详解分布式系统中如何用python实现Paxos

def execute_operation(state, operation):
     if operation.name == 'deposit':
         if not verify_signature(operation.deposit_signature):
         return state, False
         state.accounts[operation.destination_account] += operation.amount
         return state, True
     elif operation.name == 'transfer':
         if state.accounts[operation.source_account] < operation.amount:
             return state, False
             state.accounts[operation.source_account] -= operation.amount
         state.accounts[operation.destination_account] += operation.amount
         return state, True
     elif operation.name == 'get-balance':
     return state, state.accounts[operation.account]

Accepted = namedtuple('Accepted', ['slot', 'ballot_num'])
    Accept = namedtuple('Accept', ['slot', 'ballot_num', 'proposal'])
    Decision = namedtuple('Decision', ['slot', 'proposal'])
    Invoked = namedtuple('Invoked', ['client_id', 'output'])
    Invoke = namedtuple('Invoke', ['caller', 'client_id', 'input_value'])
    Join = namedtuple('Join', [])
    Active = namedtuple('Active', [])
    Prepare = namedtuple('Prepare', ['ballot_num'])
    Promise = namedtuple('Promise', ['ballot_num', 'accepted_proposals'])
    Propose = namedtuple('Propose', ['slot', 'proposal'])
    Welcome = namedtuple('Welcome', ['state', 'slot', 'decisions'])
    Decided = namedtuple('Decided', ['slot'])
    Preempted = namedtuple('Preempted', ['slot', 'preempted_by'])
    Adopted = namedtuple('Adopted', ['ballot_num', 'accepted_proposals'])
    Accepting = namedtuple('Accepting', ['leader'])

msg = Accepted(slot=10, ballot_num=30)

got_ballot_num = msg.ballot_num

JOIN_RETRANSMIT = 0.7
    CATCHUP_INTERVAL = 0.6
    ACCEPT_RETRANSMIT = 1.0
    PREPARE_RETRANSMIT = 1.0
    INVOKE_RETRANSMIT = 0.5
    LEADER_TIMEOUT = 1.0
    NULL_BALLOT = Ballot(-1, -1)  # sorts before all real ballots
    NOOP_PROPOSAL = Proposal(None, None, None)  # no-op to fill otherwise empty slots

Proposal = namedtuple('Proposal', ['caller', 'client_id', 'input'])
    Ballot = namedtuple('Ballot', ['n', 'leader'])

class Role(object):

    def __init__(self, node):
        self.node = node
        self.node.register(self)
        self.running = True
        self.logger = node.logger.getChild(type(self).__name__)

    def set_timer(self, seconds, callback):
        return self.node.network.set_timer(self.node.address, seconds,
                                           lambda: self.running and callback())

    def stop(self):
        self.running = False
        self.node.unregister(self)

class Node(object):
    unique_ids = itertools.count()

    def __init__(self, network, address):
        self.network = network
        self.address = address or 'N%d' % self.unique_ids.next()
        self.logger = SimTimeLogger(
            logging.getLogger(self.address), {'network': self.network})
        self.logger.info('starting')
        self.roles = []
        self.send = functools.partial(self.network.send, self)

    def register(self, roles):
        self.roles.append(roles)

    def unregister(self, roles):
        self.roles.remove(roles)

    def receive(self, sender, message):
        handler_name = 'do_%s' % type(message).__name__

        for comp in self.roles[:]:
            if not hasattr(comp, handler_name):
                continue
            comp.logger.debug("received %s from %s", message, sender)
            fn = getattr(comp, handler_name)
            fn(sender=sender, **message._asdict())

class Member(object):

    def __init__(self, state_machine, network, peers, seed=None,
                 seed_cls=Seed, bootstrap_cls=Bootstrap):
        self.network = network
        self.node = network.new_node()
        if seed is not None:
            self.startup_role = seed_cls(self.node, initial_state=seed, peers=peers,
                                      execute_fn=state_machine)
        else:
            self.startup_role = bootstrap_cls(self.node,
                                      execute_fn=state_machine, peers=peers)
        self.requester = None

