6.824 Lab 2: Raft

I’m learning MIT 6.824: Distributed Systems recently. This post is my solution to lab 2. This lab requires to implement a Raft system.

Part 2A: leader election

• Election ticker and heartbeat ticker

  For simplity, each Raft server starts two goroutines, responsing for timeout elections and heartbeats respectively. The Raft server controls its behavior via channels.

  For example, the election ticker has 3 channels: reset, pause and continue. It loops to select a random timeout, reset signal or pause signal. For (1)timeout, it converts to candidate (which starts a election); for (2)reset, it restarts a loop; for (3)pause, it blocks to wait for another signal from continue.

  

  The heartbeat ticker is almost the same, except it has no reset channel.

• convertTo

  A difficulty of this part is how to handle states switching. I define a helper function func (rf *Raft) convertTo(s State).

  To enum all possible (oldState, newState) pairs, I use a trick here: State type is defined as int and each state has a unique bit.

  type State int
  const (
  	Follower State = 1 << iota /* 0x1 */
  	Candidate                  /* 0x2 */
  	Leader                     /* 0x4 */
  	__nState = iota            /* 3 */
  )

  So that each pair has a unique integer and thus can be used in switch-case.

  func (s State) to(t State) int { return int(s | t<<__nState) }
  
  switch oldState.to(newState) {
  case Follower.to(Candidate): /* ... */
  case Candidate.to(Candidate): /* ... */
  /* ... */
  }

  In Figure 4 of the paper, 5 edges are defined. For convinience, I add Follower->Follower to deal with election timer reset.

  

• Election, RequestVote RPC

  To do an election, sendRequestVote to peers and wait for reply. In case some connections are too slow to reply in a timeout, election goroutine should not wait for all peers replying. Instead, when a peer reply, atomic.Add the number of votes, if it just reaches the majority, call the convertTo(Follower).

• Heartbeat, i.e. empty AppendEntries RPC