今天我们用配置变更来结束整个EtcdRaft源码分析系列。横向扩展能力是衡量分布式系统优劣的决定性指标，而能否轻松，快捷，有效，及时的变更集群成员是其中的关键。下面我们一起来看看EtcdRaft是怎么实现的。

接口

type Node interface {
    ...
   // ProposeConfChange proposes config change.
   // At most one ConfChange can be in the process of going through consensus.
   // Application needs to call ApplyConfChange when applying EntryConfChange type entry.
   ProposeConfChange(ctx context.Context, cc pb.ConfChange) error
    ...
   // ApplyConfChange applies config change to the local node.
   // Returns an opaque ConfState protobuf which must be recorded
   // in snapshots. Will never return nil; it returns a pointer only
   // to match MemoryStorage.Compact.
   ApplyConfChange(cc pb.ConfChange) *pb.ConfState
    ...
}

可以看到有两个方法跟配置变更相关，看过前面的知道，外部跟Raft打交道的方式。先提案(propose), 然后等内部达成一致，再落地(Apply)。

struct

type ConfChange struct {
   ID               uint64         `protobuf:"varint,1,opt,name=ID" json:"ID"`
   Type             ConfChangeType `protobuf:"varint,2,opt,name=Type,enum=raftpb.ConfChangeType" json:"Type"`
   NodeID           uint64         `protobuf:"varint,3,opt,name=NodeID" json:"NodeID"`
   Context          []byte         `protobuf:"bytes,4,opt,name=Context" json:"Context,omitempty"`
   XXX_unrecognized []byte         `json:"-"`
}


const (
    ConfChangeAddNode        ConfChangeType = 0
    ConfChangeRemoveNode     ConfChangeType = 1
    ConfChangeUpdateNode     ConfChangeType = 2
    ConfChangeAddLearnerNode ConfChangeType = 3
)

以上是提案内容，很清晰，但有个地方需要注意，一次只能变更一个节点。至于为什么，有兴趣的可以去看论文哈。

Propose

func (n *node) ProposeConfChange(ctx context.Context, cc pb.ConfChange) error {
   data, err := cc.Marshal()
   if err != nil {
      return err
   }
   return n.Step(ctx, pb.Message{Type: pb.MsgProp, Entries: []pb.Entry{{Type: pb.EntryConfChange, Data: data}}})
}

• 基本就是走的提交数据的流程，唯一需要注意的是这里用pb.EntryConfChange将它与其他提案区别开来。
• 接下来，我们再走一遍数据提交的流程

Leader

...
for i, e := range m.Entries {
   if e.Type == pb.EntryConfChange {
      if r.pendingConfIndex > r.raftLog.applied {
         r.logger.Infof("propose conf %s ignored since pending unapplied configuration [index %d, applied %d]",
            e.String(), r.pendingConfIndex, r.raftLog.applied)
         m.Entries[i] = pb.Entry{Type: pb.EntryNormal}
      } else {
         r.pendingConfIndex = r.raftLog.lastIndex() + uint64(i) + 1
      }
   }
}
...
if !r.appendEntry(m.Entries...) {
    return ErrProposalDropped
}
r.bcastAppend()

• 如果当前还有配置更新没有处理完，那么这次新的变更将丢弃，用一个空的entry来替换它
• 如果都处理完了，那么记下这个配置变更的位置到pendingConfIndex
• 后面就一样了，累加到本地，而且群发给其他人。
• 问题来了，配置变更都同步给成员了，怎么确认都收到了，可以开始apply了呢？我想也猜得到，会通过Ready的committedIndex来通知应用层。

apply

应用层

case raftpb.EntryConfChange:
   var cc raftpb.ConfChange
   if err := cc.Unmarshal(ents[i].Data); err != nil {
      c.logger.Warnf("Failed to unmarshal ConfChange data: %s", err)
      continue
   }

   c.confState = *c.Node.ApplyConfChange(cc)

   switch cc.Type {
   case raftpb.ConfChangeAddNode:
      c.logger.Infof("Applied config change to add node %d, current nodes in channel: %+v", cc.NodeID, c.confState.Nodes)
   case raftpb.ConfChangeRemoveNode:
      c.logger.Infof("Applied config change to remove node %d, current nodes in channel: %+v", cc.NodeID, c.confState.Nodes)
   default:
      c.logger.Panic("Programming error, encountered unsupported raft config change")
   }

