fix: used weighted RTT calculation for server selection

Author: mbroadst

lib/core/sdam/server.js

@@ -142,7 +142,7 @@ class Server extends EventEmitter {
       this.emit(
         'descriptionReceived',
         new ServerDescription(this.description.address, null, {
-          roundTripTime: event.duration,
+          roundTripTime: calculateRoundTripTime(this.description.roundTripTime, event.duration),
           error: event.failure
         })
       );
@@ -157,7 +157,7 @@ class Server extends EventEmitter {
       this.emit(
         'descriptionReceived',
         new ServerDescription(this.description.address, event.reply, {
-          roundTripTime: event.duration
+          roundTripTime: calculateRoundTripTime(this.description.roundTripTime, event.duration)
         })
       );
 
@@ -450,6 +450,11 @@ Object.defineProperty(Server.prototype, 'clusterTime', {
   }
 });
 
+function calculateRoundTripTime(oldRtt, duration) {
+  const alpha = 0.2;
+  return alpha * duration + (1 - alpha) * oldRtt;
+}
+
 function basicReadValidations(server, options) {
   if (options.readPreference && !(options.readPreference instanceof ReadPreference)) {
     return new MongoError('readPreference must be an instance of ReadPreference');

lib/core/sdam/server_description.js

@@ -69,7 +69,7 @@ class ServerDescription {
 
     this.address = address;
     this.error = options.error || null;
-    this.roundTripTime = options.roundTripTime || 0;
+    this.roundTripTime = options.roundTripTime || -1;
     this.lastUpdateTime = Date.now();
     this.lastWriteDate = ismaster.lastWrite ? ismaster.lastWrite.lastWriteDate : null;
     this.opTime = ismaster.lastWrite ? ismaster.lastWrite.opTime : null;