MongoDB adapter

How it works

The MongoDB adapter relies on MongoDB's Change Streams (and thus requires a replica set or a sharded cluster).

Every packet that is sent to multiple clients (e.g. io.to("room1").emit() or socket.broadcast.emit()) is:

• sent to all matching clients connected to the current server
• inserted in a MongoDB capped collection, and received by the other Socket.IO servers of the cluster

The source code of this adapter can be found here.

Supported features

Feature	socket.io version	Support
Socket management	4.0.0	✅ YES (since version 0.1.0)
Inter-server communication	4.1.0	✅ YES (since version 0.1.0)
Broadcast with acknowledgements	4.5.0	✅ YES (since version 0.2.0)
Connection state recovery	4.6.0	✅ YES (since version 0.3.0)

Installation

npm install @socket.io/mongo-adapter mongodb

For TypeScript users, you might also need @types/mongodb.

Usage

Broadcasting packets within a Socket.IO cluster is achieved by creating MongoDB documents and using a change stream on each Socket.IO server.

There are two ways to clean up the documents in MongoDB:

• a capped collection
• a TTL index

Usage with a capped collection

import { Server } from "socket.io";
import { createAdapter } from "@socket.io/mongo-adapter";
import { MongoClient } from "mongodb";

const DB = "mydb";
const COLLECTION = "socket.io-adapter-events";

const io = new Server();

const mongoClient = new MongoClient("mongodb://localhost:27017/?replicaSet=rs0");

await mongoClient.connect();

try {
  await mongoClient.db(DB).createCollection(COLLECTION, {
    capped: true,
    size: 1e6
  });
} catch (e) {
  // collection already exists
}
const mongoCollection = mongoClient.db(DB).collection(COLLECTION);

io.adapter(createAdapter(mongoCollection));
io.listen(3000);

Usage with a TTL index

import { Server } from "socket.io";
import { createAdapter } from "@socket.io/mongo-adapter";
import { MongoClient } from "mongodb";

const DB = "mydb";
const COLLECTION = "socket.io-adapter-events";

const io = new Server();

const mongoClient = new MongoClient("mongodb://localhost:27017/?replicaSet=rs0");

await mongoClient.connect();

const mongoCollection = mongoClient.db(DB).collection(COLLECTION);

await mongoCollection.createIndex(
  { createdAt: 1 },
  { expireAfterSeconds: 3600, background: true }
);

io.adapter(createAdapter(mongoCollection, {
  addCreatedAtField: true
}));

io.listen(3000);

Options

Name	Description	Default value	Added in
uid	the ID of this node	a random id	v0.1.0
requestsTimeout	the timeout for inter-server requests such as fetchSockets() or serverSideEmit() with ack	5000	v0.1.0
heartbeatInterval	the number of ms between two heartbeats	5000	v0.1.0
heartbeatTimeout	the number of ms without heartbeat before we consider a node down	10000	v0.1.0
addCreatedAtField	whether to add a createdAt field to each MongoDB document	false	v0.2.0

Common questions

Do I still need to enable sticky sessions when using the MongoDB adapter?

Yes. Failing to do so will result in HTTP 400 responses (you are reaching a server that is not aware of the Socket.IO session).

More information can be found here.

What happens when the MongoDB cluster is down?

In case the connection to the MongoDB cluster is severed, the behavior will depend on the value of the bufferMaxEntries option of the MongoDB client:

• if its value is -1 (default), the packets will be buffered until reconnection.
• if its value is 0, the packets will only be sent to the clients that are connected to the current server.

Documentation: http://mongodb.github.io/node-mongodb-native/3.6/api/global.html#MongoClientOptions

Latest releases

Version	Release date	Release notes	Diff
0.3.2	January 2024	link	0.3.1...0.3.2
0.3.1	January 2024	link	0.3.0...0.3.1
0.3.0	February 2023	link	0.2.1...0.3.0
0.2.1	May 2022	link	0.2.0...0.2.1
0.2.0	April 2022	link	0.1.0...0.2.0
0.1.0	June 2021	link

Complete changelog

Emitter

The MongoDB emitter allows sending packets to the connected clients from another Node.js process:

Installation

npm install @socket.io/mongo-emitter mongodb

Usage

const { Emitter } = require("@socket.io/mongo-emitter");
const { MongoClient } = require("mongodb");

const mongoClient = new MongoClient("mongodb://localhost:27017/?replicaSet=rs0");

const main = async () => {
  await mongoClient.connect();

  const mongoCollection = mongoClient.db("mydb").collection("socket.io-adapter-events");
  const emitter = new Emitter(mongoCollection);

  setInterval(() => {
    emitter.emit("ping", new Date());
  }, 1000);
}

main();

Please refer to the cheatsheet here.