Assignment: Transaction Management | Database Management System (DBMS) - Software Development PDF Download

Multiple Choice Questions (MCQ)

Q.1. Which of the following is the correct definition of a transaction in a database management system?
(a) A single SQL statement executed on a database
(b) A set of SQL statements executed together as a unit
(c) A single data manipulation operation
(d) A single data definition operation

Ans. (b)

Q.2. In a database system, the ACID properties of a transaction are used to ensure:
(a) Atomicity, Correctness, Integrity, Durability
(b) Atomicity, Consistency, Isolation, Durability
(c) Accuracy, Completeness, Isolation, Durability
(d) Accuracy, Consistency, Integration, Durability

Ans. (b)

Q.3. Which of the following is a characteristic of the serializability of transactions in a database system?
(a) Transactions can be executed concurrently without any conflicts
(b) Transactions can be interleaved in any order without affecting the final result
(c) Transactions can be rolled back if a conflict occurs
(d) Transactions can be scheduled in a way that produces the same result as if they were executed serially

Ans. (d)

Q.4. In a database system, a lock is acquired on a data item during a transaction to:
(a) Ensure atomicity of the transaction
(b) Ensure isolation among multiple transactions
(c) Prevent unauthorized access to the data item
(d) Ensure durability of the data item

Ans. (b)

Q.5. Which of the following concurrency control techniques allows transactions to read uncommitted data?
(a) Strict two-phase locking
(b) Optimistic concurrency control
(c) Multi-version concurrency control
(d) Two-phase locking with shared locks

Ans. (b)

Higher Order Thinking Questions

Q.1. Explain the concept of a transaction in a database management system. What are the ACID properties and why are they important?

A transaction in a database management system is a logical unit of work that consists of one or more database operations, such as read, write, or modify. The ACID properties (Atomicity, Consistency, Isolation, and Durability) ensure the reliability and consistency of transactions. Atomicity guarantees that a transaction is treated as a single unit of work, meaning that either all the operations within the transaction are completed successfully, or none of them are. Consistency ensures that a transaction brings the database from one consistent state to another. Isolation ensures that concurrent transactions do not interfere with each other. Durability guarantees that once a transaction is committed, its effects are permanent and will survive any subsequent failures.

Q.2. What is a schedule in the context of transaction management? Provide an example of a schedule and explain how it ensures serializability.

In the context of transaction management, a schedule is an ordering of the operations of multiple transactions. For example:

T1: Read(A), Write(B)

T2: Write(A), Read(B)

This schedule ensures serializability by maintaining strict precedence between conflicting operations. In this case, the Write(B) operation of T1 conflicts with the Read(B) operation of T2. To ensure serializability, the schedule must preserve the order of these conflicting operations. If T1's Write(B) operation is allowed to execute before T2's Read(B) operation, the final result may be different from what would be obtained if the transactions were executed serially.

Q.3. Describe the two-phase locking protocol used for concurrency control in database systems. What are the advantages and disadvantages of this protocol?

The two-phase locking protocol is a concurrency control technique used in database systems. It consists of two phases:

• The "growing" phase: During this phase, a transaction can acquire locks but cannot release any locks.
• The "shrinking" phase: During this phase, a transaction can release locks but cannot acquire any new locks.

Advantages of the two-phase locking protocol include ensuring serializability of transactions and preventing conflicts among concurrent transactions. However, it may suffer from the problem of deadlock and may lead to delays in acquiring locks, resulting in lower concurrency and performance.

Q.4. Discuss the concept of deadlock in transaction management. Explain the necessary conditions for a deadlock to occur and describe two commonly used techniques for deadlock detection and resolution.

Deadlock in transaction management occurs when two or more transactions are waiting indefinitely for each other to release resources, leading to a state where no transaction can proceed.
The necessary conditions for a deadlock are:

• Mutual exclusion: Resources can only be used by one transaction at a time.
• Hold and wait: Transactions hold resources while waiting for others.
• No preemption: Resources cannot be forcibly taken from a transaction.
• Circular wait: There is a circular chain of transactions waiting for resources.

Two commonly used techniques for deadlock detection and resolution are:

• Deadlock detection: This involves periodically checking the system for the presence of deadlocks. Once a deadlock is detected, appropriate actions can be taken to resolve it, such as aborting one or more transactions or rolling them back to a safe state.
• Deadlock prevention: This involves implementing strategies to ensure that at least one of the necessary conditions for deadlock does not hold. For example, by enforcing a policy that avoids circular wait or by employing a resource allocation algorithm that guarantees the absence of deadlock.

