Raft：etcd・Consul・TiDB の中核を支える分散合意アルゴリズム

import random
class RaftNode:    def __init__(self, node_id, n_peers):        self.id = node_id        self.term = 0        self.voted_for = None        self.peers = list(range(n_peers))        self.role = 'follower'
    def request_vote(self, candidate_id, candidate_term):        '''投票要求を受け取った時の判定'''        if candidate_term > self.term and self.voted_for is None:            self.term = candidate_term            self.voted_for = candidate_id            return True        return False
    def become_candidate(self):        '''選挙開始'''        self.role = 'candidate'        self.term += 1        self.voted_for = self.id  # 自分に投票
    def collect_votes(self, peers):        votes = 1  # 自票        for peer in peers:            if peer.request_vote(self.id, self.term):                votes += 1        return votes
# シナリオ: 5 ノードで 1 台が立候補random.seed(42)nodes = [RaftNode(i, 5) for i in range(5)]candidate = nodes[0]candidate.become_candidate()votes = candidate.collect_votes(nodes[1:])
print(f'Term {candidate.term}: candidate {candidate.id} got {votes}/5 votes')print(f'過半数 (3) 以上? {votes >= 3} → 勝者は {candidate.id}')

class RaftWithLog:    def __init__(self, node_id):        self.id = node_id        self.term = 0        self.log = []  # [(term, command), ...]        self.commit_index = -1
    def append_entries(self, leader_term, prev_log_index, prev_log_term, entries, leader_commit):        '''Leader からのログ複製要求'''        # 古い term は拒否        if leader_term < self.term:            return False
        # ログの整合性チェック        if prev_log_index >= 0:            if prev_log_index >= len(self.log):                return False  # 自分のログが短い            if self.log[prev_log_index][0] != prev_log_term:                return False  # term が合わない
        # ログ追加        self.log = self.log[:prev_log_index + 1] + entries
        # コミット        if leader_commit > self.commit_index:            self.commit_index = min(leader_commit, len(self.log) - 1)
        return True
# シナリオ: Leader → Follower にログ複製follower = RaftWithLog(1)follower.term = 5ok = follower.append_entries(    leader_term=5, prev_log_index=-1, prev_log_term=0,    entries=[(5, 'set x=1'), (5, 'set y=2')], leader_commit=1)print(f'AppendEntries OK: {ok}, log: {follower.log}, commit: {follower.commit_index}')