    def start(self):
        self.startup_role.start()
        self.thread = threading.Thread(target=self.network.run)
        self.thread.start()

    def invoke(self, input_value, request_cls=Requester):
        assert self.requester is None
        q = Queue.Queue()
        self.requester = request_cls(self.node, input_value, q.put)
        self.requester.start()
        output = q.get()
        self.requester = None
        return output

class Acceptor(Role):

    def __init__(self, node):
        super(Acceptor, self).__init__(node)
        self.ballot_num = NULL_BALLOT
        self.accepted_proposals = {}  # {slot: (ballot_num, proposal)}

    def do_Prepare(self, sender, ballot_num):
        if ballot_num > self.ballot_num:
            self.ballot_num = ballot_num
            # we've heard from a scout, so it might be the next leader
            self.node.send([self.node.address], Accepting(leader=sender))

        self.node.send([sender], Promise(
            ballot_num=self.ballot_num, 
            accepted_proposals=self.accepted_proposals
        ))

    def do_Accept(self, sender, ballot_num, slot, proposal):
        if ballot_num >= self.ballot_num:
            self.ballot_num = ballot_num
            acc = self.accepted_proposals
            if slot not in acc or acc[slot][0] < ballot_num:
                acc[slot] = (ballot_num, proposal)

        self.node.send([sender], Accepted(
            slot=slot, ballot_num=self.ballot_num))

BootStrap     Replica        Replica       Replica
     X---------->|             |             |    Join
     |<----------X             X             |    Welcome
     X------------------------>|             |    Join
     |<------------------------X             |    Welcome
     X-------------------------------------->|    Join
     |<--------------------------------------X    Welcome      
class Replica(Role):

    def __init__(self, node, execute_fn, state, slot, decisions, peers):
        super(Replica, self).__init__(node)
        self.execute_fn = execute_fn
        self.state = state
        self.slot = slot
        self.decisions = decisions
        self.peers = peers
        self.proposals = {}
        # next slot num for a proposal (may lead slot)
        self.next_slot = slot
        self.latest_leader = None
        self.latest_leader_timeout = None

    # making proposals

    def do_Invoke(self, sender, caller, client_id, input_value):
        proposal = Proposal(caller, client_id, input_value)
        slot = next((s for s, p in self.proposals.iteritems() if p == proposal), None)
        # propose, or re-propose if this proposal already has a slot
        self.propose(proposal, slot)

    def propose(self, proposal, slot=None):
        """Send (or resend, if slot is specified) a proposal to the leader"""
        if not slot:
            slot, self.next_slot = self.next_slot, self.next_slot + 1
        self.proposals[slot] = proposal
        # find a leader we think is working - either the latest we know of, or
        # ourselves (which may trigger a scout to make us the leader)
        leader = self.latest_leader or self.node.address
        self.logger.info(
            "proposing %s at slot %d to leader %s" % (proposal, slot, leader))
        self.node.send([leader], Propose(slot=slot, proposal=proposal))

    # handling decided proposals

    def do_Decision(self, sender, slot, proposal):
        assert not self.decisions.get(self.slot, None), \
                "next slot to commit is already decided"
        if slot in self.decisions:
            assert self.decisions[slot] == proposal, \
                "slot %d already decided with %r!" % (slot, self.decisions[slot])
            return
        self.decisions[slot] = proposal
        self.next_slot = max(self.next_slot, slot + 1)

        # re-propose our proposal in a new slot if it lost its slot and wasn't a no-op
        our_proposal = self.proposals.get(slot)
        if (our_proposal is not None and 
            our_proposal != proposal and our_proposal.caller):
            self.propose(our_proposal)

