feat: revisit of .to(nodeGroup) command

Provides ability to send arbitrary commands too all nodes in a requested group or
just a handy helper to select nodes from such a group

Supports 'all', 'masters' and 'slaves' groups exposing `call` and `callBuffer` command
semantics similar to Commander

README.md

@@ -664,6 +664,43 @@ but a few so that if one is unreachable the client will try the next one, and th
     * `retryDelayOnClusterDown`: When a cluster is down, all commands will be rejected with the error of `CLUSTERDOWN`. If this option is a number (by default, it is 1000), the client
     will resend the commands after the specified time (in ms).
 
+### Running same operation on multiple nodes
+
+Some operations, such as `flushdb` and `flushall`, can only be performed on a single node.
+There is a helper function for this case:
+
+```javascript
+var redis = new redis.Cluster();
+
+// groups are: all, masters and slaves
+// returns object or throws an error on invalid group
+// {
+//   nodes: Array,
+//   call: Function,
+//   callBuffer: function,
+// }
+redis.to('group')
+
+// will be performed on all master nodes
+Promise.map(redis.to('masters').nodes, function(node) {
+  return node.flushdb();
+});
+
+// exactly the same as above
+redis.to('masters').call('flushdb').then(function (results) {
+  //
+})
+
+redis.to('slaves').call('flushdb', function (err, results) {
+
+});
+
+// in case of buffer
+redis.to('slaves').callBuffer('get', 'key').catch(function (err) {
+  // likely rejected, because operations would only succeed partially due to slot | moved error
+});
+```
+
 ### Transaction and pipeline in Cluster mode
 Almost all features that are supported by `Redis` are also supported by `Redis.Cluster`, e.g. custom commands, transaction and pipeline.
 However there are some differences when using transaction and pipeline in Cluster mode:

lib/cluster.js

@@ -399,6 +399,59 @@ Cluster.prototype.executeClusterDownCommands = function () {
   }
 };
 
+Cluster.prototype.to = function (name) {
+  var fnName = '_select' + name[0].toUpperCase() + name.slice(1);
+  var fn = typeof this[fnName];
+  if (typeof fn !== 'function') {
+    // programmatic error, can't happen in prod, so throw
+    throw new Error('to ' + name + ' is not a valid group of nodes');
+  }
+
+  // could be 0 nodes just as well
+  var nodes = fn();
+  return {
+    nodes: nodes,
+    call: this._generateCallNodes(nodes, 'call'),
+    callBuffer: this._generateCallNodes(nodes, 'callBuffer')
+  };
+};
+
+Cluster.prototype._generateCallNodes = function (nodes, op, _opts) {
+  var opts = _opts || {};
+
+  return function callNode() {
+    var argLength = arguments.length;
+    var hasCb = typeof arguments[argLength - 1] === 'function';
+    var args = new Array(argLength);
+    for (var i = 0; i < argLength; ++i) {
+      args[i] = arguments[i];
+    }
+
+    var callback = hasCb ? args.pop() : null;
+    var promise = Promise.map(function (node) {
+      return node[op].apply(node, args);
+    }, opts);
+
+    if (callback) {
+      return promise.nodeify(callback);
+    }
+
+    return promise;
+  };
+};
+
+Cluster.prototype._selectAll = function () {
+  return _.values(this.nodes);
+};
+
+Cluster.prototype._selectMasters = function () {
+  return _.values(this.masterNodes);
+};
+
+Cluster.prototype._selectSlaves = function () {
+  return _.difference(this._selectAll(), this._selectMasters());
+};
+
 Cluster.prototype.sendCommand = function (command, stream, node) {
   if (this.status === 'end') {
     command.reject(new Error('Connection is closed.'));