Draft: Simplify RaftStorage API

[drmingdrmer]

There are 3 parts of data RaftStorage that raft needs to access:

• meta:

  vote: Vote
  membership_state: {committed, effective}
  

• raft-log:

  • list of entries
  • meta: range: (purged, last]

• state-machine:

  • user defined data.
  • meta: last_applied

Membership in Raft can be considered as another slot of logs. And only at most two membership logs need to be stored:

Thus internally, raft has two log-and-state-machine structures: user log and its state machine and membership log and its state machine.
The membership state machine is simply a record of the committed membership.
Thus the membership-log and membership-state-machine can be merged into one meta-record: membership_state.
This way the user-data-state-machine does not need to store membership in it.

RaftStorage {
  // infrequently updated meta data: vote and membership.
  write_meta()
  read_meta()
  
  // log
  append()
  purge()
  truncate()
  get_log_state()

  // state machine
  apply()
  get_last_applied_log_id()
  
  // snapshot
  install_snapshot()
  build_snapshot()
}

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[drmingdrmer]

The storage API has been splitted into RaftLogStorage and RaftStateMachine since 0.9