[G1-Sync] Manual knowledge update
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Antigravity Agent
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---
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id: db-transaction-patterns-deep
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title: DB Transaction Deep — isolation / SAVEPOINT / advisory lock
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category: Coding
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status: draft
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source_trust_level: B
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verification_status: conceptual
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created_at: 2026-05-09
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updated_at: 2026-05-09
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tags: [database, transaction, vibe-coding]
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tech_stack: { language: "SQL", applicable_to: ["Database"] }
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applied_in: []
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aliases: [transaction, isolation, savepoint, advisory lock, nested transaction, explicit lock]
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---
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# DB Transaction Deep
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> ACID 의 정밀 control. **Isolation, SAVEPOINT, advisory lock, FOR UPDATE**.
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## 📖 핵심 개념
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- ACID: Atomic, Consistent, Isolated, Durable.
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- Isolation level (4 가지).
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- SAVEPOINT = nested transaction.
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- Advisory lock = app-level.
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## 💻 코드 패턴
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### Basic transaction
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```sql
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BEGIN;
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UPDATE accounts SET balance = balance - 100 WHERE id = 1;
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UPDATE accounts SET balance = balance + 100 WHERE id = 2;
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COMMIT;
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-- 또는 ROLLBACK;
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```
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### Isolation levels
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```
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READ UNCOMMITTED: dirty read 가능 (Postgres 가 안 implement).
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READ COMMITTED: dirty read 안 (Postgres default).
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REPEATABLE READ: phantom 안 (range query OK).
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SERIALIZABLE: 가장 strict (slow).
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```
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```sql
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SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
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BEGIN;
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-- ...
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COMMIT;
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```
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### SAVEPOINT (nested)
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```sql
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BEGIN;
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INSERT INTO orders (...) VALUES (...);
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SAVEPOINT after_order;
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INSERT INTO items (...) VALUES (...);
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-- Error
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ROLLBACK TO after_order;
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-- 매 order 보존, items 만 rollback.
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COMMIT;
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```
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→ Partial rollback.
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### Explicit lock (FOR UPDATE)
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```sql
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BEGIN;
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SELECT * FROM accounts WHERE id = 1 FOR UPDATE;
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-- Other transaction 가 wait.
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UPDATE accounts SET balance = balance - 100 WHERE id = 1;
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COMMIT;
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```
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→ Exclusive row lock.
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### FOR UPDATE SKIP LOCKED (queue)
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```sql
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SELECT * FROM jobs
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WHERE status = 'pending'
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ORDER BY created_at
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LIMIT 1
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FOR UPDATE SKIP LOCKED;
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-- → Lock 된 row 가 skip. Multi-worker queue.
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```
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### FOR SHARE
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```sql
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SELECT * FROM users WHERE id = 1 FOR SHARE;
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-- → Shared lock. Read 가 OK, write 가 wait.
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```
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### Advisory lock (app-level)
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```sql
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-- Acquire
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SELECT pg_advisory_lock(12345);
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-- Critical section
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-- ...
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-- Release
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SELECT pg_advisory_unlock(12345);
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```
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→ Cross-transaction. Cron / migration 친화.
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### Transaction-scoped advisory
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```sql
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SELECT pg_advisory_xact_lock(12345);
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-- → Transaction 끝 시 자동 release.
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```
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### Distributed lock (Postgres)
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```ts
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async function withDistributedLock<T>(key: number, fn: () => Promise<T>): Promise<T> {
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const client = await pool.connect();
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try {
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await client.query('SELECT pg_advisory_lock($1)', [key]);
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return await fn();
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} finally {
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await client.query('SELECT pg_advisory_unlock($1)', [key]);
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client.release();
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}
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}
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```
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→ Single Postgres = 작은 system 의 distributed lock.
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### Optimistic locking (version)
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```sql
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-- Version column
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UPDATE users
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SET name = 'Bob', version = version + 1
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WHERE id = 1 AND version = 5;
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-- Affected rows = 0 → conflict.
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```
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→ Lock 없음. Retry on conflict.
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### Read-write split (transaction)
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```ts
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// Read replica
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const reader = pool.replica;
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const writer = pool.primary;
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// Write transaction
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await writer.transaction(async (tx) => {
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await tx.query('UPDATE ...');
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});
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// Read after write 의 함정:
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// Write tx commit → replica 에 lag.
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// → Read 가 stale.
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```
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→ Sticky reads.
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### Long transaction 의 함정
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```sql
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BEGIN;
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SELECT * FROM users LIMIT 1;
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-- 1 hour 후
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COMMIT;
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```
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→ Vacuum 가 freeze X. Bloat 누적.
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```sql
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-- Auto kill
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SET statement_timeout = '30s';
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SET idle_in_transaction_session_timeout = '60s';
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```
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### Deadlock
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```
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Tx A: lock row 1, wait row 2.
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Tx B: lock row 2, wait row 1.
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→ Postgres detect + abort 1.
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```
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```ts
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try {
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await tx.query(...);
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} catch (e) {
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if (e.code === '40P01') {
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// Deadlock — retry.
