Add a presence feature flag

This change adds a presence feature flag, so that consumers have to
actively opt in to use presence. This means that servers that don't want
to worry about potential fan-out issues don't need to worry about it.

Note that the feature flag will disable presence subscription and also
disable presence broadcasting, but notably _does not_ disable
unsubscribing from presence, so that clients may unsubscribe if the flag
is toggled while the server is running.

Author: Alec Gibson

README.md

@@ -95,6 +95,8 @@ __Options__
   through this pub/sub adapter. Defaults to `ShareDB.MemoryPubSub()`.
 * `options.milestoneDb` _(instance of ShareDB.MilestoneDB`)_
   Store snapshots of documents at a specified interval of versions
+* `options.presence` _boolean_
+  Enable presence functionality. Off by default. Note that this feature is not optimized for large numbers of clients and could cause fan-out issues
 
 #### Database Adapters
 * `ShareDB.MemoryDB`, backed by a non-persistent database with no queries

examples/rich-text-presence/server.js

@@ -6,7 +6,7 @@ var WebSocket = require('ws');
 var WebSocketJSONStream = require('@teamwork/websocket-json-stream');
 
 ShareDB.types.register(richText.type);
-var backend = new ShareDB();
+var backend = new ShareDB({presence: true});
 createDoc(startServer);
 
 // Create initial document then fire callback

lib/agent.js

@@ -420,6 +420,7 @@ Agent.prototype._handleMessage = function(request, callback) {
       case 'nt':
         return this._fetchSnapshotByTimestamp(request.c, request.d, request.ts, callback);
       case 'p':
+        if (!this.backend.presenceEnabled) return;
         var presence = this._createPresence(request);
         if (presence.t && !util.supportsPresence(types.map[presence.t])) {
           return callback({
@@ -429,6 +430,7 @@ Agent.prototype._handleMessage = function(request, callback) {
         }
         return this._broadcastPresence(presence, callback);
       case 'ps':
+        if (!this.backend.presenceEnabled) return;
         return this._subscribePresence(request.ch, request.seq, callback);
       case 'pu':
         return this._unsubscribePresence(request.ch, request.seq, callback);

lib/backend.js

@@ -32,6 +32,7 @@ function Backend(options) {
 
   this.suppressPublish = !!options.suppressPublish;
   this.maxSubmitRetries = options.maxSubmitRetries || null;
+  this.presenceEnabled = !!options.presence;
 
   // Map from event name to a list of middleware
   this.middleware = {};

test/client/presence/doc-presence.js

@@ -18,7 +18,7 @@ describe('DocPresence', function() {
   var presencePauser;
 
   beforeEach(function(done) {
-    backend = new Backend();
+    backend = new Backend({presence: true});
     connection1 = backend.connect();
     connection2 = backend.connect();
 

test/client/presence/presence.js

@@ -14,7 +14,7 @@ describe('Presence', function() {
   var presencePauser;
 
   beforeEach(function(done) {
-    backend = new Backend();
+    backend = new Backend({presence: true});
     var connectedCount = 0;
     connection1 = backend.connect();
     connection2 = backend.connect();