fix: reduce default keepalive time to align with Azure defaults

The default TCP idle timeout on the Azure load balancer is 240
seconds, meaning it could silently drop connections if they begin
experiencing higher latency. In order to avoid dropped connections
we reduce our TCP keepalive initial delay to 120s, half the max
time for the load balancer.

NODE-2627

lib/cmap/connect.js

@@ -233,7 +233,7 @@ function makeConnection(family, options, cancellationToken, _callback) {
   const useSsl = typeof options.ssl === 'boolean' ? options.ssl : false;
   const keepAlive = typeof options.keepAlive === 'boolean' ? options.keepAlive : true;
   let keepAliveInitialDelay =
-    typeof options.keepAliveInitialDelay === 'number' ? options.keepAliveInitialDelay : 300000;
+    typeof options.keepAliveInitialDelay === 'number' ? options.keepAliveInitialDelay : 120000;
   const noDelay = typeof options.noDelay === 'boolean' ? options.noDelay : true;
   const connectionTimeout =
     typeof options.connectionTimeout === 'number'

lib/mongo_client.js

@@ -94,7 +94,7 @@ const PromiseProvider = require('./promise_provider');
  * @param {boolean} [options.autoReconnect=true] Enable autoReconnect for single server instances
  * @param {boolean} [options.noDelay=true] TCP Connection no delay
  * @param {boolean} [options.keepAlive=true] TCP Connection keep alive enabled
- * @param {number} [options.keepAliveInitialDelay=30000] The number of milliseconds to wait before initiating keepAlive on the TCP socket
+ * @param {number} [options.keepAliveInitialDelay=120000] The number of milliseconds to wait before initiating keepAlive on the TCP socket
  * @param {number} [options.connectTimeoutMS=10000] How long to wait for a connection to be established before timing out
  * @param {number} [options.socketTimeoutMS=360000] How long a send or receive on a socket can take before timing out
  * @param {number} [options.family] Version of IP stack. Can be 4, 6 or null (default).
@@ -345,7 +345,7 @@ MongoClient.prototype.isConnected = function(options) {
  * @param {boolean} [options.autoReconnect=true] Enable autoReconnect for single server instances
  * @param {boolean} [options.noDelay=true] TCP Connection no delay
  * @param {boolean} [options.keepAlive=true] TCP Connection keep alive enabled
- * @param {boolean} [options.keepAliveInitialDelay=30000] The number of milliseconds to wait before initiating keepAlive on the TCP socket
+ * @param {number} [options.keepAliveInitialDelay=120000] The number of milliseconds to wait before initiating keepAlive on the TCP socket
  * @param {number} [options.connectTimeoutMS=10000] How long to wait for a connection to be established before timing out
  * @param {number} [options.socketTimeoutMS=360000] How long a send or receive on a socket can take before timing out
  * @param {number} [options.family] Version of IP stack. Can be 4, 6 or null (default).