fix: compose tool, fix leaner node zero generation

compose/compose.go

@@ -266,39 +266,45 @@ func getZero(idx int, raft string) service {
 	return svc
 }
 
-func getAlpha(idx int, raft string) service {
-	basename := "alpha"
-	// internalPort is used for Raft communication between nodes.
-	internalPort := alphaBasePort + opts.PortOffset + getOffset(idx)
-	// grpcPort is the public-facing port for clients. It is offset from the internal port.
-	grpcPort := internalPort + 1000
-	svc := initService(basename, idx, grpcPort)
-
-	if opts.TmpFS {
-		svc.TmpFS = append(svc.TmpFS, fmt.Sprintf("/data/%s/w", svc.name))
-	}
-
+// makeZeroList builds a comma-separated --zero= value covering `count` zeros
+// starting at zero index `firstIdx` (1-based). Used by main alphas (pointing
+// at the main zero quorum, firstIdx=1) and by learner alphas (pointing at the
+// learner zero quorum, firstIdx=opts.NumZeros+1). On Dgraph versions older
+// than 1.2.3 / 20.03.0 (which only accept a single --zero), the list is
+// truncated to the first address.
+func makeZeroList(firstIdx, count int) string {
 	isMultiZeros := true
-	var isInvalidVersion, err = semverCompare("< 1.2.3 || 20.03.0", opts.Tag)
+	isInvalidVersion, err := semverCompare("< 1.2.3 || 20.03.0", opts.Tag)
 	if err != nil || isInvalidVersion {
 		if opts.Tag != "latest" {
 			isMultiZeros = false
 		}
 	}
 
-	maxZeros := 1
-	if isMultiZeros {
-		maxZeros = opts.NumZeros
+	if !isMultiZeros {
+		count = 1
 	}
 
-	zeroHostAddr := fmt.Sprintf("zero%d:%d", 1, zeroBasePort+opts.PortOffset)
-	zeros := []string{zeroHostAddr}
-	for i := 2; i <= maxZeros; i++ {
-		zeroHostAddr = fmt.Sprintf("zero%d:%d", i, zeroBasePort+opts.PortOffset+getOffset(i))
-		zeros = append(zeros, zeroHostAddr)
+	zeros := make([]string, 0, count)
+	for i := 0; i < count; i++ {
+		idx := firstIdx + i
+		port := zeroBasePort + opts.PortOffset + getOffset(idx)
+		zeros = append(zeros, fmt.Sprintf("zero%d:%d", idx, port))
 	}
+	return strings.Join(zeros, ",")
+}
 
-	zerosOpt := strings.Join(zeros, ",")
+func getAlpha(idx int, raft, zerosOpt string) service {
+	basename := "alpha"
+	// internalPort is used for Raft communication between nodes.
+	internalPort := alphaBasePort + opts.PortOffset + getOffset(idx)
+	// grpcPort is the public-facing port for clients. It is offset from the internal port.
+	grpcPort := internalPort + 1000
+	svc := initService(basename, idx, grpcPort)
+
+	if opts.TmpFS {
+		svc.TmpFS = append(svc.TmpFS, fmt.Sprintf("/data/%s/w", svc.name))
+	}
 
 	offset := getOffset(idx)
 	if (opts.PortOffset + offset) != 0 {
@@ -663,6 +669,16 @@ func main() {
 
 	services := make(map[string]service)
 
+	// Main alphas point at the main zero quorum; learner alphas point at
+	// the learner zero quorum (the whole reason for deploying learner zeros
+	// in the first place — they're the edge contact point for nearby learner
+	// alphas, see compose docs / customer feedback on this).
+	mainZerosOpt := makeZeroList(1, opts.NumZeros)
+	var learnerZerosOpt string
+	if opts.NumLearners > 0 {
+		learnerZerosOpt = makeZeroList(opts.NumZeros+1, opts.NumLearners)
+	}
+
 	for i := 1; i <= opts.NumZeros; i++ {
 		svc := getZero(i, fmt.Sprintf("idx=%d", i))
 		services[svc.name] = svc
@@ -671,7 +687,7 @@ func main() {
 	for i := 1; i <= opts.NumAlphas; i++ {
 		gid := int(math.Ceil(float64(i) / float64(opts.NumReplicas)))
 		rs := fmt.Sprintf("idx=%d; group=%d", i, gid)
-		svc := getAlpha(i, rs)
+		svc := getAlpha(i, rs, mainZerosOpt)
 		// Don't make Alphas depend on each other.
 		svc.DependsOn = nil
 		services[svc.name] = svc
@@ -690,7 +706,7 @@ func main() {
 		for i := 1; i <= opts.NumLearners; i++ {
 			lidx++
 			rs := fmt.Sprintf("idx=%d; group=%d; learner=true", lidx, gid)
-			svc := getAlpha(lidx, rs)
+			svc := getAlpha(lidx, rs, learnerZerosOpt)
 			services[svc.name] = svc
 		}
 	}