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How to emit to a list of sockets

In Socket.IO, there are several ways to emit an event to a specific subset of connected sockets:

Room​

In most applications, the recommended approach is to use rooms: create rooms based on your domain model, such as user IDs, organizations, projects, conversations, or subscriptions, then emit to those rooms.

Reference: Rooms

Example targeting a specific user:

function computeUserId(handshake) {
  // compute the user ID based on the headers/cookies/etc.
}

io.use(async (socket, next) => {
  const userId = await computeUserId(socket.handshake);

  socket.join(`user:${userId}`);

  next();
});

// then later
io.to("user:123").emit("event", "to this specific user");

Similarly, to target a specific project:

function computeUserId(handshake) {
  // compute the user ID based on the headers/cookies/etc.
}

io.use(async (socket, next) => {
  const userId = await computeUserId(socket.handshake);
  const projects = await getProjectsForUser(userId);

  for (const project of projects) {
    socket.join(`project:${project.id}`);
  }

  next();
});

// then later
io.to("project:123").emit("event", "to all users working on this project");

Pros​

  • easy to implement
  • performant

Cons​

  • you need to keep the rooms up-to-date:

  • there is no way to do an intersection between rooms (for example, all sockets with a specific role in the organization and assigned to a particular project)

info

io.to("org:123:manager").to("project:456").emit("event") will do a union of the rooms and emit to all sockets which are in either the org:123:manager or project:456 rooms (or both).

Namespace​

Namespaces are another way to organize your Socket.IO server into logical groups.

Reference: Namespaces

Example with a namespace for admin users:

const adminNamespace = io.of("/admin");

async function isAdmin(handshake) {
  // ...
}

adminNamespace.use(async (socket, next) => {
  if (await isAdmin(socket.handshake)) {
    next();
  } else {
    next(new Error("not authorized"));
  }
});

// then later
adminNamespace.emit("event", "to all admin users");

Or with one namespace per organization:

const namespaces = io.of(/^\/org:\d+$/);

async function isMemberOfOrganization(handshake, orgId) {
  // ...
}

namespaces.use(async (socket, next) => {
  const orgId = socket.nsp.name.slice("/org:".length);
  
  if (await isMemberOfOrganization(socket.handshake, orgId)) {
    next();
  } else {
    next(new Error("not authorized"));
  }
});

namespaces.on("connection", (socket) => {
  const namespace = socket.nsp;

  namespace.emit("event", "to all users in this organization");
});

Pros​

  • easy to implement
  • performant

Cons​

  • you need to keep the namespaces up-to-date:
    • call socket.disconnect() or io.disconnectSockets() (in the example above, when the user is removed from an organization)
    • connect to the namespace from the client side when the user is added to an organization

Dynamic subset of sockets​

If the solutions above don't suit your needs, you can always loop through all connected sockets and filter the sockets you want to emit to:

Standalone version​

Example if you have a single Socket.IO server:

function emitToMatchingSockets(filter, event, arg) {
  for (const socket of io.of("/").sockets.values()) {
    if (filter(socket)) {
      socket.emit(event, arg);
    }
  }
}

emitToMatchingSockets(
  (socket) => {
    return (
      socket.data.orgId === 123 &&
      socket.data.role === "manager" &&
      socket.data.projectIds.includes(456)
    );
  },
  "event",
  "to this specific subset of users",
);

Cluster version​

Example if you have multiple Socket.IO servers:

function localEmit({ orgId, role, projectId }, event, arg) {
  function filter(socket) {
    return (
      socket.data.orgId === orgId &&
      socket.data.role === role &&
      socket.data.projectIds.includes(projectId)
    );
  }

  for (const socket of io.of("/").sockets.values()) {
    if (filter(socket)) {
      socket.emit(event, arg);
    }
  }
}

io.on("emitToMatchingSockets", (filters, event, arg) => {
  localEmit(filters, event, arg);
});

function emitToMatchingSockets(filters, event, arg) {
  // emit to sockets connected on this server
  localEmit(filters, event, arg);

  // notify the other servers
  io.serverSideEmit("emitToMatchingSockets", filters, event, arg);
}

emitToMatchingSockets(
  {
    orgId: 123,
    role: "manager",
    projectId: 456,
  },
  "event",
  "to this specific subset of users",
);

References:

Pros​

  • you can apply any filter

Cons​

  • a bit less performant, since the payload will be encoded for each socket

That's all folks, thanks for reading!

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