All questions of Transaction & Concurrency Control for Computer Science Engineering (CSE) Exam

Both schedules S1 and S2 are not conflict serializable because they contain conflicting operations.


S1 : r1(x) r1(y) r2(x) r2(y) w2(y) w1(x)


A schedule is conflict serializable if its precedence graph is acyclic. The precedence graph can be constructed by considering the conflicting operations in the schedule.





From the precedence graph, it is clear that there is a cycle present in the graph (r1(x) -> w1(x) -> r1(y) -> r2(y) -> w2(y) -> r2(x) -> r1(x)). Therefore, schedule S1 is not conflict serializable.


S2 : r1(x) r2(x) r2(y) w2(y) r1(y) w1(x)





From the precedence graph, it is clear that there is no cycle present in the graph. Therefore, schedule S2 is conflict serializable.


Based on the analysis, the correct option is (b) S1 is not conflict serializable, and S2 is conflict serializable.

Explanation:

Non-serial interleaving:
- In the given transaction T1 and T2, the operations are interleaved in a non-serial manner, meaning they are executed concurrently or in an order different from the original sequence.

Conflict Serializable Schedule:
- A schedule is said to be conflict serializable if it is equivalent to a serial schedule, meaning the final result is the same as if the transactions were executed in serial order.
- In this case, the given non-serial interleaving of T1 and T2 does not result in a conflict serializable schedule.

Precedence Graph:
- In conflict serializability, a precedence graph can be drawn to determine if the schedule is conflict serializable.
- However, in this case, a precedence graph cannot be drawn for the given schedule due to conflicting operations.

Conclusion:
- The non-serial interleaving of T1 and T2 leads to a schedule that is not conflict serializable. The conflicting operations and lack of a precedence graph indicate that the schedule does not maintain the same final result as a serial execution.

Yes, if a schedule S can be transformed into a schedule S' by a series of valid operations, then it is possible to transform S into S'. This transformation can involve rearranging the order of operations, rescheduling activities, or making adjustments to the timing or duration of tasks. The specific transformations will depend on the constraints and requirements of the schedule, but as long as they maintain the overall integrity and consistency of the schedule, it is possible to transform S into S'.

Explanation:

Durability:
Durability is one of the ACID properties of transactions in database systems. It ensures that once a transaction is committed, the changes made by that transaction persist in the database even in the event of a system failure. This means that the changes must be permanent and cannot be lost or undone due to any kind of failure.

Explanation of the given statement:
The statement "The changes applied to the database by a committed transaction must persist in the database, and these changes must not be lost because of any failure" perfectly describes the durability property of transactions. It emphasizes the importance of ensuring that the changes made by a committed transaction are durable and remain in the database even if there is a system failure.

Importance of Durability:
- Durability is crucial for maintaining the integrity and consistency of the database.
- It ensures that the database remains in a consistent state even after unexpected events such as power outages or hardware failures.
- Without durability, users may experience data loss or inconsistencies in the database, leading to potential errors and loss of trust in the system.
In conclusion, durability is a critical aspect of transaction processing in databases, as it guarantees that the changes made by committed transactions are permanent and will not be lost due to any failure.

Explanation:

Conflict Serializable Schedules:
- Conflict serializability is a property of a schedule that ensures that the final result of the schedule is the same as the result of some serial execution of the transactions in the schedule.
- A schedule is conflict-serializable if it is equivalent to a serial schedule of the same transactions, where the order of non-conflicting operations is preserved.

Checking for Conflict Serializability:
To determine if a schedule is conflict-serializable, we can use the precedence graph method. We draw a directed edge from Ti to Tj if there is a conflicting operation between Ti and Tj in the schedule.

Analysis:
- For schedule S1:
- The precedence graph for S1 is T1 -> T3, T3 -> T2, T2 -> T1.
- As there is a cycle in the precedence graph, the schedule S1 is not conflict-serializable.
- For schedule S2:
- The precedence graph for S2 is T1 -> T3, T3 -> T2, T2 -> T1.
- As there is a cycle in the precedence graph, the schedule S2 is not conflict-serializable.
Therefore, only schedule S1 is conflict-serializable, according to the precedence graph method.

