⚡️ Deduplicate Backend.getOps() calls

Calls to `Backend.getOps()` can be quite expensive. In order to minimize
the impact of these calls on the server, this change deduplicates
concurrent calls with the same arguments, and makes only a single call,
then invoking all the callbacks with the single result.

lib/backend.js

@@ -57,6 +57,11 @@ function Backend(options) {
     function(error, context) {
       logger.error(error);
     };
+
+  var backend = this;
+  this._dbGetOps = util.deduplicateRequests(function() {
+    backend.db.getOps.apply(backend.db, arguments);
+  });
 }
 module.exports = Backend;
 emitter.mixin(Backend);
@@ -324,7 +329,7 @@ Backend.prototype._getSanitizedOps = function(agent, projection, collection, id,
   var backend = this;
   if (!opsOptions) opsOptions = {};
   if (agent) opsOptions.agentCustom = agent.custom;
-  backend.db.getOps(collection, id, from, to, opsOptions, function(err, ops) {
+  this._dbGetOps(collection, id, from, to, opsOptions, function(err, ops) {
     if (err) return callback(err);
     backend._sanitizeOps(agent, projection, collection, id, ops, function(err) {
       if (err) return callback(err);

lib/util.js

@@ -98,6 +98,35 @@ exports.clone = function(obj) {
   return (obj === undefined) ? undefined : JSON.parse(JSON.stringify(obj));
 };
 
+exports.deduplicateRequests = function(fn) {
+  var callbacksByArgs = {};
+  return function() {
+    var callback = arguments[arguments.length - 1];
+    var args = [];
+    for (var i = 0; i < arguments.length - 1; i++) {
+      args.push(arguments[i]);
+    }
+    var argString = JSON.stringify(args);
+
+    var callbacks = exports.digOrCreate(callbacksByArgs, argString, function() {
+      return [];
+    });
+    callbacks.push(callback);
+
+    if (callbacks.length > 1) return;
+
+    args.push(function() {
+      while (callbacks.length) {
+        var cb = callbacks.shift();
+        cb.apply(null, arguments);
+      }
+      delete callbacksByArgs[argString];
+    });
+
+    fn.apply(null, args);
+  };
+};
+
 var objectProtoPropNames = Object.create(null);
 Object.getOwnPropertyNames(Object.prototype).forEach(function(prop) {
   if (prop !== '__proto__') {

test/backend.js

@@ -92,6 +92,23 @@ describe('Backend', function() {
           done();
         });
       });
+
+      it('deduplicates concurrent requests', function(done) {
+        var getOps = sinon.spy(backend.db, 'getOps');
+        var count = 0;
+        var callback = function(error, ops) {
+          if (error) return done(error);
+          expect(ops).to.have.length(2);
+          expect(ops[0].create.data).to.eql({title: '1984'});
+          expect(ops[1].op).to.eql([{p: ['author'], oi: 'George Orwell'}]);
+          count++;
+          expect(getOps).to.have.been.calledOnce;
+          if (count === 2) done();
+        }
+
+        backend.getOps(agent, 'books', '1984', 0, null, callback);
+        backend.getOps(agent, 'books', '1984', 0, null, callback);
+      });
     });
 
     describe('getOpsBulk', function() {