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Basic-Paxos.md

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Basic Paxos

Introduction

This is my attempt to describe the basic paxos algorithm in a programmer friendly way. I try to merge the description in Leslie Lamport's The Part-Time Parliament paper and subsequent TLA+ specification.

Terms

  • Process - a Paxos participant (priest in PTP paper).
  • Ballot - a referendum on a single decree. Each Ballot is identified by a unique ballot number, and Ballots are ordered by the ballot number.
  • Decree - represents the value being agreed upon, i.e. the value being voted on.
  • Ballot Number - a unique number made up of a pair - proposal number, and process id. Ballot Numbers are sorted by proposal number, followed by process id.
  • Process id - a unique id given to each process.
  • Proposal number - a monotonically increasing sequence number maintained by each process, -1 indicates none, valid values are >= 0.
  • Vote v - a vote cast by a process, is a tuple containing the process id of the voter, the ballot number, and the decree being voted. Votes are ordered by ballot numbers.
  • Ledger - each process must maintain some data in persistent storage - the ledger represents the storage data structure.
  • quorumSize - (Number of Participants + 1) / 2, where Number of participants is an odd number.

Functions

  • MaxVote(votes) - function that returns the max vote cast, where max() is based on the ordering of votes by ballot number.
  • owner(b) - returns the process id that initiated the ballot number b.

Data Maintained in the Ledger

  • outcome - this is value of the decree in the ledger, or NULL if there is nothing written yet.
  • lastTried - The ballot number that the process p last tried to initiate, or (-1,p.id) if none.
  • maxBal - The maximum ballot number that process p ever agreed to participate in, or (-1,p.id) if p has never agreed to participate in a ballot.
  • maxVBal - The ballot number in which p last voted or (-1,p.id) if p never voted.
  • maxVal - The value of the decree associated with maxVBal, i.e. the decree that p last voted, or NULL if p never voted.

Data Maintained by a Process p in memory

  • status - the status of process p, which can be one of the following:

    • idle - not conducting or trying to begin a ballot
    • trying - trying to begin ballot number ledger.lastTried
    • polling - Conducting ballot number ledger.lastTried
    • On startup the status is assumed to be idle.

  • prevVotes - the set of votes received in LastVote messages for the current ballot (i.e. ballot number in ledger.lastTried).

  • quorum - the set of processes including p, that responded with LastVote messages for current ballot, only meaningful when status == polling.

  • voters - the set of processes including p, from whom p has received Voted messages in the current ballot, only meaningful when status == polling.

  • decree - if status == polling, then the decree of the current ballot, otherwise meaningless.

Messages

  • NextBallot - aka PREPARE 1a - message sent by the ballot conductor.
  • LastVote - aka PROMISE 1b - message sent by participant to ballot conductor.
  • BeginBallot - aka ACCEPT 2a - messages sent by the ballot conductor.
  • Voted - aka ACCEPTED 2b - message sent by participant to ballot conductor.
  • Success - message sent by ballot conductor to all processes once the ballot is successfully completed.

The content and timing of each message is described below.

Algorithm for Basic Paxos.

The following algorithm must be implemented for each process that is participating in a Basic Paxos run.

Start New Ballot (Phase 1)

This step is invoked when a new ballot must be started, perhaps on client request.

  • If status != idle, reject request. If status != idle, p is already conducting a ballot.
  • Let b = ledger.lastTried + 1, where 1 is added to the proposal number. First valid proposal number is thus 0, as initial value of ledger.lastTried is (-1, p.id).
  • Set ledger.lastTried to b.
  • Set p.status to trying.
  • Set p.prevVotes to empty set.
  • To each process participating in basic paxos, send NextBallot(ledger.lastTried) PREPARE 1a message, including to itself.

Receive NextBallot(b) PREPARE 1a, conditionally send LastVote PROMISE 1b

This is executed by each process that receives the NextBallot message.

  • Let b = NextBallot.b.
  • if b > ledger.maxBal then
    • Set ledger.maxBal to b
    • To the sender of ballot b, i.e. owner(b), send LastVote (PROMISE 1b) message with following contents
      • voter - p.id (i.e. the id of the process sending the LastVote)
      • v Vote containing
        • b - ballot number
        • ledger.maxVBal
        • ledger.maxVal

Receive LastVote(owner,b,v) PROMISE 1b message

  • if b == ledger.lastTried and status == trying then
    • Set add vote v to the set prevVotes. Note that two votes that are from different participants (i.e. LastVote.voter is different), must be considered as distinct votes here.
    • If count of prevVotes with ballot b is >= to quorumSize then start polling.

Start Polling (Phase 2) - Send BeginBallot(b,decree) ACCEPT 2a

This step is enabled when status=trying and there is a quorum of votes in prevVotes as described above.

  • Set status to polling
  • Set quorum to the set of processes in prevVotes where v.b=ledger.lastTried
  • Set voters to the empty set.
  • Let maxVote = MaxVote(prevVotes)
  • If maxVote has a ballot b with proposal number -1, then set decree to the value requested by client.
  • Else if maxVote has a ballot b with proposal number >=0 then set decree to maxVote.decree.
  • Send BeginBallot(b,decree) ACCEPT 2a to all the participants, including itself.

Receive BeginBallot(b, decree) ACCEPT 2a message, conditionally send Voted ACCEPTED 2b

This is executed by each process that receives the BeginBallot message.

  • if BeginBallot.b >= ledger.maxBal
    • Set ledger.maxBal to BeginBallot.b
    • Set ledger.maxVBal to BeginBallot.b
    • Set ledger.maxVal to BeginBallot.decree
    • Send to the owner of ballot owner(BeginBallot.b), a Voted(b, id) ACCEPTED 2b message where b is ledger.maxBal, and id is the process sending the Voted message.

Receive Voted(b,voter) ACCEPTED 2b message

This step is enabled when status=polling.

  • if Voted.b == ledger.lastTried and status == polling
    • Add voter process Voted.voter to the set of voters.
    • if count of voters is >= to quorumSize then
      • If ledger.outcome is NULL then set ledger.outcome to decree
      • Send Success(ledger.outcome) to all participants, including itself
      • Set status to idle

Receive Success(outcome) message

  • If ledger.outcome is NULL then set ledger.outcome to Success.outcome