sc-stateless-presence

Plugin for doing stateless presence in SC.

This plugin lets you do real-time channel presence at scale without having to use an external memory store or database. It's ideal for tracking the presence of users within channels that have fewer than 100 concurrent subscribers online at any given time.

Requires SocketCluster version 6.0.1 or higher.

You may want to read the SocketCluster authentication guide before using this plugin: https://socketcluster.io/#!/docs/authentication

Installing

npm install sc-stateless-presence

On the server

On the server side, in worker.js:

const scStatelessPresence = require('sc-stateless-presence');

// Pass the SCWorker instance as argument.
// The options object is optional.
//
// Valid properties of the options object include:
// presenceInterval: Number of milliseconds interval to ping out user presence.
//                   Defaults to 10000.
// presenceTimeout: Number of milliseconds without a ping which signifies that
//                  a user has gone offline. Defaults to presenceInterval * 1.3.

scStatelessPresence.attach(worker, options);

Note that sc-stateless-presence tracks users based on a username property inside JWT tokens attached to sockets.

To create a JWT token from SC on the server side, you will need to do something like this:

socket.setAuthToken({username: 'alice123'});

It doesn't matter how the JWT token is created though (e.g. you can create it over HTTP before you connect the socket), so long as:

• SC can verify the JWT with its own authKey (see the authKey option here: https://socketcluster.io/#!/docs/api-socketcluster).
• The JWT token contains a JSON object which has a username property (make sure that the letter case matches exactly and that the value is a string which represents the user's username).

For details about creating the JWT with HTTP before connecting the real-time socket in SocketCluster, see this discussion: SocketCluster/socketcluster#233 (comment)

On the client

On the client side, you need to require client.js (if using webpack or Browserify) or include the standalone script sc-stateless-presence-client.js on your front end, then:

Initialize

// Pass an existing socketcluster-client socket as argument.
// The options object is optional.
//
// Valid properties of the options object include:
// presenceCheckInterval: This is an interval (in milliseconds)
//                        that the client uses to evaluate the presence
//                        status of users. Defaults to 1000.
//                        Using a larger value means that the client
//                        will take more time to detect when a user has
//                        gone offline because of a connection failure.

let presence = scStatelessPresenceClient.create(socket, options);

Track user presence changes

To track user presence on a channel on the client side:

// The only argument is the channel name to track.
// The object returned by the method is a Stream that can be consumed in a for-await loop.
for await (let action of presence.trackPresence('sample')) {
  // The action argument can be in the form:
  // { action: 'join', username: 'bob123' } or { action: 'leave', username: 'alice456' }
  console.log('PRESENCE:', action);
};

Note that to get the presence status in real-time for a given channel, you need to make sure that all clients which are subscribed to that channel are also tracking it; otherwise there may be a delay based on the presenceInterval option provided on the server side.

Stop tracking user presence changes

To untrack a channel or remove a tracking listener, you can use:

// The only argument is the channel name to stop tracking.
// All loops consuming the channel stream will break

presence.untrackPresence('sample');

Get the user presence list for a channel

// Returns an array of usernames that are subscribed to the sample channel.

presence.getPresenceList('sample');

Get the listeners which are tracking a channel

// The presence.channelListeners property is an object which maps a channel name to
// an array of listener functions which are currently tracking the channel.

presence.channelListeners['sample']

Contributing

To build the standalone client:

browserify -s scStatelessPresenceClient client.js > sc-stateless-presence-client.js