concurrent connections
-l: display a latency histogram

In case you see errors in autocannon output, use --debug to see the errors like below

Error: connect ECONNREFUSED
    at TCPConnectWrap.afterConnect [as oncomplete] (node:net:1278:16) {
  errno: -61,
  syscall: 'connect',
  address: '',
  port: 3001

Reverse proxy concerns

Create a HAProxy config to test out perf difference with and without proxy

    mode http

frontend inbound
    bind localhost:4001
    default_backend native-http

backend native-http
    server native-http-1 localhost:4000

Now start the test

node benchmark/native-http.js - Run the server
haproxy -f haproxy/benchmark-basic.cfg - Start proxy
autocannon -d 60 -c 10 -l http://localhost:4001 - Start loadtest

HTTP Compression

Add passthru.cfg to the haproxy folder. With this we can measure the cost of performing http compression in HAProxy vs Nodejs

    mode tcp
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

frontend inbound
    bind localhost:3000
    default_backend server-api

backend server-api
    server server-api-1 localhost:3001

Run below commands

Create simple index.html to be served

rm index.html ; curl -o index.html https://thomashunter.name

Start nodejs server which has compression

PORT=3001 node server-gzip.js```

To be continued with part-2...