What is Deadlock?
Like two people blocking each other in a narrow hallway - neither can move forward.
Deadlock Example:
Transaction T3: Transaction T4:
1. Lock-X(B) ✓ 1. Lock-S(A) ✓
2. Read(B) 2. Read(A)
3. Lock-X(A) ⏳ WAIT 3. Lock-S(B) ⏳ WAIT
T3 waits for A (held by T4), T4 waits for B (held by T3) = DEADLOCK!
Solution: Roll back one transaction to break the cycle.