feat(zero): allow --rebalance_interval=0 to disable automatic tablet …

dgraph/cmd/zero/run.go

@@ -83,7 +83,8 @@ instances to achieve high-availability.
 		" The count includes the original shard.")
 	flag.String("peer", "", "Address of another dgraphzero server.")
 	flag.StringP("wal", "w", "zw", "Directory storing WAL.")
-	flag.Duration("rebalance_interval", 8*time.Minute, "Interval for trying a predicate move.")
+	flag.Duration("rebalance_interval", 8*time.Minute, "Interval for trying a predicate move. "+
+		"Set to 0 to disable automatic tablet rebalancing.")
 	flag.String("enterprise_license", "", "(deprecated) Path to the enterprise license file.")
 	flag.String("cid", "", "Cluster ID")
 
@@ -282,9 +283,9 @@ func run() {
 			"WAL directory and Audit output cannot be the same ('%s').", opts.audit.Output)
 	}
 
-	if opts.rebalanceInterval <= 0 {
-		log.Fatalf("ERROR: Rebalance interval must be greater than zero. Found: %d",
-			opts.rebalanceInterval)
+	if opts.rebalanceInterval < 0 {
+		log.Fatalf("ERROR: Rebalance interval must be zero (to disable automatic tablet "+
+			"rebalancing) or positive. Found: %d", opts.rebalanceInterval)
 	}
 
 	addr := "localhost"

dgraph/cmd/zero/tablet.go

@@ -75,8 +75,14 @@ This would trigger G1 to get latest state. Wait for it.
 */
 
 // TODO: Have a event log for everything.
-func (s *Server) rebalanceTablets() {
-	ticker := time.Tick(opts.rebalanceInterval)
+func (s *Server) rebalanceTablets(interval time.Duration) {
+	// A zero interval disables automatic rebalancing; it must not reach time.Tick,
+	// which panics on non-positive durations.
+	if interval <= 0 {
+		glog.Infof("Automatic tablet rebalancing is disabled (rebalance_interval=%v).", interval)
+		return
+	}
+	ticker := time.Tick(interval)
 	for range ticker {
 		predicate, srcGroup, dstGroup := s.chooseTablet()
 		if len(predicate) == 0 {

dgraph/cmd/zero/tablet_test.go

@@ -66,3 +66,20 @@ func TestMoveBackoff(t *testing.T) {
 	s.recordMoveResult(pred, time.Minute, nil)
 	require.False(t, s.skipMove(pred))
 }
+
+func TestRebalanceTabletsDisabled(t *testing.T) {
+	// A non-positive interval disables automatic rebalancing: the loop must return
+	// immediately instead of ticking (time.Tick would panic on these values).
+	for _, interval := range []time.Duration{0, -time.Minute} {
+		done := make(chan struct{})
+		go func() {
+			(&Server{}).rebalanceTablets(interval)
+			close(done)
+		}()
+		select {
+		case <-done:
+		case <-time.After(5 * time.Second):
+			t.Fatalf("rebalanceTablets(%v) did not return; rebalancing not disabled", interval)
+		}
+	}
+}

dgraph/cmd/zero/zero.go

@@ -99,7 +99,7 @@ func (s *Server) Init() {
 			opts.limiterConfig.RefillAfter, s.closer)
 	}
 
-	go s.rebalanceTablets()
+	go s.rebalanceTablets(opts.rebalanceInterval)
 }
 
 func (s *Server) periodicallyPostTelemetry() {