feat(cluster): support scaling reads to slaves

The new option scaleReads is used to specify where to send the reads.

Add two new events:

    1. "+node": a new node is discovered.
    2. "-node": a node is disconnected.

BREAKING CHANGE:
    1. Cluster#masterNodes and Cluster#nodes is removed. Use Cluster#nodes('masters') and Cluster#nodes('all') instead.
    2. Cluster#to() is removed. Use
Promise.all(Cluster#nodes().map(function (node) {})) instead.

Closes #170.

Author: luin

API.md

@@ -2,8 +2,6 @@
 <dl>
 <dt><a href="#Redis">Redis</a> ⇐ <code>[EventEmitter](http://nodejs.org/api/events.html#events_class_events_eventemitter)</code></dt>
 <dd></dd>
-<dt><a href="#Cluster">Cluster</a> ⇐ <code>[EventEmitter](http://nodejs.org/api/events.html#events_class_events_eventemitter)</code></dt>
-<dd></dd>
 <dt><a href="#Commander">Commander</a></dt>
 <dd></dd>
 </dl>
@@ -12,9 +10,6 @@
 <dt><a href="#defaultOptions">defaultOptions</a></dt>
 <dd><p>Default options</p>
 </dd>
-<dt><a href="#defaultOptions">defaultOptions</a></dt>
-<dd><p>Default options</p>
-</dd>
 </dl>
 <a name="Redis"></a>
 ## Redis ⇐ <code>[EventEmitter](http://nodejs.org/api/events.html#events_class_events_eventemitter)</code>
@@ -190,81 +185,6 @@ Define a custom command using lua script
 Create a Redis instance
 
 **Kind**: static method of <code>[Redis](#Redis)</code>  
-<a name="Cluster"></a>
-## Cluster ⇐ <code>[EventEmitter](http://nodejs.org/api/events.html#events_class_events_eventemitter)</code>
-**Kind**: global class  
-**Extends:** <code>[EventEmitter](http://nodejs.org/api/events.html#events_class_events_eventemitter)</code>, <code>[Commander](#Commander)</code>  
-
-* [Cluster](#Cluster) ⇐ <code>[EventEmitter](http://nodejs.org/api/events.html#events_class_events_eventemitter)</code>
-  * [new Cluster(startupNodes, options)](#new_Cluster_new)
-  * [.disconnect()](#Cluster+disconnect)
-  * [.getBuiltinCommands()](#Commander+getBuiltinCommands) ⇒ <code>Array.&lt;string&gt;</code>
-  * [.createBuiltinCommand(commandName)](#Commander+createBuiltinCommand) ⇒ <code>object</code>
-  * [.defineCommand(name, definition)](#Commander+defineCommand)
-  * *[.sendCommand()](#Commander+sendCommand)*
-
-<a name="new_Cluster_new"></a>
-### new Cluster(startupNodes, options)
-Creates a Redis Cluster instance
-
-
-| Param | Type | Default | Description |
-| --- | --- | --- | --- |
-| startupNodes | <code>Array.&lt;Object&gt;</code> |  | An array of nodes in the cluster, [{ port: number, host: string }] |
-| options | <code>Object</code> |  |  |
-| [options.enableOfflineQueue] | <code>boolean</code> | <code>true</code> | See Redis class |
-| [options.lazyConnect] | <code>boolean</code> | <code>false</code> | See Redis class |
-| [options.readOnly] | <code>boolean</code> | <code>false</code> | Connect in READONLY mode |
-| [options.maxRedirections] | <code>number</code> | <code>16</code> | When a MOVED or ASK error is received, client will redirect the command to another node. This option limits the max redirections allowed to send a command. |
-| [options.clusterRetryStrategy] | <code>function</code> |  | See "Quick Start" section |
-| [options.retryDelayOnFailover] | <code>number</code> | <code>2000</code> | When an error is received when sending a command(e.g. "Connection is closed." when the target Redis node is down), |
-| [options.retryDelayOnClusterDown] | <code>number</code> | <code>1000</code> | When a CLUSTERDOWN error is received, client will retry if `retryDelayOnClusterDown` is valid delay time. |
-
-<a name="Cluster+disconnect"></a>
-### cluster.disconnect()
-Disconnect from every node in the cluster.
-
-**Kind**: instance method of <code>[Cluster](#Cluster)</code>  
-**Access:** public  
-<a name="Commander+getBuiltinCommands"></a>
-### cluster.getBuiltinCommands() ⇒ <code>Array.&lt;string&gt;</code>
-Return supported builtin commands
-
-**Kind**: instance method of <code>[Cluster](#Cluster)</code>  
-**Returns**: <code>Array.&lt;string&gt;</code> - command list  
-**Access:** public  
-<a name="Commander+createBuiltinCommand"></a>
-### cluster.createBuiltinCommand(commandName) ⇒ <code>object</code>
-Create a builtin command
-
-**Kind**: instance method of <code>[Cluster](#Cluster)</code>  
-**Returns**: <code>object</code> - functions  
-**Access:** public  
-
-| Param | Type | Description |
-| --- | --- | --- |
-| commandName | <code>string</code> | command name |
-
-<a name="Commander+defineCommand"></a>
-### cluster.defineCommand(name, definition)
-Define a custom command using lua script
-
-**Kind**: instance method of <code>[Cluster](#Cluster)</code>  
-
-| Param | Type | Default | Description |
-| --- | --- | --- | --- |
-| name | <code>string</code> |  | the command name |
-| definition | <code>object</code> |  |  |
-| definition.lua | <code>string</code> |  | the lua code |
-| [definition.numberOfKeys] | <code>number</code> | <code></code> | the number of keys. If omit, you have to pass the number of keys as the first argument every time you invoke the command |
-
-<a name="Commander+sendCommand"></a>
-### *cluster.sendCommand()*
-Send a command
-
-**Kind**: instance abstract method of <code>[Cluster](#Cluster)</code>  
-**Overrides:** <code>[sendCommand](#Commander+sendCommand)</code>  
-**Access:** public  
 <a name="Commander"></a>
 ## Commander
 **Kind**: global class  
@@ -331,9 +251,3 @@ Default options
 
