2nd/10_Wiki/Topics/Coding/DB_Read_Replica_Patterns.md

id, title, category, status, source_trust_level, verification_status, created_at, updated_at, tags, tech_stack, applied_in, aliases

id	title	category	status	source_trust_level	verification_status	created_at	updated_at	tags	tech_stack	applied_in	aliases
db-read-replica-patterns	Read Replica — Replication Lag / 일관성	Coding	draft	B	conceptual	2026-05-09	2026-05-09	database replication read-replica consistency vibe-coding	language applicable_to SQL / Postgres / MySQL Backend		replication lag eventual consistency write-then-read read-your-writes

Read Replica

Primary 1대 + Replica N대. Read 분산 → Primary 부하↓. Replication lag (보통 ms~s) 이 함정. 방금 쓴 데이터 즉시 read 시 미반영 가능 — read-your-writes 패턴 필요.

📖 핵심 개념

• Async replication (보통): primary 가 commit 후 replica 로 stream.
• Replication lag: primary→replica 도달 시간.
• Read-your-writes: 자기가 쓴 건 자기가 읽을 때 보여야.
• Strong vs eventual: 모든 쿼리가 강할 필요 없음.

💻 코드 패턴

라우팅 — Prisma

import { PrismaClient } from '@prisma/client';

const writer = new PrismaClient({ datasources: { db: { url: process.env.PRIMARY_URL } } });
const reader = new PrismaClient({ datasources: { db: { url: process.env.REPLICA_URL } } });

async function getUserPosts(userId: string) {
  return reader.post.findMany({ where: { userId } }); // read = replica
}

async function createPost(input: NewPost) {
  return writer.post.create({ data: input }); // write = primary
}

Read-your-writes — sticky after write

class DbRouter {
  private lastWriteAt = 0;

  reader() {
    if (Date.now() - this.lastWriteAt < 2000) return writer; // 최근 2초 = primary
    return reader;
  }

  async write(fn: (db) => Promise<void>) {
    await fn(writer);
    this.lastWriteAt = Date.now();
  }
}

좀 더 정확 — LSN tracking (Postgres)

-- Primary 에서 commit 후 LSN 받음
SELECT pg_current_wal_lsn();

-- Replica 에서 검사
SELECT pg_last_wal_replay_lsn() >= 'X/Y'::pg_lsn AS caught_up;

async function readAfterWrite(query: () => Promise<R>): Promise<R> {
  const lsn = await writer.queryRaw('SELECT pg_current_wal_lsn() AS lsn');
  
  for (let i = 0; i < 10; i++) {
    const caught = await reader.queryRaw(
      `SELECT pg_last_wal_replay_lsn() >= '${lsn}'::pg_lsn AS ok`
    );
    if (caught.ok) return query.call(reader);
    await sleep(50);
  }
  return query.call(writer); // fallback
}

라우팅 — request scope

// Express middleware
app.use((req, res, next) => {
  req.db = req.method === 'GET' ? reader : writer;
  next();
});

트랜잭션 안 — primary 만

// 트랜잭션 내 read 도 primary — replica 는 다른 view 가능
await writer.$transaction(async (tx) => {
  const user = await tx.user.findUnique(...);   // primary
  await tx.post.create({ data: { userId: user.id, ...} });
});

Replication lag 모니터링

-- Postgres
SELECT now() - pg_last_xact_replay_timestamp() AS lag;

-- MySQL
SHOW REPLICA STATUS\G
-- Seconds_Behind_Source

알람: lag > 5s.

🤔 의사결정 기준

상황	라우팅
Write	Primary
즉시 read after write	Primary (2초 sticky)
일반 list / detail	Replica
분석 / 리포트	Replica (분리된 분석용)
트랜잭션 내 read	Primary (같은 connection)
Cache 가능	Cache 우선, 미스 시 replica

❌ 안티패턴

• 모든 read 를 무조건 replica: read-after-write 깨짐.
• 트랜잭션 안 read 를 replica: stale.
• Lag 모니터링 없음: 100s lag 도 모름.
• Replica failover 안 함: replica 1대 죽으면 모두 실패. health check + 다음 replica.
• Primary write 성공 → 그 자리에서 replica read: 거의 무조건 stale.
• GROUP BY count 같은 무거운 쿼리 primary: primary 부하. analytic replica 분리.

🤖 LLM 활용 힌트

• 기본 read = replica, write = primary.
• Read-your-writes = 2초 sticky 또는 LSN.
• Lag 모니터링 + alarm 필수.

🔗 관련 문서

• DB_Connection_Pooling_Patterns
• DB_Sharding_Strategies