how-paxos-acceptor.py

Accepted = namedtuple('Accepted', ['n', 'state'])
class Acceptor:
    def __init__(self, node_id, hdd):
        self.node_id = node_id
        if hdd.is_empty():
            hdd.write(promise=0, accepted=Accepted(0,None))
            # emit_* functions are used to record a event and capture a bit of context
            # The proof is based on reasoning on the structure of the sequence of the event
            # that the program can generate
            emit_accepted(0, node_id=self.node_id, ts=now(), accepted_n=0, state=self.hdd.accepted.state)
        self.hdd = hdd
    @endpoint
    @synchronized
    def prepare(self, ballot_n):
        if self.hdd.promise < ballot_n:
            self.hdd.write(promise=ballot_n)
            emit_promised(ballot_n, node_id=self.node_id, ts=now(), accepted_n=self.hdd.accepted.n)
            return self.hdd.accepted
        return Conflict(promise=self.hdd.promise)
    @endpoint
    @synchronized
    def accept(self, ballot_n, state):
        if self.hdd.promise == ballot_n:
            # always true, but it simplifies the proof
            # the ballot number monotonicity assert:
            assert self.hdd.accepted.n < ballot_n
            self.hdd.write(accepted=Accepted(ballot_n, state))
            emit_accepted(ballot_n, node_id=self.node_id, ts=now(), accepted_n=ballot_n, state=self.hdd.accepted.state)
            return OK()
        return Conflict(promise=self.hdd.promise)