   // This ConfChange was introduced by a previously committed config block,
   // we can now unblock submitC to accept envelopes.
   if c.confChangeInProgress != nil &&
      c.confChangeInProgress.NodeID == cc.NodeID &&
      c.confChangeInProgress.Type == cc.Type {

      if err := c.configureComm(); err != nil {
         c.logger.Panicf("Failed to configure communication: %s", err)
      }

      c.confChangeInProgress = nil
      c.configInflight = false
      // report the new cluster size
      c.Metrics.ClusterSize.Set(float64(len(c.opts.RaftMetadata.Consenters)))
   }

   if cc.Type == raftpb.ConfChangeRemoveNode && cc.NodeID == c.raftID {
      c.logger.Infof("Current node removed from replica set for channel %s", c.channelID)
      // calling goroutine, since otherwise it will be blocked
      // trying to write into haltC
      go c.Halt()
   }
}

• 这里，我举的是Fabric的例子，只关注关键流程就好
• 收到Raft的ConfChange，第一件事，我们就要Node.ApplyConfChange(cc)
• Raft的通讯层是需要应用层托管的，所以不是Raft那边做完配置变更，就可以收工了。
  • Fabric要根据最新的集群成员数据，去做grpc的连接
  • 如果有删除的节点，还要去停掉这个成员，这个后面会讲。

Raft

ApplyConfChange

case cc := <-n.confc:
            if cc.NodeID == None {
                select {
                case n.confstatec <- pb.ConfState{
                    Nodes:    r.nodes(),
                    Learners: r.learnerNodes()}:
                case <-n.done:
                }
                break
            }
            switch cc.Type {
            case pb.ConfChangeAddNode:
                r.addNode(cc.NodeID)
            case pb.ConfChangeAddLearnerNode:
                r.addLearner(cc.NodeID)
            case pb.ConfChangeRemoveNode:
                // block incoming proposal when local node is
                // removed
                if cc.NodeID == r.id {
                    propc = nil
                }
                r.removeNode(cc.NodeID)
            case pb.ConfChangeUpdateNode:
            default:
                panic("unexpected conf type")
            }
            select {
            case n.confstatec <- pb.ConfState{
                Nodes:    r.nodes(),
                Learners: r.learnerNodes()}:
            case <-n.done:
            }

• 首先，这里有个调用技巧，如果调用的时候传入的NodeID为None，那么会返回当前Raft的成员
• 下面我们具体看下这几种变更类型具体是在干嘛

addNode&addLearner

func (r *raft) addNodeOrLearnerNode(id uint64, isLearner bool) {
   pr := r.getProgress(id)
   if pr == nil {
      r.setProgress(id, 0, r.raftLog.lastIndex()+1, isLearner)
   } else {
      if isLearner && !pr.IsLearner {
         // can only change Learner to Voter
         r.logger.Infof("%x ignored addLearner: do not support changing %x from raft peer to learner.", r.id, id)
         return
      }

      if isLearner == pr.IsLearner {
         // Ignore any redundant addNode calls (which can happen because the
         // initial bootstrapping entries are applied twice).
         return
      }

      // change Learner to Voter, use origin Learner progress
      delete(r.learnerPrs, id)
      pr.IsLearner = false
      r.prs[id] = pr
   }

   if r.id == id {
      r.isLearner = isLearner
   }

   // When a node is first added, we should mark it as recently active.
   // Otherwise, CheckQuorum may cause us to step down if it is invoked
   // before the added node has a chance to communicate with us.
   pr = r.getProgress(id)
   pr.RecentActive = true
}

• 如果是全新的节点，初始化Progress，这里match=0，next=r.raftLog.lastIndex()+1
• 如果之前是learner，那么从learner里面转移到正常节点里面

removeNode

func (r *raft) removeNode(id uint64) {
   r.delProgress(id)

   // do not try to commit or abort transferring if there is no nodes in the cluster.
   if len(r.prs) == 0 && len(r.learnerPrs) == 0 {
      return
   }

   // The quorum size is now smaller, so see if any pending entries can
   // be committed.
   if r.maybeCommit() {
      r.bcastAppend()
   }
   // If the removed node is the leadTransferee, then abort the leadership transferring.
   if r.state == StateLeader && r.leadTransferee == id {
      r.abortLeaderTransfer()
   }
}

• 真的删除Progress就够了么？想象下整个系统运转是靠什么？
• 还记得应用层会tick么？Leader靠这个发心跳，非Leader靠这个选举超时。
• 我们再回顾下应用层还做了些什么

应用层

if cc.Type == raftpb.ConfChangeRemoveNode && cc.NodeID == c.raftID {
    // calling goroutine, since otherwise it will be blocked
    // trying to write into haltC
    go c.Halt()
}

case <-n.chain.haltC:
   ticker.Stop()
   n.Stop()
   n.storage.Close()
   n.logger.Infof("Raft node stopped")
   close(n.chain.doneC) // close after all the artifacts are closed
   return
}

• 可以看到如果是删除当前节点的消息，会最终会让该节点的ticker.Stop。这也导致该节点最终会被Raft抛弃。