 **Kind**: global variable  
 **Access:** protected  
-<a name="defaultOptions"></a>
-## defaultOptions
-Default options
-
-**Kind**: global variable  
-**Access:** protected  

lib/cluster/connection_pool.js

@@ -0,0 +1,89 @@
+'use strict';
+
+var util = require('util');
+var EventEmitter = require('events').EventEmitter;
+var _ = require('lodash');
+var Redis = require('../redis');
+
+function ConnectionPool(redisOptions) {
+  EventEmitter.call(this);
+  this.redisOptions = redisOptions;
+
+  // this.masters + this.slaves = this.nodes
+  this.nodes = {};
+  this.masters = {};
+  this.slaves = {};
+
+  this.specifiedOptions = {};
+}
+
+util.inherits(ConnectionPool, EventEmitter);
+
+ConnectionPool.prototype.findOrCreate = function (node, readOnly) {
+  node.port = node.port || 6379;
+  node.host = node.host || '127.0.0.1';
+  node.key = node.key || node.host + ':' + node.port;
+
+  if (this.specifiedOptions[node.key]) {
+    _.assign(node, this.specifiedOptions[node.key]);
+  } else {
+    this.specifiedOptions[node.key] = node;
+  }
+
+  if (this.nodes[node.key] && this.nodes[node.key].options.readOnly !== readOnly) {
+    this.remove(node.key);
+  }
+
+  if (!this.nodes[node.key]) {
+    var redis = this.nodes[node.key] = new Redis(_.defaults({
+      retryStrategy: null,
+      enableOfflineQueue: true,
+      readOnly: readOnly
+    }, node, this.redisOptions, { lazyConnect: true }));
+    this[readOnly ? 'slaves' : 'masters'][node.key] = redis;
+
+    var _this = this;
+    redis.once('end', function () {
+      delete _this.nodes[node.key];
+      delete _this.masters[node.key];
+      delete _this.slaves[node.key];
+      _this.emit('-node', redis);
+      if (!Object.keys(_this.nodes).length) {
+        _this.emit('drain');
+      }
+    });
+
+    this.emit('+node', redis);
+  }
+
+  return this.nodes[node.key];
+};
+
+ConnectionPool.prototype.remove = function (key) {
+  if (this.nodes[key]) {
+    this.nodes[key].disconnect();
+    delete this.nodes[key];
+    delete this.masters[key];
+    delete this.slaves[key];
+  }
+};
+
+ConnectionPool.prototype.reset = function (nodes) {
+  var newNodes = {};
+  for (var i = 0; i < nodes.length; i++) {
+    var node = nodes[i];
+    node.key = node.host + ':' + node.port;
+    newNodes[node.key] = node;
+  }
+  var _this = this;
+  Object.keys(this.nodes).forEach(function (key) {
+    if (!newNodes[key]) {
+      _this.remove(key);
+    }
+  });
+  Object.keys(newNodes).forEach(function (key) {
+    _this.findOrCreate(newNodes[key], newNodes[key].readOnly);
+  });
+};
+
+module.exports = ConnectionPool;