Distributed Systems

Paxos protocol

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Who uses Paxos

  • Google uses it in building Chubby, it’s highly available and distributed lock service
  • Zookeeper is based on Paxos and used by very large number of products and organizations.

Paxos Protocol

PROPOSERS:
  1. Submit a proposal numbered \(n\) to a majority of acceptors. Wait for a majority of acceptors to reply.
  2. If the majority reply ‘agree’, they will also send back the value of any proposals they have already accepted. Pick one of these values, and send a ‘commit’ message with the proposal number and the value. If no values have already been accepted, use your own. If instead a majority reply ‘reject’, or fail to reply, abandon the proposal and start again.
  3. If a majority reply to your commit request with an ‘accepted’ message, consider the protocol terminated. Otherwise, abandon the proposal and start again.
ACCEPTORS:
  1. Once a proposal is received, compare its number to the highest numbered proposal you have already agreed to. If the new proposal is higher, reply ‘agree’ with the value of any proposals you have already accepted. If it is lower, reply ‘reject’, along with the sequence number of the highest proposal.
  2. When a ‘commit’ message is received, accept it if a) the value is the same as any previously accepted proposal and b) its sequence number is the highest proposal number you have agreed to. Otherwise, reject it.

One of the excellent explanations which made Paxos a bit easier to understand for me is below.

 

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