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Version: 4.x

Using multiple nodes

When deploying multiple Socket.IO servers, there are two things to take care of:

  • enabling sticky session, if HTTP long-polling is enabled (which is the default): see below
  • using a compatible adapter, see here

Sticky load balancing​

If you plan to distribute the load of connections among different processes or machines, you have to make sure that all requests associated with a particular session ID reach the process that originated them.

Why is sticky-session required​

This is because the HTTP long-polling transport sends multiple HTTP requests during the lifetime of the Socket.IO session.

In fact, Socket.IO could technically work without sticky sessions, with the following synchronization (in dashed lines):

Using multiple nodes without sticky sessions

While obviously possible to implement, we think that this synchronization process between the Socket.IO servers would result in a big performance hit for your application.


  • without enabling sticky-session, you will experience HTTP 400 errors due to "Session ID unknown"
  • the WebSocket transport does not have this limitation, since it relies on a single TCP connection for the whole session. Which means that if you disable the HTTP long-polling transport (which is a perfectly valid choice in 2021), you won't need sticky sessions:
const socket = io("", {
// WARNING: in that case, there is no fallback to long-polling
transports: [ "websocket" ] // or [ "websocket", "polling" ] (the order matters)

Documentation: transports

Enabling sticky-session​

To achieve sticky-session, there are two main solutions:

  • routing clients based on a cookie (recommended solution)
  • routing clients based on their originating address

You will find below some examples with common load-balancing solutions:

For other platforms, please refer to the relevant documentation:

Important note: if you are in a CORS situation (the front domain is different from the server domain) and session affinity is achieved with a cookie, you need to allow credentials:


const io = require("")(httpServer, {
cors: {
origin: "",
methods: ["GET", "POST"],
credentials: true


const io = require("");
const socket = io("", {
withCredentials: true

Without it, the cookie will not be sent by the browser and you will experience HTTP 400 "Session ID unknown" responses. More information here.

NginX configuration​

Within the http { } section of your nginx.conf file, you can declare a upstream section with a list of Socket.IO process you want to balance load between:

http {
server {
listen 3000;

location / {
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $host;

proxy_pass http://nodes;

# enable WebSockets
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";

upstream nodes {
# enable sticky session with either "hash" (uses the complete IP address)
hash $remote_addr consistent;
# or "ip_hash" (uses the first three octets of the client IPv4 address, or the entire IPv6 address)
# ip_hash;
# or "sticky" (needs commercial subscription)
# sticky cookie srv_id expires=1h path=/;

server app01:3000;
server app02:3000;
server app03:3000;

Notice the hash instruction that indicates the connections will be sticky.

Make sure you also configure worker_processes in the topmost level to indicate how many workers NginX should use. You might also want to look into tweaking the worker_connections setting within the events { } block.


Apache HTTPD configuration​

Header add Set-Cookie "SERVERID=sticky.%{BALANCER_WORKER_ROUTE}e; path=/" env=BALANCER_ROUTE_CHANGED

<Proxy "balancer://nodes_polling">
BalancerMember "http://app01:3000" route=app01
BalancerMember "http://app02:3000" route=app02
BalancerMember "http://app03:3000" route=app03
ProxySet stickysession=SERVERID

<Proxy "balancer://nodes_ws">
BalancerMember "ws://app01:3000" route=app01
BalancerMember "ws://app02:3000" route=app02
BalancerMember "ws://app03:3000" route=app03
ProxySet stickysession=SERVERID

RewriteEngine On
RewriteCond %{HTTP:Upgrade} =websocket [NC]
RewriteRule /(.*) balancer://nodes_ws/$1 [P,L]
RewriteCond %{HTTP:Upgrade} !=websocket [NC]
RewriteRule /(.*) balancer://nodes_polling/$1 [P,L]

ProxyTimeout 3


HAProxy configuration​

# Reference:

listen chat
bind *:80
default_backend nodes

backend nodes
option httpchk HEAD /health
http-check expect status 200
cookie io prefix indirect nocache # using the `io` cookie set upon handshake
server app01 app01:3000 check cookie app01
server app02 app02:3000 check cookie app02
server app03 app03:3000 check cookie app03



Using container labels:

# docker-compose.yml
image: traefik:2.4
- /var/run/docker.sock:/var/run/docker.sock
- server

image: my-image:latest
- "`/`)"

With the File provider:

## Dynamic configuration
rule: "PathPrefix(`/`)"
name: server_id
httpOnly: true


Using Node.js Cluster​

Just like NginX, Node.js comes with built-in clustering support through the cluster module.

There are several solutions, depending on your use case:

NPM packageHow it works routing is based on the sid query parameter
sticky-sessionthe routing is based on connection.remoteAddress
socketio-sticky-sessionthe routing based on the x-forwarded-for header)

Example with

const cluster = require("cluster");
const http = require("http");
const { Server } = require("");
const numCPUs = require("os").cpus().length;
const { setupMaster, setupWorker } = require("");
const { createAdapter, setupPrimary } = require("");

if (cluster.isMaster) {
console.log(`Master ${} is running`);

const httpServer = http.createServer();

// setup sticky sessions
setupMaster(httpServer, {
loadBalancingMethod: "least-connection",

// setup connections between the workers

// needed for packets containing buffers (you can ignore it if you only send plaintext objects)
// Node.js < 16.0.0
serialization: "advanced",
// Node.js > 16.0.0
// cluster.setupPrimary({
// serialization: "advanced",
// });


for (let i = 0; i < numCPUs; i++) {

cluster.on("exit", (worker) => {
console.log(`Worker ${} died`);
} else {
console.log(`Worker ${} started`);

const httpServer = http.createServer();
const io = new Server(httpServer);

// use the cluster adapter

// setup connection with the primary process

io.on("connection", (socket) => {
/* ... */

Passing events between nodes​

Now that you have multiple Socket.IO nodes accepting connections, if you want to broadcast events to all clients (or to the clients in a certain room) you’ll need some way of passing messages between processes or computers.

The interface in charge of routing messages is what we call the Adapter.