---
id: db-read-replica-patterns
title: Read Replica — Replication Lag / 일관성
category: Coding
status: draft
source_trust_level: B
verification_status: conceptual
created_at: 2026-05-09
updated_at: 2026-05-09
tags: [database, replication, read-replica, consistency, vibe-coding]
tech_stack: { language: "SQL / Postgres / MySQL", applicable_to: ["Backend"] }
applied_in: []
aliases: [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
```ts
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
```ts
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)
```sql
-- 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;
```

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

### 트랜잭션 안 — primary 만
```ts
// 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
```sql
-- 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]]
