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Version: 4.x


An Adapter is a server-side component which is responsible for broadcasting events to all or a subset of clients.

When scaling to multiple Socket.IO servers, you will need to replace the default in-memory adapter by another implementation, so the events are properly routed to all clients.

Besides the in-memory adapter, there are four official implementations:

There are also several other options which are maintained by the (awesome!) community:

Please note that enabling sticky sessions is still needed when using multiple Socket.IO servers and HTTP long-polling. More information here.


You can have access to the adapter instance with:

// main namespaceconst mainAdapter = io.of("/").adapter; // WARNING! io.adapter() will not work// custom namespaceconst adminAdapter = io.of("/admin").adapter;

Starting with, each Adapter instance emits the following events:

  • create-room (argument: room)
  • delete-room (argument: room)
  • join-room (argument: room, id)
  • leave-room (argument: room, id)


io.of("/").adapter.on("create-room", (room) => {  console.log(`room ${room} was created`);});
io.of("/").adapter.on("join-room", (room, id) => {  console.log(`socket ${id} has joined room ${room}`);});


Most adapter implementations come with their associated emitter package, which allows communicating to the group of Socket.IO servers from another Node.js process.

Emitter diagram

This may be useful for example in a microservice setup, where all clients connect to the microservice M1, while the microservice M2 uses the emitter to broadcast packets (uni-directional communication).

Emitter cheatsheet#

// to all clientsemitter.emit(/* ... */);
// to all clients in "room1""room1").emit(/* ... */);
// to all clients in "room1" except those in "room2""room1").except("room2").emit(/* ... */);
const adminEmitter = emitter.of("/admin");
// to all clients in the "admin" namespaceadminEmitter.emit(/* ... */);
// to all clients in the "admin" namespace and in the "room1""room1").emit(/* ... */);

The emitter also supports the utility methods that were added in

  • socketsJoin()
// make all Socket instances join the "room1" roomemitter.socketsJoin("room1");
// make all Socket instances of the "admin" namespace in the "room1" room join the "room2" roomemitter.of("/admin").in("room1").socketsJoin("room2");
  • socketsLeave()
// make all Socket instances leave the "room1" roomemitter.socketsLeave("room1");
// make all Socket instances in the "room1" room leave the "room2" and "room3""room1").socketsLeave(["room2", "room3"]);
// make all Socket instances in the "room1" room of the "admin" namespace leave the "room2" roomemitter.of("/admin").in("room1").socketsLeave("room2");
  • disconnectSockets()
// make all Socket instances disconnectemitter.disconnectSockets();
// make all Socket instances in the "room1" room disconnect (and discard the low-level connection)"room1").disconnectSockets(true);
// make all Socket instances in the "room1" room of the "admin" namespace disconnectemitter.of("/admin").in("room1").disconnectSockets();
// this also works with a single socket IDemitter.of("/admin").in(theSocketId).disconnectSockets();
  • serverSideEmit()
// emit an event to all the Socket.IO servers of the clusteremitter.serverSideEmit("hello", "world");
// Socket.IO server (server-side)io.on("hello", (arg) => {  console.log(arg); // prints "world"});