Paxos

Single-instance consensus

• different problem abstraction from Raft
• same environment—asynchrony
  • node/network failure/slowness
• “equivalent” to multi-instance consensus
• proposer/acceptor/learner: logical roles; each server/replica has all

Preliminaries

• starting point
  • different replicas have different ids; can generate distinguished numbers
  • a value is attached with a number to distinguish
• goal
  • The system eventually “chooses” a value; or, a value gets chosen
  • Chosen (by definition) if it is “accepted” by a majority of servers

Thinking

• a server has to accept the first value it sees.
  • otherwise the system will never choose any value
  • consequence: each server may accept different values
• a server has to accept multiple values
  • otherwise “split”
  • consequences multiple values can be chosen
• if two values get chosen, they must be the same value
  • otherwise unsafe
• equivalent to: if a value with number n is chosen, any value with greater number m (m>n) chosen must be the same value
• further: if value-n is chosen, any value-m accepted must be the same value
• further: if value-n is chosen, any value-m proposed must be the same value
• if value-n is chosen, before value-m is proposed, the system replies value-n to the value-m proposer.
• if value-n is possibly chosen, before value-m is proposed, the system
  • replies value-n to the value-m proposer,
  • or, the system promises not to accept/choose value-n anymore.

Protocol

• prepare phase:
  • pick number n, send prepare messages (phase-1a) to all
  • on receiving a prepare request, each replica updates its highest seen ballot, reply (phase-1b) with the highest number value it has ever accepted.
  • collect replies (phase-1b) until get at least a majority, pick the value with the greatest number to propose, if no value in replies, pick any value
• accept phase:
  • use the value picked, send accept messages to all
  • on receiving an accept message, check highest seen number, accept and reply
  • a majority of replies -> chosen (commit)
• commit phase:
  • broadcast commit messages notifying the chosen value

Questions

• can a server accept different values?
  • yes
• does the majority of phase-1 (prepare) and phase-2 (accept) have to be the same?
  • no
• can each server only keep 1 ballot number instead of 2?
  • no
• in a 3-replica system, if (v1, b1) is accepted on server s1, (v1, b3) is accepted on s2, is the value v1 considered chosen (committed)?
  • no, think about the case where:
    • s1, s2 prepared b1; s1 accepted (v1, b1)
    • s2, s3 prepared b2; s2 accepted (v2, b2)
    • s1, s3 prepared b3; s3 accepted (v1, b3)
    • s1, s2 prepared b4; s1, s2 accepted (v2, b4)

Multi-Paxos

• each log entry maps to a Paxos instance
• batch prepare request
  • normally 1 roundtrip to commit a value on the leader, same as Raft
• log entry has more content than Raft
  • highest seen ballot, accepted value ballot, value
• unlike Raft, log entry could be modified
  • otherwise what could happen?
• reconfiguration
  • add view information into each log entry.

Compare Multi-Paxos and Raft

• similarities:
  • log structure
  • performance (w.r.t. 1 message roundtrip to commit)
• differences:
  • currentTerm is not highest seen ballot
  • term in each log entry is not the same as accepted ballot
  • Raft has two extra rules

Paxos->Raft

• change phase-1: move comparison from proposer to acceptor.
  • proposer send its accepted ballot to others to request a “vote”
• “super-prepare/accept”
  • each super-prepare prepares all instances
  • each super-accept accepts all instances
  • “super-reject”, if any instances refuses to prepare/accept, then reject
• global ballot number?
  • a single highest seen ballot for all instances
  • a single highest accepted ballot number for all instances
    • what about the ballot numbers in each log entry?
    • if we keep them, “sometimes”, they are the same as the highest accepted ballot number
      • sometimes = when highest seen ballot = highest accepted ballot.
    • if the proposing server always use the ballot in a new log entry:
      • for a recipient, the last log entry log ballot is always the same as the global highest accepted ballot.
      • so we can remove the global highest accepted ballot.
  • revisit Raft rules
    • “up-to-date” rule in leader election
    • “cannot count” rule in failure recovery
• shrink the super-prepare/accept message size.
  • prepare
    • send a digest of all previously accepted entries
    • this digest is accepted ballot + log length (index).
  • accept
    • prefix of the committed logs are the same.
    • inductively, a ballot at the same index is the same => all previous are the same
    • send a “delta”: what is different between the leader/follower?
    • should guarantee no “holes”.