        # execute any pending, decided proposals
        while True:
            commit_proposal = self.decisions.get(self.slot)
            if not commit_proposal:
                break  # not decided yet
            commit_slot, self.slot = self.slot, self.slot + 1

            self.commit(commit_slot, commit_proposal)

    def commit(self, slot, proposal):
        """Actually commit a proposal that is decided and in sequence"""
        decided_proposals = [p for s, p in self.decisions.iteritems() if s < slot]
        if proposal in decided_proposals:
            self.logger.info(
                "not committing duplicate proposal %r, slot %d", proposal, slot)
            return  # duplicate

        self.logger.info("committing %r at slot %d" % (proposal, slot))
        if proposal.caller is not None:
            # perform a client operation
            self.state, output = self.execute_fn(self.state, proposal.input)
            self.node.send([proposal.caller], 
                Invoked(client_id=proposal.client_id, output=output))

    # tracking the leader

    def do_Adopted(self, sender, ballot_num, accepted_proposals):
        self.latest_leader = self.node.address
        self.leader_alive()

    def do_Accepting(self, sender, leader):
        self.latest_leader = leader
        self.leader_alive()

    def do_Active(self, sender):
        if sender != self.latest_leader:
            return
        self.leader_alive()

    def leader_alive(self):
        if self.latest_leader_timeout:
            self.latest_leader_timeout.cancel()

        def reset_leader():
            idx = self.peers.index(self.latest_leader)
            self.latest_leader = self.peers[(idx + 1) % len(self.peers)]
            self.logger.debug("leader timed out; tring the next one, %s", 
                self.latest_leader)
        self.latest_leader_timeout = self.set_timer(LEADER_TIMEOUT, reset_leader)

    # adding new cluster members

    def do_Join(self, sender):
        if sender in self.peers:
            self.node.send([sender], Welcome(
                state=self.state, slot=self.slot, decisions=self.decisions))

class Leader(Role):

    def __init__(self, node, peers, commander_cls=Commander, scout_cls=Scout):
        super(Leader, self).__init__(node)
        self.ballot_num = Ballot(0, node.address)
        self.active = False
        self.proposals = {}
        self.commander_cls = commander_cls
        self.scout_cls = scout_cls
        self.scouting = False
        self.peers = peers

    def start(self):
        # reminder others we're active before LEADER_TIMEOUT expires
        def active():
            if self.active:
                self.node.send(self.peers, Active())
            self.set_timer(LEADER_TIMEOUT / 2.0, active)
        active()

    def spawn_scout(self):
        assert not self.scouting
        self.scouting = True
        self.scout_cls(self.node, self.ballot_num, self.peers).start()

    def do_Adopted(self, sender, ballot_num, accepted_proposals):
        self.scouting = False
        self.proposals.update(accepted_proposals)
        # note that we don't re-spawn commanders here; if there are undecided
        # proposals, the replicas will re-propose
        self.logger.info("leader becoming active")
        self.active = True

    def spawn_commander(self, ballot_num, slot):
        proposal = self.proposals[slot]
        self.commander_cls(self.node, ballot_num, slot, proposal, self.peers).start()

    def do_Preempted(self, sender, slot, preempted_by):
        if not slot:  # from the scout
            self.scouting = False
        self.logger.info("leader preempted by %s", preempted_by.leader)
        self.active = False
        self.ballot_num = Ballot((preempted_by or self.ballot_num).n + 1, 
                                 self.ballot_num.leader)

    def do_Propose(self, sender, slot, proposal):
        if slot not in self.proposals:
            if self.active:
                self.proposals[slot] = proposal
                self.logger.info("spawning commander for slot %d" % (slot,))
                self.spawn_commander(self.ballot_num, slot)
            else:
                if not self.scouting:
                    self.logger.info("got PROPOSE when not active - scouting")
                    self.spawn_scout()
                else:
                    self.logger.info("got PROPOSE while scouting; ignored")
        else:
            self.logger.info("got PROPOSE for a slot already being proposed")

def __init__(self, node, ballot_num, peers):
        super(Scout, self).__init__(node)
        self.ballot_num = ballot_num
        self.accepted_proposals = {}
        self.acceptors = set([])
        self.peers = peers
        self.quorum = len(peers) / 2 + 1
        self.retransmit_timer = None

