server: add max one peer count balance support. (#173)

server/balancer.go

@@ -247,18 +247,29 @@ func (rb *resourceBalancer) selectRemovePeer(cluster *clusterInfo, peers map[uin
 }
 
 func (rb *resourceBalancer) doLeaderBalance(cluster *clusterInfo, stores []*storeInfo, region *metapb.Region, leader *metapb.Peer, newPeer *metapb.Peer) (*balanceOperator, error) {
+	if !rb.checkScore(cluster, leader, newPeer) {
+		return nil, nil
+	}
+
+	regionID := region.GetId()
+
+	// If cluster max peer count config is 1, we cannot do leader transfer,
+	// only need to add new peer and remove leader peer.
+	meta := cluster.getMeta()
+	if meta.GetMaxPeerCount() == 1 {
+		addPeerOperator := newAddPeerOperator(regionID, newPeer)
+		removePeerOperator := newRemovePeerOperator(regionID, leader)
+
+		return newBalanceOperator(region, addPeerOperator, removePeerOperator), nil
+	}
+
 	followerPeers, _ := getFollowerPeers(region, leader)
 	newLeader := rb.selectNewLeaderPeer(cluster, followerPeers)
 	if newLeader == nil {
 		log.Warn("new leader peer cannot be found to do balance, try to do follower peer balance")
 		return nil, nil
 	}
 
-	if !rb.checkScore(cluster, leader, newPeer) {
-		return nil, nil
-	}
-
-	regionID := region.GetId()
 	leaderTransferOperator := newTransferLeaderOperator(regionID, leader, newLeader, maxWaitCount)
 	addPeerOperator := newAddPeerOperator(regionID, newPeer)
 	removePeerOperator := newRemovePeerOperator(regionID, leader)

server/balancer_test.go

@@ -330,4 +330,49 @@ func (s *testBalancerSuite) TestResourceBalancer(c *C) {
 	bop, err = cb.Balance(clusterInfo)
 	c.Assert(err, IsNil)
 	c.Assert(bop, IsNil)
+
+	// If cluster max peer count config is 1, we can only add peer and remove leader peer.
+	s.updateStore(c, clusterInfo, 1, 100, 60, 0, 0)
+	s.updateStore(c, clusterInfo, 2, 100, 70, 0, 0)
+	s.updateStore(c, clusterInfo, 3, 100, 80, 0, 0)
+	s.updateStore(c, clusterInfo, 4, 100, 90, 0, 0)
+
+	// Set cluster config.
+	oldMeta := clusterInfo.getMeta()
+	meta := &metapb.Cluster{
+		Id:           proto.Uint64(0),
+		MaxPeerCount: proto.Uint32(1),
+	}
+	clusterInfo.setMeta(meta)
+
+	testCfg.MinCapacityUsedRatio = 0.3
+	testCfg.MaxCapacityUsedRatio = 0.9
+	cb = newResourceBalancer(testCfg)
+	bop, err = cb.Balance(clusterInfo)
+	c.Assert(err, IsNil)
+	c.Assert(bop, IsNil)
+
+	// Set region peers to one peer.
+	peers := region.GetPeers()
+	region.Peers = []*metapb.Peer{leaderPeer}
+	clusterInfo.regions.updateRegion(region)
+
+	cb = newResourceBalancer(testCfg)
+	bop, err = cb.Balance(clusterInfo)
+	c.Assert(err, IsNil)
+	c.Assert(bop, NotNil)
+
+	newOp1 = bop.Ops[0].(*changePeerOperator)
+	c.Assert(newOp1.ChangePeer.GetChangeType(), Equals, raftpb.ConfChangeType_AddNode)
+	c.Assert(newOp1.ChangePeer.GetPeer().GetStoreId(), Equals, uint64(4))
+
+	newOp2 = bop.Ops[1].(*changePeerOperator)
+	c.Assert(newOp2.ChangePeer.GetChangeType(), Equals, raftpb.ConfChangeType_RemoveNode)
+	c.Assert(newOp2.ChangePeer.GetPeer().GetStoreId(), Equals, uint64(1))
+
+	// Reset cluster config and region peers.
+	clusterInfo.setMeta(oldMeta)
+
+	region.Peers = peers
+	clusterInfo.regions.updateRegion(region)
 }