Understand what deadlocks are and how they are detected and handled.
Deadlock is a critical problem that can arise in systems that use lock-based concurrency control. A deadlock is a state in which two or more competing transactions are waiting for each other to release the locks that they hold. Because all transactions are waiting, none of them can proceed, and they will wait indefinitely, effectively freezing a part of the system. A classic deadlock scenario involves two transactions, T1 and T2, and two data items, A and B. T1 acquires an exclusive lock on A and then requests a lock on B. At the same time, T2 acquires an exclusive lock on B and then requests a lock on A. Now, T1 is waiting for T2 to release its lock on B, and T2 is waiting for T1 to release its lock on A. Neither can proceed. DBMSs handle deadlocks primarily in two ways: deadlock prevention and deadlock detection. Deadlock prevention involves protocols that ensure a deadlock can never occur, for instance, by requiring transactions to acquire all their locks at once or by imposing a specific order for acquiring locks. These methods can be restrictive and reduce concurrency. A more common approach is deadlock detection and recovery. The DBMS periodically checks for deadlocks, often by building a 'wait-for graph' where nodes are transactions and an edge from T1 to T2 means T1 is waiting for T2. If a cycle is detected in this graph, a deadlock exists. The system then resolves the deadlock by selecting a 'victim' transaction, aborting it (rolling it back), and releasing its locks, allowing the other transactions to proceed.