In concurrency control for databases and operating systems, what is time stamping (timestamp based protocol) and how is it used to order transactions?

Introduction / Context:
When multiple transactions or processes access shared data concurrently, the system must maintain consistency while still providing good performance. One approach, in both database management systems and some operating systems, is timestamp based concurrency control. This question checks whether you understand that time stamping is about assigning logical time values to transactions to control the allowed order of operations, not about hardware clocks, compression or backup strategies.

Given Data / Assumptions:

• Several transactions may read and write the same data items concurrently.

• The system must avoid conflicts that would break consistency, such as lost updates or dirty reads.

• Each transaction can be associated with a logical timestamp that represents its order in a serial schedule.

• The question focuses on the conceptual role of time stamping in concurrency control.

Concept / Approach:
Timestamp based concurrency control protocols assign a unique monotonically increasing timestamp to each transaction when it starts. The system keeps, for each data item, the timestamps of the latest read and write operations. When a transaction attempts a read or a write, its own timestamp is compared with these stored timestamps to decide whether the operation is allowed, should be delayed or should cause the transaction to roll back. In this way, the actual concurrent execution is forced to be equivalent to some serial order based on timestamps, preserving serializability.

Step-by-Step Solution:
Step 1: Recall that a timestamp is a logical or physical time value that can be used to order transactions. Step 2: In a timestamp ordering protocol, each transaction T is given a timestamp TS(T) when it begins. Step 3: For each data item X, the system maintains read_TS(X) and write_TS(X) that record the timestamps of the latest read and write of X. Step 4: When a transaction tries to read or write X, the protocol compares TS(T) with read_TS(X) and write_TS(X). If the operation would violate the desired order, the transaction may be rolled back and restarted with a new timestamp. Step 5: Choose the option that explicitly mentions assigning unique timestamps and using them to impose a serializable order on reads and writes.

Verification / Alternative check:
You can verify the understanding by imagining two transactions that both want to update the same bank account. If T1 has an earlier timestamp than T2, then any schedule produced by the timestamp protocol should appear as if T1 executed completely before T2. Any access by T2 that would make it appear earlier than T1 is rejected or rolled back. This idea of enforcing a logical serial order is the core purpose of timestamp based control, so any correct explanation should refer to this property in some form.

Why Other Options Are Wrong:
Option B is wrong because time stamping in concurrency control is not a compression method and it certainly does not store only time values instead of full records. Option C is wrong because it describes a backup policy, which may also refer to time but has nothing to do with ordering reads and writes of transactions. Option D is wrong because it talks about hardware features for raising CPU clock speed, which is unrelated to the logical timestamps used by database or operating system schedulers to enforce serializability.

Common Pitfalls:
A common pitfall is to assume that timestamp protocols always use physical clock times, which can lead to confusion about clock synchronization. In many systems, the timestamps are simply logical counters that increase for each new transaction and do not depend on real time. Another mistake is to think that time stamping only orders transaction start times, whereas in reality the protocol uses timestamps to validate each individual read and write operation. Some learners also mix up timestamp ordering with locking protocols, but in pure timestamp schemes there are no traditional locks; instead, access is accepted or rejected based on comparison with previously recorded timestamps.

Final Answer:
Thus, time stamping is a method that assigns a unique increasing timestamp to each transaction and uses these timestamps to order reads and writes so that the schedule is equivalent to a serial order.