feat(pool): Minimum connections, auto open connections, and close idle connections

This adds a new configuration for desired minimum connections, to complement the connection limit.

If the autoOpenConnections is enabled (default true), the pool will immediately open the desired
minimum number of connections.

Any connection opened above the minimum, who sits unused for a time specified in idleTimeout, will
be automatically closed.

This allows you to set up a minimum expectation of standard load for connections to your database,
but also allow you to configure a higher maximum to handle unexpected burst traffic, and be able to
close them automatically when the burst subsides.

Author: aikar

lib/connection.js

@@ -946,8 +946,8 @@ Connection.prototype.serverHandshake = function serverHandshake(args) {
 Connection.prototype.end = function(callback) {
   var connection = this;
 
+  connection._closing = true;
   if (this.config.isServer) {
-    connection._closing = true;
     var quitCmd = new EventEmitter();
     setImmediate(function() {
       connection.stream.end();

lib/pool.js

@@ -1,6 +1,7 @@
 var mysql = require('../index.js');
 
 var EventEmitter = require('events').EventEmitter;
+var Timers = require('timers');
 var Util = require('util');
 var PoolConnection = require('./pool_connection.js');
 var Queue = require('denque');
@@ -18,6 +19,14 @@ function Pool(options) {
   this._freeConnections = new Queue();
   this._connectionQueue = new Queue();
   this._closed = false;
+  if (this.config.autoOpenConnections) {
+    var self = this;
+    for (var i = 0; i < this.config.minConnections; i++) {
+      this.getConnection(function (conn) {
+        self.releaseConnection(conn);
+      })
+    }
+  }
 }
 
 Pool.prototype.getConnection = function(cb) {
@@ -82,6 +91,7 @@ Pool.prototype.getConnection = function(cb) {
 Pool.prototype.releaseConnection = function(connection) {
   var cb;
 
+  connection._lastReleased = Date.now();
   if (!connection._pool) {
     // The connection has been removed from the pool and is no longer good.
     if (this._connectionQueue.length) {
@@ -95,6 +105,7 @@ Pool.prototype.releaseConnection = function(connection) {
     process.nextTick(cb.bind(null, null, connection));
   } else {
     this._freeConnections.push(connection);
+    this._manageExpiredTimer();
   }
 };
 
@@ -111,14 +122,14 @@ Pool.prototype.end = function(cb) {
 
   var calledBack = false;
   var closedConnections = 0;
-  var connection;
+  var numConnections = this._allConnections.length;
 
   var endCB = function(err) {
     if (calledBack) {
       return;
     }
 
-    if (err || ++closedConnections >= this._allConnections.length) {
+    if (err || ++closedConnections >= numConnections) {
       calledBack = true;
       cb(err);
       return;
@@ -130,9 +141,9 @@ Pool.prototype.end = function(cb) {
     return;
   }
 
-  for (var i = 0; i < this._allConnections.length; i++) {
-    connection = this._allConnections.get(i);
-    connection._realEnd(endCB);
+  var connection;
+  while ((connection = this._allConnections.shift())) {
+    this._closeConnection(connection, endCB);
   }
 };
 
@@ -184,14 +195,58 @@ Pool.prototype.execute = function(sql, values, cb) {
   });
 };
 
+Pool.prototype._manageExpiredTimer = function() {
+  var hasExtra = this._allConnections.length > this.config.minConnections;
+  if (hasExtra && !this._expiredTimer) {
+    this._expiredTimer = Timers.setInterval(
+      Pool.prototype._closeIdleConnections.bind(this),
+      15000
+    );
+  } else if (!hasExtra && this._expiredTimer) {
+    Timers.clearInterval(this._expiredTimer);
+    this._expiredTimer = null;
+  }
+};
+
+Pool.prototype._closeIdleConnections = function() {
+  var now = Date.now();
+  var timeout = this.config.idleTimeout * 1000;
+  var length = this._freeConnections.length;
+  var numExtra = this._allConnections.length - this.config.minConnections;
+  for (var i = 0; i < length; i++) {
+    if (numExtra <= 0) {
+      break;
+    }
+    var conn = this._freeConnections.get(i);
+
+    if (now > conn._lastReleased + timeout) {
+      // This connection has been unused for longer than the timeout
+      this._closeConnection(conn);
+      // decrement i and length as the length will be reduced by 1 by closeConnection
+      i--;
+      length--;
+    }
+  }
+  this._manageExpiredTimer();
+};
+
 Pool.prototype._removeConnection = function(connection) {
   // Remove connection from all connections
   spliceConnection(this._allConnections, connection);
 
   // Remove connection from free connections
   spliceConnection(this._freeConnections, connection);
 
-  this.releaseConnection(connection);
+  if (!connection._closing) {
+    this.releaseConnection(connection);
+  }
+
+  this._manageExpiredTimer();
+};
+
+Pool.prototype._closeConnection = function (connection, cb) {
+  connection._realEnd(cb);
+  this._removeConnection(connection);
 };
 
 Pool.prototype.format = function(sql, values) {

lib/pool_config.js

@@ -6,13 +6,23 @@ function PoolConfig(options) {
     options = ConnectionConfig.parseUrl(options);
   }
   this.connectionConfig = new ConnectionConfig(options);
-  this.waitForConnections = options.waitForConnections === undefined
+  this.waitForConnections = options.waitForConnections == null
     ? true
     : Boolean(options.waitForConnections);
-  this.connectionLimit = options.connectionLimit === undefined
+  this.connectionLimit = options.connectionLimit == null
     ? 10
-    : Number(options.connectionLimit);
-  this.queueLimit = options.queueLimit === undefined
+    : Number(options.connectionLimit) || 10;
+  this.queueLimit = options.queueLimit == null
     ? 0
-    : Number(options.queueLimit);
+    : Number(options.queueLimit) || 0;
+  this.minConnections = Math.min(
+    this.connectionLimit,
+    Number(options.minConnections) || 0
+  );
+  this.autoOpenConnections = options.autoOpenConnections == null
+    ? true
+    : Boolean(options.autoOpenConnections);
+  this.idleTimeout = options.idleTimeout == null
+    ? 300
+    : Number(options.idleTimeout) || 300;
 }