@socket.io/mongo-adapter

Socket.IO MongoDB adapter

The @socket.io/mongo-adapter package allows broadcasting packets between multiple Socket.IO servers.

Unlike the existing socket.io-adapter-mongo package which uses tailable cursors, this package relies on change streams and thus requires a replica set or a sharded cluster.

Table of contents

• Supported features
• Installation
• Usage
  • Usage with a capped collection
  • Usage with a TTL index
• Known errors
• License

Supported features

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

Installation

npm install @socket.io/mongo-adapter 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);

Known errors

• MongoError: The $changeStream stage is only supported on replica sets

Change streams are only available for replica sets and sharded clusters.

More information here.

Please note that, for development purposes, you can have a single MongoDB process acting as a replica set by running rs.initiate() on the node.

• TypeError: this.mongoCollection.insertOne is not a function

You probably passed a MongoDB client instead of a MongoDB collection to the createAdapter method.

License

MIT

Changelog

0.4.0 (2025-08-10)

Features

• allow to specify additional change stream options (#26) (eae849b)