issue-1423: cluster-osc-slaves API may not return all required slaves

go/inst/instance_dao.go

@@ -68,6 +68,30 @@ func (this InstancesByCountReplicas) Less(i, j int) bool {
 	return len(this[i].Replicas) < len(this[j].Replicas)
 }
 
+// InstancesByDc is a sortable type for Instance
+// 1. Instances are sorted by DC
+// 2. Within DC group instances are sorted by replicas count
+// 3. Within ReplicasCount group insances are:
+//    a) not sorted if ReplicasCount > 0
+//    b) sorted by replication lag if ReplicasCount == 0
+//
+// DC1 < DC2
+// if DC1 == DC2 => len(Replicas1) < len (Replicas2)
+// if Replicas.cnt == 0 => replicationLag1 < replicatonLag2
+type InstancesByDc [](*Instance)
+
+func (this InstancesByDc) Len() int      { return len(this) }
+func (this InstancesByDc) Swap(i, j int) { this[i], this[j] = this[j], this[i] }
+func (this InstancesByDc) Less(i, j int) bool {
+	if this[i].DataCenter == this[j].DataCenter {
+		if len(this[i].Replicas) == 0 && len(this[j].Replicas) == 0 {
+			return this[i].ReplicationLagSeconds.Int64 < this[j].ReplicationLagSeconds.Int64
+		}
+		return len(this[i].Replicas) < len(this[j].Replicas)
+	}
+	return (this[i].DataCenter < this[j].DataCenter)
+}
+
 // Constant strings for Group Replication information
 // See https://dev.mysql.com/doc/refman/8.0/en/replication-group-members-table.html for additional information.
 const (
@@ -1665,62 +1689,91 @@ func filterOSCInstances(instances [](*Instance)) [](*Instance) {
 	return result
 }
 
+// Get two busiest instances per DC
+func getTwoBusiestPerDC(all [](*Instance)) [](*Instance) {
+	result := [](*Instance){}
+
+	// sort by DC and replicas count
+	sort.Sort(sort.Reverse(InstancesByDc(all)))
+
+	currentDCInstances := 0
+	var currentDC *string = nil
+
+	for _, im := range all {
+		if currentDC == nil || *currentDC != im.DataCenter {
+			currentDCInstances = 0
+			currentDC = &im.DataCenter
+		}
+		if currentDCInstances > 1 {
+			continue
+		}
+		currentDCInstances++
+		result = append(result, im)
+	}
+	return result
+}
+
 // GetClusterOSCReplicas returns a heuristic list of replicas which are fit as controll replicas for an OSC operation.
 // These would be intermediate masters
 func GetClusterOSCReplicas(clusterName string) ([](*Instance), error) {
-	intermediateMasters := [](*Instance){}
-	result := [](*Instance){}
-	var err error
-	if strings.Index(clusterName, "'") >= 0 {
+	if strings.Contains(clusterName, "'") {
 		return [](*Instance){}, log.Errorf("Invalid cluster name: %s", clusterName)
 	}
+
+	result := [](*Instance){}
+	// Stage 1: 1st tier servers.
+	// We get up to two 1st tier servers from each DC in the following order:
+	// 1. Most busiest IMs
+	// 2. Most lagging leaf nodes
+	// Examples:
+	// 1. If there are N > 1 IMs in the DC, we will use 2 busiest ones
+	// (having the highest number of replicas)
+	// 2. If there is only 1 IM in the DC, but there are some leaf nodes,
+	// we will use IM + most lagging leaf node
+	// 3. If there are no IMs in the DC, but there are leaf nodes, we will use
+	// up to two most lagging leaf nodes
+	//
+	// So this stage will collect at most 2 servers per DC
 	{
-		// Pick up to two busiest IMs
 		condition := `
 			replication_depth = 1
-			and num_slave_hosts > 0
 			and cluster_name = ?
 		`
-		intermediateMasters, err = readInstancesByCondition(condition, sqlutils.Args(clusterName), "")
+		firstTierServers, err := readInstancesByCondition(condition, sqlutils.Args(clusterName), "")
 		if err != nil {
 			return result, err
 		}
-		sort.Sort(sort.Reverse(InstancesByCountReplicas(intermediateMasters)))
-		intermediateMasters = filterOSCInstances(intermediateMasters)
-		intermediateMasters = intermediateMasters[0:math.MinInt(2, len(intermediateMasters))]
-		result = append(result, intermediateMasters...)
+
+		firstTierServers = filterOSCInstances(firstTierServers)
+		result = append(result, getTwoBusiestPerDC(firstTierServers)...)
 	}
+
+	// Stage 2: 2nd tier servers
+	// Examine all selected 1st tier servers, and if they are IMs, get at most
+	// two of their busiest replicas (2nd tier servers).
+	// So this stage will collect at most 2 replicas per IM. If we collected 2 IMs
+	// per DC in the 1st stage, here we will get 4 servers per DC
 	{
-		// Get 2 replicas of found IMs, if possible
-		if len(intermediateMasters) == 1 {
-			// Pick 2 replicas for this IM
-			replicas, err := ReadReplicaInstances(&(intermediateMasters[0].Key))
+		// Get at most 2 replicas of found IMs
+		for _, im := range result {
+			if len(im.Replicas) == 0 {
+				// this is 1st tier leaf
+				continue
+			}
+			replicas, err := ReadReplicaInstances(&im.Key)
 			if err != nil {
 				return result, err
 			}
 			sort.Sort(sort.Reverse(InstancesByCountReplicas(replicas)))
 			replicas = filterOSCInstances(replicas)
 			replicas = replicas[0:math.MinInt(2, len(replicas))]
 			result = append(result, replicas...)
-
 		}
-		if len(intermediateMasters) == 2 {
-			// Pick one replica from each IM (should be possible)
-			for _, im := range intermediateMasters {
-				replicas, err := ReadReplicaInstances(&im.Key)
-				if err != nil {
-					return result, err
-				}
-				sort.Sort(sort.Reverse(InstancesByCountReplicas(replicas)))
-				replicas = filterOSCInstances(replicas)
-				if len(replicas) > 0 {
-					result = append(result, replicas[0])
 				}
-			}
-		}
-	}
+
+	// Stage 3: 3rd tier servers
+	// Get 2 busiest 3rd tier replicas per DC
 	{
-		// Get 2 3rd tier replicas, if possible
 		condition := `
 			replication_depth = 3
 			and cluster_name = ?
@@ -1729,25 +1782,8 @@ func GetClusterOSCReplicas(clusterName string) ([](*Instance), error) {
 		if err != nil {
 			return result, err
 		}
-		sort.Sort(sort.Reverse(InstancesByCountReplicas(replicas)))
-		replicas = filterOSCInstances(replicas)
-		replicas = replicas[0:math.MinInt(2, len(replicas))]
-		result = append(result, replicas...)
-	}
-	{
-		// Get 2 1st tier leaf replicas, if possible
-		condition := `
-			replication_depth = 1
-			and num_slave_hosts = 0
-			and cluster_name = ?
-		`
-		replicas, err := readInstancesByCondition(condition, sqlutils.Args(clusterName), "")
-		if err != nil {
-			return result, err
-		}
 		replicas = filterOSCInstances(replicas)
-		replicas = replicas[0:math.MinInt(2, len(replicas))]
-		result = append(result, replicas...)
+		result = append(result, getTwoBusiestPerDC(replicas)...)
 	}
 
 	return result, nil