Adds support for Round Robin Replica selection

Author: Unnikrishnan G

osscluster.go

@@ -48,6 +48,9 @@ type ClusterOptions struct {
 	// Allows routing read-only commands to the random master or slave node.
 	// It automatically enables ReadOnly.
 	RouteRandomly bool
+	// Allows routing read-only commands to the replica nodes in ronund-robin.
+	// It automatically enables ReadOnly
+	RouteRoundRobinReplicas bool
 
 	// Optional function that returns cluster slots information.
 	// It is useful to manually create cluster of standalone Redis servers
@@ -584,6 +587,9 @@ func (c *clusterNodes) Random() (*clusterNode, error) {
 type clusterSlot struct {
 	start, end int
 	nodes      []*clusterNode
+
+	// Allows node selection to use round-robin selection strategy.
+	next uint32
 }
 
 type clusterSlotSlice []*clusterSlot
@@ -767,7 +773,44 @@ func (c *clusterState) slotRandomNode(slot int) (*clusterNode, error) {
 	return nodes[randomNodes[0]], nil
 }
 
+// slotRoundRobinReplicaNode tries to select a node from the list of replica nodes.
+// if no replica nodes are available, returns the primary node.
+func (c *clusterState) slotRoundRobinReplicaNode(slot int) (*clusterNode, error) {
+	cs := c.slotCluster(slot)
+	if cs == nil {
+		return c.nodes.Random()
+	}
+
+	switch len(cs.nodes) {
+	case 0:
+		return c.nodes.Random()
+	case 1:
+		return cs.nodes[0], nil
+	case 2:
+		if replica := cs.nodes[1]; !replica.Failing() {
+			return replica, nil
+		}
+		return cs.nodes[0], nil
+	default:
+		var replica *clusterNode
+		for i := 0; i < 10; i++ {
+			next := atomic.AddUint32(&cs.next, 1)
+			n := (int(next))%(len(cs.nodes)-1) + 1
+			replica = cs.nodes[n]
+			if !replica.Failing() {
+				return replica, nil
+			}
+		}
+		// All slaves are loading - use master.
+		return cs.nodes[0], nil
+	}
+}
+
 func (c *clusterState) slotNodes(slot int) []*clusterNode {
+	return c.slotCluster(slot).nodes
+}
+
+func (c *clusterState) slotCluster(slot int) *clusterSlot {
 	i := sort.Search(len(c.slots), func(i int) bool {
 		return c.slots[i].end >= slot
 	})
@@ -776,8 +819,9 @@ func (c *clusterState) slotNodes(slot int) []*clusterNode {
 	}
 	x := c.slots[i]
 	if slot >= x.start && slot <= x.end {
-		return x.nodes
+		return x
 	}
+
 	return nil
 }
 
@@ -1824,6 +1868,9 @@ func (c *ClusterClient) slotReadOnlyNode(state *clusterState, slot int) (*cluste
 	if c.opt.RouteRandomly {
 		return state.slotRandomNode(slot)
 	}
+	if c.opt.RouteRoundRobinReplicas {
+		return state.slotRoundRobinReplicaNode(slot)
+	}
 	return state.slotSlaveNode(slot)
 }
 

osscluster_test.go

@@ -1282,6 +1282,66 @@ var _ = Describe("ClusterClient", func() {
 
 		assertClusterClient()
 	})
+
+	Describe("ClusterClient with RouteRoundRobinReplicas and ClusterSlots with multiple nodes per slot", func() {
+		BeforeEach(func() {
+			failover = true
+
+			opt = redisClusterOptions()
+			opt.RouteRoundRobinReplicas = true
+			opt.ClusterSlots = func(ctx context.Context) ([]redis.ClusterSlot, error) {
+				slots := []redis.ClusterSlot{{
+					Start: 0,
+					End:   4999,
+					Nodes: []redis.ClusterNode{{
+						Addr: ":8220",
+					}, {
+						Addr: ":8223",
+					}},
+				}, {
+					Start: 5000,
+					End:   9999,
+					Nodes: []redis.ClusterNode{{
+						Addr: ":8221",
+					}, {
+						Addr: ":8224",
+					}},
+				}, {
+					Start: 10000,
+					End:   16383,
+					Nodes: []redis.ClusterNode{{
+						Addr: ":8222",
+					}, {
+						Addr: ":8225",
+					}},
+				}}
+				return slots, nil
+			}
+			client = cluster.newClusterClient(ctx, opt)
+
+			err := client.ForEachMaster(ctx, func(ctx context.Context, master *redis.Client) error {
+				return master.FlushDB(ctx).Err()
+			})
+			Expect(err).NotTo(HaveOccurred())
+
+			err = client.ForEachSlave(ctx, func(ctx context.Context, slave *redis.Client) error {
+				Eventually(func() int64 {
+					return client.DBSize(ctx).Val()
+				}, 30*time.Second).Should(Equal(int64(0)))
+				return nil
+			})
+			Expect(err).NotTo(HaveOccurred())
+		})
+
+		AfterEach(func() {
+			failover = false
+
+			err := client.Close()
+			Expect(err).NotTo(HaveOccurred())
+		})
+
+		assertClusterClient()
+	})
 })
 
 var _ = Describe("ClusterClient without nodes", func() {