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}
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}
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```
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### Deadlock 방지
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```
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- 모든 transaction 가 같은 order 로 lock.
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- Short transaction.
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- 최소 lock.
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```
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### Two-phase commit (XA, 2PC)
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```sql
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PREPARE TRANSACTION 'tx-1';
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-- 다른 system 도 PREPARE.
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-- Coordinator 가 모두 OK.
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COMMIT PREPARED 'tx-1';
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-- 또는
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ROLLBACK PREPARED 'tx-1';
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```
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→ Cross-DB. 매우 느린. 거의 안 사용.
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→ [[Backend_Saga_Choreography_vs_Orchestration]] 가 modern.
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### Idempotent transaction
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```sql
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INSERT INTO transactions (id, ...) VALUES ($idempKey, ...)
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ON CONFLICT (id) DO NOTHING;
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-- Retry-safe.
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```
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### Outbox pattern
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```sql
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BEGIN;
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INSERT INTO orders (...);
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INSERT INTO outbox (event_type, payload) VALUES ('OrderCreated', ...);
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COMMIT;
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-- Background process: outbox → publish.
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```
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→ DB write + event 가 atomic.
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→ [[Backend_Outbox_Pattern]].
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### MVCC (Multi-Version Concurrency Control)
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```
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Postgres / MySQL InnoDB:
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- 매 row 가 version 가 가짐.
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- Read 가 lock X (snapshot).
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- Write 가 새 version.
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→ "Reader 가 writer 가 막지 않음".
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```
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→ [[CS_MVCC_Concurrency]].
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### Connection pool + transaction
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```ts
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// 매 transaction 가 dedicated connection.
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const client = await pool.connect();
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try {
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await client.query('BEGIN');
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await client.query('UPDATE ...');
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await client.query('COMMIT');
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} finally {
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client.release();
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}
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// 또는 helper
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await pool.transaction(async (tx) => {
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await tx.query('UPDATE ...');
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});
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```
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### PgBouncer 함정 (transaction mode)
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```
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PgBouncer transaction mode:
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- 매 transaction = 다른 connection.
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- Session-bound feature 안 됨 (LISTEN, prepared, advisory lock).
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→ Session mode 또는 raw query.
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```
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→ [[DB_Connection_Pooling_Patterns]].
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### Phantom read (REPEATABLE READ 가 막음)
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```
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Tx A: SELECT count(*) FROM users WHERE age > 18 → 100.
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Tx B: INSERT new user (age 20).
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Tx A: SELECT count(*) → 101 (REPEATABLE READ X) / 100 (REPEATABLE READ).
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```
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### Serializable conflict
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```sql
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-- Postgres detect + abort.
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ERROR: could not serialize access due to concurrent update
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SQLSTATE: 40001
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```
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→ Retry on 40001.
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### Performance
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```
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SERIALIZABLE: 가장 slow.
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REPEATABLE READ: 약간 slow.
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READ COMMITTED: default + 빠름.
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→ Use case 의 적절.
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```
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### Real-world tip
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```
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1. Default = READ COMMITTED.
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2. Transfer / financial = SERIALIZABLE 또는 explicit lock.
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3. Long read = SET TRANSACTION READ ONLY.
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4. Idempotency key 매 critical write.
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5. Optimistic lock + version (web app).
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6. Advisory lock (distributed lock).
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```
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### Distributed transaction (saga)
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```
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Cross-DB / cross-service:
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- 2PC 안 사용 (slow, brittle).
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- Saga + compensation.
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- Idempotent step.
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```
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→ [[Backend_Saga_Choreography_vs_Orchestration]].
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### 함정
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```
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- Long transaction: bloat / lock contention.
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- Implicit transaction: ORM 의 auto-commit 무시.
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- Lock order 다름: deadlock.
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- Retry 무한: 내려.
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- 2PC: 느린, 거의 안 사용.
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```
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## 🤔 의사결정 기준
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| 작업 | 추천 |
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| Read | Default isolation |
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| Critical write | SERIALIZABLE / FOR UPDATE |
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| Queue | FOR UPDATE SKIP LOCKED |
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| Web app | Optimistic + version |
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| Distributed lock | pg_advisory_lock |
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| Cross-service | Saga (NOT 2PC) |
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| Idempotent | Idempotency key |
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## ❌ 안티패턴
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- **Long transaction**: bloat.
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- **2PC**: brittle.
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- **No retry on serialize fail**: lost.
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- **모든 거 SERIALIZABLE**: slow.
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- **Lock order inconsistent**: deadlock.
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- **No timeout**: hang.
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## 🤖 LLM 활용 힌트
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- READ COMMITTED 가 default.
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- FOR UPDATE SKIP LOCKED 가 queue 의 답.
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- Advisory lock 가 app-level.
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- Saga > 2PC.
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## 🔗 관련 문서
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- [[DB_Transaction_Isolation]]
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- [[DB_Lock_Analysis]]
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- [[CS_MVCC_Concurrency]]
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