다중 마스터 복제

-- Galera Cluster (MySQL Multi-Master) 설정 예시
-- wsrep.cnf
[mysqld]
wsrep_on=ON
wsrep_cluster_name="my_cluster"
wsrep_cluster_address="gcomm://node1,node2,node3"
wsrep_node_name="node1"
wsrep_provider=/usr/lib/libgalera_smm.so

-- 모든 노드에서 쓰기 가능
-- Node 1에서
INSERT INTO users (name) VALUES ('Alice');

-- Node 2에서 (동시)
INSERT INTO users (name) VALUES ('Bob');

-- 충돌 발생 시나리오
-- Node 1: UPDATE products SET stock = 10 WHERE id = 1;
-- Node 2: UPDATE products SET stock = 5 WHERE id = 1;
-- Last Write Wins 또는 애플리케이션 정의 로직 필요

# Python: 충돌 감지 및 재시도 패턴
def update_with_conflict_retry(product_id, new_stock, max_retries=3):
    for attempt in range(max_retries):
        try:
            with db.begin():
                product = db.query("SELECT * FROM products WHERE id = %s FOR UPDATE", product_id)
                # 낙관적 락: version 체크
                db.execute("""
                    UPDATE products
                    SET stock = %s, version = version + 1
                    WHERE id = %s AND version = %s
                """, (new_stock, product_id, product['version']))
                if db.rowcount == 0:
                    raise ConflictError("Version mismatch")
            return True
        except ConflictError:
            time.sleep(0.1 * (2 ** attempt))
    raise Exception("Max retries exceeded")