Adds load balancing to connection pool

lib/connection/pool.js

@@ -141,8 +141,6 @@ var Pool = function(topology, options) {
 
   // Number of consecutive timeouts caught
   this.numberOfConsecutiveTimeouts = 0;
-  // Current pool Index
-  this.connectionIndex = 0;
 
   // event handlers
   const pool = this;
@@ -1069,15 +1067,30 @@ function _execute(self) {
         break;
       }
 
-      var connection = null;
-      const connections = self.availableConnections.filter(conn => conn.workItems.length === 0);
+      // Load balance our pool connections, sending next workItem to connection with least number of already-pending workItems
+      var tmpConn = null;
+      var workItemsLen = 0;
+      var min = Number.POSITIVE_INFINITY;
+      var connections = [];
+
+      for (var i = 0; i < self.availableConnections.length; i++) {
+        tmpConn = self.availableConnections[i];
+        workItemsLen = tmpConn.workItems.length;
+
+        if (workItemsLen < min) {
+          min = workItemsLen;
+          connections = [tmpConn];
+        } else if (workItemsLen === min) {
+          connections.push(tmpConn);
+        }
+      }
 
-      // No connection found that has no work on it, just pick one for pipelining
-      if (connections.length === 0) {
-        connection =
-          self.availableConnections[self.connectionIndex++ % self.availableConnections.length];
+      var connection = null;
+      if (connections.length === 1) {
+        connection = connections[0];
       } else {
-        connection = connections[self.connectionIndex++ % connections.length];
+        // We have multiple connections with an equal low-number of workItems, pick a random one
+        connection = connections[Math.floor(Math.random() * connections.length)];
       }
 
       // Is the connection connected