What is the two-phase locking protocol? How does it guarantee serializability?
What are some variations of the two-phase locking protocol? Why is strict or rigorous two-phase locking often preferred?
Discuss the problems of deadlock and starvation, and the different approaches to dealing with these problems.
Compare binary locks to exclusive/shared locks. Why is the latter type of locks preferable?
The Correct Answer and Explanation is :
The two-phase locking protocol (2PL) is a concurrency control mechanism used in databases to ensure transaction serializability. It works by requiring that a transaction acquire all the necessary locks before it starts releasing any locks. The protocol is divided into two phases:
- The Growing Phase: A transaction can acquire locks but cannot release any locks.
- The Shrinking Phase: A transaction can release locks but cannot acquire any new locks.
By following this protocol, the system ensures that no other transaction can access data being modified by the current transaction until it is completed, thereby preventing conflicts.
Guaranteeing Serializability: The two-phase locking protocol guarantees serializability by ensuring that transactions are executed in a way that results in a serial order of execution (i.e., no interleaving of conflicting operations). Since no transaction can release a lock until it has finished acquiring all the locks it needs, transactions that conflict are forced to execute in a non-interfering sequence, preventing non-serializable executions.
Variations of the Two-Phase Locking Protocol:
- Strict Two-Phase Locking: A stricter form where a transaction holds all of its locks until it commits or aborts. This ensures that no other transaction can access any data until the current one completes.
- Rigorous Two-Phase Locking: This variation enforces strict two-phase locking but with additional controls to ensure that no transaction can release locks until it has committed or aborted.
Why Strict or Rigorous Two-Phase Locking is Preferred: These variations are preferred because they avoid cascading rollbacks (where one transaction’s failure causes others to fail) and guarantee that no transaction can read uncommitted data. This ensures consistency in the system, as transactions only interact with fully committed data.
Deadlock and Starvation:
- Deadlock occurs when two or more transactions are waiting for each other to release locks, causing a cycle where no transaction can proceed. Techniques to handle deadlock include timeout-based approaches, wait-for graphs, and deadlock detection algorithms.
- Starvation happens when a transaction is perpetually delayed because other transactions continue to acquire the necessary resources. This can be addressed by using fair lock acquisition policies, such as a priority queue or aging mechanisms, to prevent indefinite blocking of a transaction.
Binary Locks vs. Exclusive/Shared Locks:
- Binary Locks are simple locks with only two states: locked or unlocked. They are easy to implement but not very flexible for transactions that need to read data without modifying it.
- Exclusive/Shared Locks are more advanced. Exclusive locks prevent other transactions from accessing the locked resource, while shared locks allow multiple transactions to read but not write to the resource.
Why Shared/Exclusive Locks are Preferable: Shared/exclusive locks offer more flexibility and improve concurrency. By allowing multiple transactions to share a lock when reading, they reduce the need for full exclusive locking, which can lead to unnecessary blocking and reduced system throughput.