Q.5. Explain the difference between pessimistic and optimistic concurrency control. Provide examples of situations where each approach would be suitable.

Pessimistic concurrency control involves acquiring locks on data items before performing any operations, assuming that conflicts will occur and transactions need to wait for their turn to access the data. This approach is suitable when conflicts are frequent and the cost of rollbacks or aborts is high. For example, in a banking system where multiple transactions may attempt to withdraw funds from the same account simultaneously, pessimistic concurrency control can prevent incorrect account balances.

Optimistic concurrency control, on the other hand, assumes that conflicts are rare and allows transactions to proceed without acquiring locks initially. Conflicts are checked at the time of committing the transaction, and if conflicts are detected, appropriate actions are taken to resolve them. This approach is suitable when conflicts are infrequent, and the cost of checking for conflicts at the end of the transaction is lower than acquiring and releasing locks throughout the transaction. For example, in a system where multiple users can edit different sections of a document simultaneously, optimistic concurrency control can provide better concurrency and responsiveness.

Fill in the Blanks

1. A transaction is a ________________ of operations that must be executed as a unit.

sequence

2. The __________ properties of a transaction ensure its reliability and consistency.

ACID

3. In a database system, a __________ is a mechanism to prevent conflicts among concurrent transactions.

lock

4. _______________ is a technique used to ensure that transactions are executed in a serializable order.

Serializability

5. A _______________ occurs when two or more transactions are waiting indefinitely for each other to release resources.

deadlock

True/False

1. A transaction can consist of multiple SQL statements executed together.

True

2. The ACID properties of a transaction include Atomicity, Consistency, Isolation, and Durability.

True

3. In multi-version concurrency control, transactions read the latest committed version of data items.

False

4. The two-phase locking protocol guarantees deadlock-free execution of transactions.

True

5. Optimistic concurrency control is more suitable for systems with a high read-to-write ratio.

False

Hands-On Questions

Q.1. Consider the following schedule of transactions:

• T1: Read(A), Write(A), Write(B)
• T2: Read(B), Write(A), Read(A)

Identify any conflict operations and indicate if the schedule is conflict-serializable.

The conflict operations in the given schedule are:

• T1 Write(A) and T2 Read(A)
• T1 Write(B) and T2 Read(B)

The schedule is conflict-serializable because the order of the conflict operations can be rearranged without changing the final result.

Q.2. Using the two-phase locking protocol, show the locks acquired and released for the following schedule of transactions:

• T1: Read(A), Write(B)
• T2: Write(A), Read(B)

Locks acquired and released for the given schedule:

• T1: Read(A), Write(B)
• T2: Write(A), Read(B)

T1 acquires a shared lock on A (S(A)), releases the lock on A, acquires an exclusive lock on B (X(B)).

T2 acquires an exclusive lock on A (X(A)), releases the lock on A, acquires a shared lock on B (S(B)).

Q.3. Perform a schedule analysis to determine if the following schedule is conflict-serializable:

• T1: Read(A), Write(B)
• T2: Write(A), Read(B)

The given schedule is conflict-serializable because the order of operations can be rearranged without changing the final result. The conflict operations are:

• T1 Write(B) and T2 Read(B)
• T1 Write(A) and T2 Read(A)

Q.4. Explain the concept of optimistic concurrency control and its implementation using validation.

Optimistic concurrency control is a technique where transactions are allowed to execute without acquiring locks initially. Instead of blocking transactions, potential conflicts are detected at the time of committing the transaction. This is done through validation, where the database system checks if the changes made by a transaction conflict with the changes made by other concurrent transactions. If conflicts are detected, appropriate actions are taken to resolve them, such as aborting the transaction or restarting it.

Q.5. Consider a database system that uses strict two-phase locking for concurrency control. Describe the potential issues that may arise due to lock contention and how these issues can be mitigated.

In a database system that uses strict two-phase locking, lock contention can occur when multiple transactions try to acquire conflicting locks on the same data items. This can result in delays and reduced concurrency.
To mitigate lock contention, the following techniques can be employed:

• Use lock escalation: If a transaction needs to acquire a large number of locks, the system can escalate the locks to a higher level, reducing the overall number of locks.
• Use lock timeouts: If a transaction cannot acquire a lock within a specified time limit, it can be aborted or rolled back to free up the locks for other transactions.
• Use deadlock detection and resolution: By periodically checking for deadlocks and resolving them promptly, the system can avoid situations where transactions are waiting indefinitely for locks.
• Optimize data access patterns: Analyze and optimize the queries and data access patterns to minimize the number of locks required and reduce contention.