Timestamp-based protocols in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the purpose of timestamp-based protocols

What is the main goal of using timestamp-based protocols in database management systems?

A To ensure transactions are executed in the order of their timestamps to maintain serializability

B To lock data items before transactions access them to prevent conflicts

C To allow transactions to execute without any order restrictions for maximum concurrency

D To prioritize transactions based on their size rather than their start time

💡 Hint

Think about how timestamps help maintain a consistent order of transactions.

Timestamp-based protocols assign each transaction a unique timestamp and ensure that conflicting operations follow the timestamp order. This maintains serializability without using locks.

📋 Factual

intermediate

Timestamp components in protocols

Which two timestamps are maintained for each data item in a timestamp-based concurrency control protocol?

A Lock timestamp and unlock timestamp

B Start timestamp and commit timestamp

C Read timestamp and write timestamp

D Access timestamp and release timestamp

💡 Hint

Consider timestamps related to reading and writing operations on data items.

Each data item keeps track of the largest timestamp of any transaction that has successfully read it (read timestamp) and the largest timestamp of any transaction that has successfully written it (write timestamp).

🔍 Analysis

advanced

Result of conflicting operations in timestamp protocols

Consider two transactions T1 and T2 with timestamps TS(T1) = 5 and TS(T2) = 10. If T2 tries to write a data item that T1 has already read, what will happen under a timestamp-based protocol?

A T2 waits until T1 commits

B T1 is rolled back because it started earlier

C Both transactions proceed without interruption

D T2 is rolled back because it violates the timestamp order

💡 Hint

Recall that in timestamp ordering, a younger transaction can write an item read by an older transaction.

In timestamp-based protocols, if a younger transaction (higher timestamp) tries to write a data item previously read by an older transaction (lower timestamp), the write is allowed because it follows the timestamp order (older read before younger write). Thus, both transactions proceed.

❓ Comparison

advanced

Difference between basic timestamp protocol and Thomas' write rule

How does Thomas' write rule improve upon the basic timestamp ordering protocol?

A It assigns timestamps dynamically during transaction execution

B It allows some write operations to be ignored if they are obsolete, increasing concurrency

C It introduces locking to prevent conflicts between transactions

D It enforces stricter ordering by aborting more transactions

💡 Hint

Consider how ignoring certain writes can help reduce unnecessary rollbacks.

Thomas' write rule allows a write operation to be ignored if the data item has already been written by a newer transaction, thus avoiding unnecessary aborts and improving concurrency.

❓ Reasoning

expert

Determining the number of possible serial schedules

Given 3 transactions T1, T2, and T3, each with unique timestamps, how many possible serial schedules are there that respect the timestamp order in a timestamp-based protocol?

A 1

B 3

C 6

D 0

💡 Hint

Think about how timestamp ordering restricts the order of transactions.

Timestamp-based protocols enforce a strict order based on timestamps, so only one serial schedule is possible that respects this order.