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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 the Redis adapter (or another compatible Adapter): see below

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 package How it works the routing is based on the sid query parameter
sticky-session the routing is based on connection.remoteAddress
socketio-sticky-session the routing based on the x-forwarded-for header)

Example with

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

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

const httpServer = http.createServer();
setupMaster(httpServer, {
loadBalancingMethod: "least-connection", // either "random", "round-robin" or "least-connection"

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);
io.adapter(redisAdapter({ host: "localhost", port: 6379 }));

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

Passing events between nodes

The Redis adapter

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. You can implement your own on top of the (by inheriting from it) or you can use the one we provide on top of Redis:

const io = require("")(3000);
const redis = require("");
io.adapter(redis({ host: "localhost", port: 6379 }));

Then the following call:

io.emit("hi", "all sockets");

will be broadcast to every clients through the Pub/Sub mechanism of Redis:

Broadcasting with Redis

Sending messages from the outside world

Using the Redis adapter has another benefit: you can now emit events from outside the context of your Socket.IO processes.

Diagram with Redis adapter and external emitter

This emitter is available in several languages:

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