Transaction usage

What is a Transaction?

• A transaction is a group of operations that must be treated as one unit.
• In database terms, it follows ACID:
  • Atomicity → all succeed OR all fail
  • Consistency → data stays correct
  • Isolation → concurrent transactions don’t break each other
  • Durability → once committed, data is permanent

Why Transaction Alone Is NOT Enough to solve high concurrency ?

• Because under very high concurrency:
  • DB locks become bottlenecks
  • too many transactions slow DB
  • row lock contention increases
  • deadlocks appear
  • DB CPU explodes
• Example : 100k users claiming coupon. If every request directly:
  1. begin transaction
  2. query stock
  3. update stock
  4. insert records
  5. commit
• Database may collapse.

What is @Transactional?

• @Transactional is implemented by AOP to automatically manage transaction lifecycle for business methods.

  @Service
  public class CouponService {
  
      @Transactional
      public void claimCoupon(Long userId, Long campaignId) {
          // 1. check stock
          // 2. deduct stock
          // 3. create user coupon
          // 4. create claim record
      }
  }

  • Spring automatically start transaction before method, commit if success or rollback if exception.

• How it actually works (very important for debugging)

  • Step 1: Spring creates a proxy object —— YourService → Proxy → Real Method
  • Step 2: AOP intercepts method call —— call method() → [AOP intercept] → start transaction → execute method → success>commit/error>rollback

• Transaction only works on PUBLIC methods

• Self-call will FAIL (classic bug) —— because AOP proxy is bypassed

  public void A() {
      B(); // ❌ transaction NOT applied
  }
  
  @Transactional
  public void B() {}

• Default rollback only for RuntimeException