Deadlock occurs when two transactions are waiting for a lock on an item held by the other.

Example:

T1: Lw(A) R(A)            Lw(B) ......
T2:            Lw(B) R(B)       Lw(A) .....

How to deal with deadlock?

• prevent it happening in the first place
• let it happen, detect it, recover from it