feat(cluster): add the option for a custom node selector in scaleReads

#250

README.md

@@ -671,10 +671,11 @@ but a few so that if one is unreachable the client will try the next one, and th
 
 A typical redis cluster contains three or more masters and several slaves for each master. It's possible to scale out redis cluster by sending read queries to slaves and write queries to masters by setting the `scaleReads` option.
 
-`scaleReads` is "master" by default, which means ioredis will never send any queries to slaves. There are other two available options:
+`scaleReads` is "master" by default, which means ioredis will never send any queries to slaves. There are other three available options:
 
 1. "all": Send write queries to masters and read queries to masters or slaves randomly.
 2. "slave": Send write queries to masters and read queries to slaves.
+3. a custom `function(nodes, command): node`: Will choose the custom function to select to which node to send read queries (write queries keep being sent to master). The first node in `nodes` is always the master serving the relevant slots. If the function returns an array of nodes, a random node of that list will be selected.
 
 For example:
 

lib/cluster/index.js

@@ -40,9 +40,10 @@ function Cluster(startupNodes, options) {
   this.options = _.defaults(this.options, options, Cluster.defaultOptions);
 
   // validate options
-  if (['all', 'master', 'slave'].indexOf(this.options.scaleReads) === -1) {
+  if (typeof this.options.scaleReads !== 'function' &&
+      ['all', 'master', 'slave'].indexOf(this.options.scaleReads) === -1) {
     throw new Error('Invalid option scaleReads "' + this.options.scaleReads +
-      '". Expected "all", "master" or "slave"');
+      '". Expected "all", "master", "slave" or a custom function');
   }
 
   if (!Array.isArray(startupNodes) || startupNodes.length === 0) {
@@ -453,15 +454,30 @@ Cluster.prototype.sendCommand = function (command, stream, node) {
         if (!random) {
           if (typeof targetSlot === 'number' && _this.slots[targetSlot]) {
             var nodeKeys = _this.slots[targetSlot];
-            var key;
-            if (to === 'all') {
-              key = utils.sample(nodeKeys);
-            } else if (to === 'slave' && nodeKeys.length > 1) {
-              key = utils.sample(nodeKeys, 1);
+            if (typeof to === 'function') {
+              var nodes =
+                  nodeKeys
+                  .map(function(key) {
+                    return _this.connectionPool.nodes.all[key];
+                  });
+              redis = to(nodes, command);
+              if (Array.isArray(redis)) {
+                redis = utils.sample(redis);
+              }
+              if (!redis) {
+                redis = nodes[0];
+              }
             } else {
-              key = nodeKeys[0];
+              var key;
+              if (to === 'all') {
+                key = utils.sample(nodeKeys);
+              } else if (to === 'slave' && nodeKeys.length > 1) {
+                key = utils.sample(nodeKeys, 1);
+              } else {
+                key = nodeKeys[0];
+              }
+              redis = _this.connectionPool.nodes.all[key];
             }
-            redis = _this.connectionPool.nodes.all[key];
           }
           if (asking) {
             redis = _this.connectionPool.nodes.all[asking];

test/functional/cluster.js

@@ -940,7 +940,7 @@ describe('cluster', function () {
       function handler(port, argv) {
         if (argv[0] === 'cluster' && argv[1] === 'slots') {
           return [
-            [0, 16381, ['127.0.0.1', 30001], ['127.0.0.1', 30003]],
+            [0, 16381, ['127.0.0.1', 30001], ['127.0.0.1', 30003], ['127.0.0.1', 30004]],
             [16382, 16383, ['127.0.0.1', 30002]]
           ];
         }
@@ -949,10 +949,11 @@ describe('cluster', function () {
       this.node1 = new MockServer(30001, handler.bind(null, 30001));
       this.node2 = new MockServer(30002, handler.bind(null, 30002));
       this.node3 = new MockServer(30003, handler.bind(null, 30003));
+      this.node4 = new MockServer(30004, handler.bind(null, 30004));
     });
 
     afterEach(function (done) {
-      disconnect([this.node1, this.node2, this.node3], done);
+      disconnect([this.node1, this.node2, this.node3, this.node4], done);
     });
 
     context('master', function () {
@@ -977,7 +978,7 @@ describe('cluster', function () {
         });
         cluster.on('ready', function () {
           stub(utils, 'sample', function (array, from) {
-            expect(array).to.eql(['127.0.0.1:30001', '127.0.0.1:30003']);
+            expect(array).to.eql(['127.0.0.1:30001', '127.0.0.1:30003', '127.0.0.1:30004']);
             expect(from).to.eql(1);
             return '127.0.0.1:30003';
           });
@@ -1006,14 +1007,62 @@ describe('cluster', function () {
       });
     });
 
+    context('custom', function () {
+      it('should send to selected slave', function (done) {
+        var cluster = new Redis.Cluster([{ host: '127.0.0.1', port: '30001' }], {
+          scaleReads: function(node, command) {
+            if (command.name === 'get') {
+              return node[1];
+            } else {
+              return node[2];
+            }
+          }
+        });
+        cluster.on('ready', function () {
+          stub(utils, 'sample', function (array, from) {
+            expect(array).to.eql(['127.0.0.1:30001', '127.0.0.1:30003', '127.0.0.1:30004']);
+            expect(from).to.eql(1);
+            return '127.0.0.1:30003';
+          });
+          cluster.hgetall('foo', function (err, res) {
+            utils.sample.restore();
+            expect(res).to.eql(30004);
+            cluster.disconnect();
+            done();
+          });
+        });
+      });
+
+      it('should send writes to masters', function (done) {
+        var cluster = new Redis.Cluster([{ host: '127.0.0.1', port: '30001' }], {
+          scaleReads: function(node, command) {
+            if (command.name === 'get') {
+              return node[1];
+            } else {
+              return node[2];
+            }
+          }
+        });
+        cluster.on('ready', function () {
+          stub(utils, 'sample').throws('sample is called');
+          cluster.set('foo', 'bar', function (err, res) {
+            utils.sample.restore();
+            expect(res).to.eql(30001);
+            cluster.disconnect();
+            done();
+          });
+        });
+      });
+    });
+
     context('all', function () {
       it('should send reads to all nodes randomly', function (done) {
         var cluster = new Redis.Cluster([{ host: '127.0.0.1', port: '30001' }], {
           scaleReads: 'all'
         });
         cluster.on('ready', function () {
           stub(utils, 'sample', function (array, from) {
-            expect(array).to.eql(['127.0.0.1:30001', '127.0.0.1:30003']);
+            expect(array).to.eql(['127.0.0.1:30001', '127.0.0.1:30003', '127.0.0.1:30004']);
             expect(from).to.eql(undefined);
             return '127.0.0.1:30003';
           });