nextIndex

  1. 官方描述：for each server, index of the next log entry to send to that server (initialized to leader last log index + 1)
  2. leader保存该数组变量，由leader更新。
  3. nextIndex[i]代表着对第i个follower发起Append Entries RPC时尝试replicated log的位置，仅仅是猜测并不是真实的，并且出现log inconsistency会将该值回退。所以nextIndex[i]需要比matchIndex[i]要大。
  4. leader收到Append Entries RPC回复后只有是因为log inconsistency才能更新nextIndex。
  5. 更新nextIndex的三个时机
    • leader election成功时候，需要更新nextIndex为leader last log index + 1
    • AppendEntries RPC返回success，代表已经成功replicated log
    • log inconsistency时候回退nextIndex

matchIndex

  1. 官方描述：for each server, index of highest log entry known to be replicated on server (initialized to 0, increases monotonically)
  2. leader保存该数组变量，由leader更新。
  3. 更新matchIndex时候，应该取自args的值，因为nextIndex和raft中的logEntries的值可能已经发生了变化。matchIndex = prevLogIndex + len(args.entries)
  4. 更新matchIndex的三个时机
    • leader election成功，更新matchIndex为lastLogIndex
    • AppendEntries RPC返回success，代表已经成功replicated log
    • leader接收到新的command时候，主要用于配合该规则更新commitIndex，然后触发apply操作：If there exists an N such that N > commitIndex, a majority of matchIndex[i] ≥ N, and log[N].term == currentTerm: set commitIndex = N

commitIndex

  1. 官方描述：index of highest log entry known to be committed (initialized to 0, increases monotonically)
  2. commitIndex并不是只有leader才有的，所有server的commitIndex应该是一致的。
  3. If leaderCommit > commitIndex, set commitIndex = min(leaderCommit, index of last new entry)
    • leader发送带有log的Append Entries RPC给follower，append log的follower达到majority后，leader commit该log，并更新commitIndex
    • 第二次发送RPC时候，因为leaderCommit大于follower的commitIndex，代表着leader已经commit了新的log，所以follower根据上述规则更新自己的commitIndex
    • 根据commitIndex > lastApplied，leader(如果已经达到majority的话)/follower就会apply [lastApplied+1, commitIndex]的log到state machine。这里apply的是上一次的log。因为leader commit后，follower才能commit，本次log leader还没有commit
  4. apply时候需要注意Figure8的条件，leader只能apply currentTerm的log时候顺便apply了之前的term的log，而不能直接apply之前term的log

lastApplied

  • highest applied log index，和commitIndex配合使用，标明要apply log的区间