⚡️ Remove server-side Presence transformations

This change addresses a performance issue with Presence, whereby every
single Presence update will cause a Database call to fetch the latest
ops, and transform Presence up.

This can be quite costly if there are a lot of Presence updates, and
adversely impact the "normal" operation of ShareDB. This is particularly
strange for the "happy case", where two clients are exchanging Presence
on the same version of a document (where there should be no need to
query the database at all for Presence).

We already have mechanisms in the client to transform presence, as well
as re-request stale presence, so we can remove the server-side Presence
transformations all together.

This alleviates pressure on the server, at the cost of decreased
performance in edge cases, where clients need to re-request Presence
from other clients, or catch up on cached ops. However, this should be a
sensible change to make, since we're trying to optimise for the happy
path, rather than edge cases.

Note that this changes a test case: we have to resubscribe `doc1` in
this test case, otherwise it's never up-to-date, and `presence2`
continues to throw out its presence updates. This could happen in a
real-world situation (where `doc1` stays offline), but it's arguable
that this is uninteresting presence anyway, since that client is not
"live".

lib/agent.js

@@ -748,13 +748,10 @@ Agent.prototype._broadcastPresence = function(presence, callback) {
   };
   backend.trigger(backend.MIDDLEWARE_ACTIONS.receivePresence, this, context, function(error) {
     if (error) return callback(error);
-    backend.transformPresenceToLatestVersion(agent, presence, function(error, presence) {
+    var channel = agent._getPresenceChannel(presence.ch);
+    agent.backend.pubsub.publish([channel], presence, function(error) {
       if (error) return callback(error);
-      var channel = agent._getPresenceChannel(presence.ch);
-      agent.backend.pubsub.publish([channel], presence, function(error) {
-        if (error) return callback(error);
-        callback(null, presence);
-      });
+      callback(null, presence);
     });
   });
 };

lib/backend.js

@@ -858,22 +858,6 @@ Backend.prototype._buildSnapshotFromOps = function(id, startingSnapshot, ops, ca
   callback(error, snapshot);
 };
 
-Backend.prototype.transformPresenceToLatestVersion = function(agent, presence, callback) {
-  if (!presence.c || !presence.d) return callback(null, presence);
-  this.getOps(agent, presence.c, presence.d, presence.v, null, function(error, ops) {
-    if (error) return callback(error);
-    for (var i = 0; i < ops.length; i++) {
-      var op = ops[i];
-      var isOwnOp = op.src === presence.src;
-      var transformError = ot.transformPresence(presence, op, isOwnOp);
-      if (transformError) {
-        return callback(transformError);
-      }
-    }
-    callback(null, presence);
-  });
-};
-
 function pluckIds(snapshots) {
   var ids = [];
   for (var i = 0; i < snapshots.length; i++) {

test/client/presence/doc-presence.js

@@ -232,38 +232,28 @@ describe('DocPresence', function() {
     var localPresence1 = presence1.create('presence-1');
 
     async.series([
+      presence1.subscribe.bind(presence1),
       presence2.subscribe.bind(presence2),
       doc1.unsubscribe.bind(doc1),
       doc2.submitOp.bind(doc2, {index: 5, value: 'ern'}),
       function(next) {
         expect(doc1.version).to.eql(1);
         expect(doc2.version).to.eql(2);
-
+        next();
+      },
+      localPresence1.submit.bind(localPresence1, {index: 12}),
+      function(next) {
         localPresence1.submit({index: 12}, errorHandler(done));
-        presence2.once('receive', function(id, presence) {
-          expect(doc2.version).to.eql(2);
-          expect(presence).to.eql({index: 15});
-          next();
-        });
-      }
-    ], done);
-  });
 
-  it('returns errors when failing to transform old presence to the latest doc', function(done) {
-    var localPresence1 = presence1.create('presence-1');
+        var handlePresence = function(id, presence) {
+          if (presence.index !== 15) return;
+          presence2.off('receive', handlePresence);
+          next();
+        };
 
-    async.series([
-      presence2.subscribe.bind(presence2),
-      doc1.unsubscribe.bind(doc1),
-      doc2.submitOp.bind(doc2, {index: 5, value: 'ern'}),
-      function(next) {
-        expect(doc1.version).to.eql(1);
-        expect(doc2.version).to.eql(2);
+        presence2.on('receive', handlePresence);
 
-        localPresence1.submit({badProp: 'foo'}, function(error) {
-          expect(error.code).to.equal('ERR_PRESENCE_TRANSFORM_FAILED');
-          next();
-        });
+        doc1.subscribe(errorHandler(next));
       }
     ], done);
   });

test/ot.js

@@ -316,6 +316,21 @@ describe('ot', function() {
       expect(error.code).to.eql('ERR_OT_OP_BADLY_FORMED');
     });
 
+    it('returns a transformation error if the presence cannot be transformed', function() {
+      var presence = {
+        p: {index: 5},
+        t: presenceType.uri,
+        v: 1
+      };
+
+      var op = {
+        op: {badProp: 'foo'}
+      };
+      var error = ot.transformPresence(presence, op);
+
+      expect(error.code).to.eql('ERR_PRESENCE_TRANSFORM_FAILED');
+    });
+
     it('considers isOwnOp', function() {
       var presence = {
         p: {index: 5},