SocketCluster server

This is a stand-alone server module for SocketCluster. SocketCluster's protocol is backwards compatible with the SocketCluster protocol.

Setting up

You will need to install both ``socketcluster-server` and `socketcluster-client` (https://github.com/SocketCluster/socketcluster-client).

npm install socketcluster-server

const http = require('http');
const socketClusterServer = require('socketcluster-server');

let httpServer = http.createServer();
let agServer = socketClusterServer.attach(httpServer);
(async () => {
  // Handle new inbound sockets.
  for await (let {socket} of agServer.listener('connection')) {
(async () => {
      // Set up a loop to handle and respond to RPCs for a procedure.
      for await (let req of socket.procedure('customProc')) {
        if (!req.data) {
          let error = new Error('Server failed to execute the procedure');
          error.name = 'BadCustomError';
          req.error(error);
        } else {
          req.end('Success');
        }
      }
    })();
(async () => {
      // Set up a loop to handle remote transmitted events.
      for await (let data of socket.receiver('customRemoteEvent')) {
        // ...
      }
    })();
}
})();
httpServer.listen(8000);

For more detailed examples of how to use SocketCluster, see test/integration.js. Also, see tests from the socketcluster-client module.

SocketCluster can work without the for-await-of loop; a while loop with await statements can be used instead. See https://github.com/SocketCluster/stream-demux#usage

For compatibility with existing SocketCluster clients, set the protocolVersion to 1 and make sure that the path matches your old client path:

let agServer = socketClusterServer.attach(httpServer, {
  protocolVersion: 1,
  path: '/socketcluster/'
});

Since version 21.0.0, the maxPayload option defaults to 4MB; before that it was unset, which left the ws engine default of 100MB in place. A message which exceeds the limit is rejected and the socket is closed with status code 1009.

let agServer = socketClusterServer.attach(httpServer, {
  maxPayload: 8  1024  1024
});

Set it to null to fall back to the wsEngine default.

Since version 21.0.0, the maxInboundBatchSize option limits how many packets a single batched (array) message from a client may contain; it defaults to 1000. A message which exceeds the limit is rejected and the socket is closed with status code 1009.

let agServer = socketClusterServer.attach(httpServer, {
  maxInboundBatchSize: 100
});

Set it to null to allow batches of any size.

The socketChannelLimit option limits how many channels a single socket may be subscribed to at once; since version 21.0.3 it defaults to 1000. A #subscribe request which would exceed the limit is rejected; the socket keeps its existing subscriptions and stays connected.

let agServer = socketClusterServer.attach(httpServer, {
  socketChannelLimit: 100
});

Set it to null to allow any number of subscriptions per socket. Note that each subscription costs memory on both the socket and the broker, so an unlimited setting lets a single client exhaust server memory; prefer a finite limit which suits your application.

The inboundBackpressureLimit option limits how many unprocessed inbound messages a single socket may accumulate; since version 21.0.3 it defaults to 1000. Inbound messages are processed one at a time (for example while asynchronous inbound middleware is applied), so a client which sends messages faster than the server can process them causes the backlog to grow. When the backlog exceeds the limit, the socket is closed with status code 1009.

let agServer = socketClusterServer.attach(httpServer, {
  inboundBackpressureLimit: 100
});

Set it to null to allow an unbounded backlog. Since each queued message is held in memory, an unlimited setting lets a single client exhaust server memory; prefer a finite limit which suits your application. The current backlog for a socket can be inspected at runtime with socket.getInboundBackpressure().

• Clone this repo: git clone [email protected]:SocketCluster/socketcluster-server.git
• Navigate to project directory: cd socketcluster-server
• Install all dependencies: npm install
• Run the tests: npm test

Benefits of async Iterable over EventEmitter

• More readable: Code is written sequentially from top to bottom. It avoids event handler callback hell. It's also much easier to write and read complex integration test scenarios.
• More succinct: Event streams can be easily chained, filtered and combined using a declarative syntax (e.g. using async generators).
• More manageable: No need to remember to unbind listeners with removeListener(...); just break out of the for-await-of loop to stop consuming. This also encourages a more declarative style of coding which reduces the likelihood of memory leaks and unintended side effects.
• Less error-prone: Each event/RPC/message can be processed sequentially in the same order that they were sent without missing any data; even if asynchronous calls are made inside middleware or listeners. On the other hand, with EventEmitter`, the listener function for the same event cannot be prevented from running multiple times in parallel; also, asynchronous calls within middleware and listeners can affect the final order of actions; all this can cause unintended side effects.

License

(The MIT License)

Copyright (c) 2013-2026 SocketCluster.io

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.