    def start(self):
        self.logger.info("scout starting")
        self.send_prepare()

    def send_prepare(self):
        self.node.send(self.peers, Prepare(ballot_num=self.ballot_num))
        self.retransmit_timer = self.set_timer(PREPARE_RETRANSMIT, self.send_prepare)

    def update_accepted(self, accepted_proposals):
        acc = self.accepted_proposals
        for slot, (ballot_num, proposal) in accepted_proposals.iteritems():
            if slot not in acc or acc[slot][0] < ballot_num:
                acc[slot] = (ballot_num, proposal)

    def do_Promise(self, sender, ballot_num, accepted_proposals):
        if ballot_num == self.ballot_num:
            self.logger.info("got matching promise; need %d" % self.quorum)
            self.update_accepted(accepted_proposals)
            self.acceptors.add(sender)
            if len(self.acceptors) >= self.quorum:
                # strip the ballot numbers from self.accepted_proposals, now that it
                # represents a majority
                accepted_proposals = \ 
                    dict((s, p) for s, (b, p) in self.accepted_proposals.iteritems())
                # We're adopted; note that this does *not* mean that no other
                # leader is active.  # Any such conflicts will be handled by the
                # commanders.
                self.node.send([self.node.address],
                    Adopted(ballot_num=ballot_num, 
                            accepted_proposals=accepted_proposals))
                self.stop()
        else:
            # this acceptor has promised another leader a higher ballot number,
            # so we've lost
            self.node.send([self.node.address], 
                Preempted(slot=None, preempted_by=ballot_num))
            self.stop()

def __init__(self, node, ballot_num, slot, proposal, peers):
        super(Commander, self).__init__(node)
        self.ballot_num = ballot_num
        self.slot = slot
        self.proposal = proposal
        self.acceptors = set([])
        self.peers = peers
        self.quorum = len(peers) / 2 + 1

    def start(self):
        self.node.send(set(self.peers) - self.acceptors, Accept(
            slot=self.slot, ballot_num=self.ballot_num, proposal=self.proposal))
        self.set_timer(ACCEPT_RETRANSMIT, self.start)

    def finished(self, ballot_num, preempted):
        if preempted:
            self.node.send([self.node.address], 
                           Preempted(slot=self.slot, preempted_by=ballot_num))
        else:
            self.node.send([self.node.address], 
                           Decided(slot=self.slot))
        self.stop()

    def do_Accepted(self, sender, slot, ballot_num):
        if slot != self.slot:
            return
        if ballot_num == self.ballot_num:
            self.acceptors.add(sender)
            if len(self.acceptors) < self.quorum:
                return
            self.node.send(self.peers, Decision(
                           slot=self.slot, proposal=self.proposal))
            self.finished(ballot_num, False)
        else:
            self.finished(ballot_num, True)

class Bootstrap(Role):

    def __init__(self, node, peers, execute_fn,
                 replica_cls=Replica, acceptor_cls=Acceptor, leader_cls=Leader,
                 commander_cls=Commander, scout_cls=Scout):
        super(Bootstrap, self).__init__(node)
        self.execute_fn = execute_fn
        self.peers = peers
        self.peers_cycle = itertools.cycle(peers)
        self.replica_cls = replica_cls
        self.acceptor_cls = acceptor_cls
        self.leader_cls = leader_cls
        self.commander_cls = commander_cls
        self.scout_cls = scout_cls

    def start(self):
        self.join()

    def join(self):
        self.node.send([next(self.peers_cycle)], Join())
        self.set_timer(JOIN_RETRANSMIT, self.join)

    def do_Welcome(self, sender, state, slot, decisions):
        self.acceptor_cls(self.node)
        self.replica_cls(self.node, execute_fn=self.execute_fn, peers=self.peers,
                         state=state, slot=slot, decisions=decisions)
        self.leader_cls(self.node, peers=self.peers, commander_cls=self.commander_cls,
                        scout_cls=self.scout_cls).start()
        self.stop()