refactor(sessions): Performance Improvement

Changed the sessions structure to an object instead of an array
so when we close a session we could remove it from the data structure more efficiently

Author: Tom Leibo

lib/mongo_client.js

@@ -157,7 +157,7 @@ function MongoClient(url, options) {
     options: options || {},
     promiseLibrary: null,
     dbCache: new Map(),
-    sessions: [],
+    sessions: new Set(),
     writeConcern: WriteConcern.fromOptions(options),
     namespace: new MongoDBNamespace('admin')
   };

lib/topologies/mongos.js

@@ -194,7 +194,7 @@ class Mongos extends TopologyBase {
       // Server Session Pool
       sessionPool: null,
       // Active client sessions
-      sessions: [],
+      sessions: new Set(),
       // Promise library
       promiseLibrary: options.promiseLibrary || Promise
     };

lib/topologies/replset.js

@@ -210,7 +210,7 @@ class ReplSet extends TopologyBase {
       // Server Session Pool
       sessionPool: null,
       // Active client sessions
-      sessions: [],
+      sessions: new Set(),
       // Promise library
       promiseLibrary: options.promiseLibrary || Promise
     };

lib/topologies/server.js

@@ -202,7 +202,7 @@ class Server extends TopologyBase {
       // Server Session Pool
       sessionPool: null,
       // Active client sessions
-      sessions: [],
+      sessions: new Set(),
       // Promise library
       promiseLibrary: promiseLibrary || Promise
     };

lib/topologies/topology_base.js

@@ -291,11 +291,12 @@ class TopologyBase extends EventEmitter {
 
   startSession(options, clientOptions) {
     const session = new ClientSession(this, this.s.sessionPool, options, clientOptions);
+
     session.once('ended', () => {
-      this.s.sessions = this.s.sessions.filter(s => !s.equals(session));
+      this.s.sessions.delete(session);
     });
 
-    this.s.sessions.push(session);
+    this.s.sessions.add(session);
     return session;
   }
 
@@ -382,9 +383,7 @@ class TopologyBase extends EventEmitter {
   close(forceClosed, callback) {
     // If we have sessions, we want to individually move them to the session pool,
     // and then send a single endSessions call.
-    if (this.s.sessions.length) {
-      this.s.sessions.forEach(session => session.endSession());
-    }
+    this.s.sessions.forEach(session => session.endSession());
 
     if (this.s.sessionPool) {
       this.s.sessionPool.endAllPooledSessions();

test/functional/cursor_tests.js

@@ -4369,7 +4369,7 @@ describe('Cursor', function() {
             const cursor = collection.find({});
 
             cursor.next(function() {
-              test.equal(client.topology.s.sessions.length, 0);
+              test.equal(client.topology.s.sessions.size, 0);
               client.close();
               done();
             });
@@ -4410,13 +4410,13 @@ describe('Cursor', function() {
             test.equal(null, err);
             const cursor = collection.find({}, { batchSize: 3 });
             cursor.next(function() {
-              test.equal(client.topology.s.sessions.length, 1);
+              test.equal(client.topology.s.sessions.size, 1);
               cursor.next(function() {
-                test.equal(client.topology.s.sessions.length, 1);
+                test.equal(client.topology.s.sessions.size, 1);
                 cursor.next(function() {
-                  test.equal(client.topology.s.sessions.length, 1);
+                  test.equal(client.topology.s.sessions.size, 1);
                   cursor.next(function() {
-                    test.equal(client.topology.s.sessions.length, 0);
+                    test.equal(client.topology.s.sessions.size, 0);
                     client.close();
                     done();
                   });

test/functional/sessions_tests.js

@@ -50,7 +50,7 @@ describe('Sessions', function() {
         expect(test.commands.started[0].commandName).to.equal('endSessions');
         expect(test.commands.started[0].command.endSessions).to.include.deep.members(sessions);
 
-        expect(client.s.sessions).to.have.length(0);
+        test.equal(client.s.sessions.size, 0);
         done();
       });
     }