In order to provide a complete response, the statement seems to be incomplete. Could you please provide the full statement for the transaction involving bank accounts x and y?

T2, and T3. Let's assume that T1 and T2 are executed concurrently, and T3 is executed after T2.

A serial schedule for the given transactions would be T1, T2, T3. In this schedule, T1 is executed first, followed by T2, and finally T3. Each transaction is executed in its entirety before the next transaction begins. This ensures that the transactions are executed one after the other, without any concurrency.

On the other hand, a concurrent schedule for the given transactions could be T1, T2, T3. In this schedule, T1 and T2 are executed concurrently, meaning that their operations can be interleaved. For example, T1 may perform some operations, then T2 may perform some operations, and so on. Finally, T3 is executed after T2 completes.

It is important to note that there can be multiple possible concurrent schedules for a given set of transactions, depending on how their operations are interleaved. Concurrent schedules can provide benefits such as improved performance and resource utilization but may also introduce concurrency-related issues such as conflicts and consistency problems.

The correct answer is option 'A' - Serializability order.

Explanation:
A serializability order is a linear order of transactions that is consistent with the partial order of the precedence graph. To understand this, let's first define some terms:

1. Transactions: In databases, a transaction is a logical unit of work that consists of one or more database operations, such as reads and writes.

2. Precedence graph: In concurrency control, a precedence graph is a directed graph that represents the order in which transactions are executed based on their dependencies. Each transaction is represented by a node, and there is an edge from transaction T1 to transaction T2 if T1 needs to be completed before T2 can start.

Now, let's break down the options and understand why option 'A' is the correct answer:

a) Serializability order:
- A serializability order is a linear order of transactions that is consistent with the partial order of the precedence graph.
- It represents a valid execution sequence of the transactions, where each transaction is executed one after another in a consistent order.
- Finding a serializability order ensures that the database remains in a consistent state throughout the execution of transactions.
- This order is important for ensuring isolation and maintaining the ACID properties of the database.

b) Direction graph:
- The term "Direction graph" is not a commonly used term in the context of databases or transaction processing.
- It is likely a distractor option and does not relate directly to the question or the concept of finding a linear order consistent with the partial order of the precedence graph.

c) Precedence graph:
- The precedence graph represents the order in which transactions are executed based on their dependencies.
- It helps determine conflicts and ensures that transactions are executed in a valid order.
- While the precedence graph is used to find the serializability order, it is not the answer itself.

d) Scheduling scheme:
- A scheduling scheme refers to an algorithm or strategy used to determine the order in which transactions are executed.
- While a scheduling scheme can consider the precedence graph and aim for a serializability order, it is not the direct answer to the question.

In conclusion, the correct answer is option 'A' - Serializability order because it directly relates to finding a linear order consistent with the partial order of the precedence graph, ensuring a valid execution sequence of the transactions.

Isolation

The property of isolation in database transactions ensures that concurrent transactions do not interfere with each other, creating the illusion that only one transaction is executed in the system, even though multiple transactions may be executed in parallel. It provides a way to maintain the integrity and consistency of the data in a multi-user environment.

Concurrency Control

Concurrency control is the mechanism used to manage concurrent access to the database by multiple transactions. It ensures that the execution of transactions is properly synchronized to avoid conflicts and maintain data consistency.

Isolation Levels

There are different isolation levels defined in database systems to control the level of isolation provided by transactions. The most commonly used isolation levels are:

1. Read Uncommitted: This is the lowest isolation level where transactions can read uncommitted changes made by other transactions. It does not provide any isolation and can lead to dirty reads, non-repeatable reads, and phantom reads.

2. Read Committed: In this isolation level, transactions can only read committed data. It avoids dirty reads but can still lead to non-repeatable reads and phantom reads.

3. Repeatable Read: This isolation level guarantees that a transaction will see a consistent snapshot of the database throughout its execution. It prevents dirty reads and non-repeatable reads, but phantom reads can still occur.

4. Serializable: This is the highest isolation level that provides full isolation between transactions. It ensures that transactions are executed as if they were executed one after another, in a serial manner. It prevents dirty reads, non-repeatable reads, and phantom reads, but it can lead to a high degree of contention and may impact performance.

Locking Mechanisms

To achieve isolation, database systems use locking mechanisms to control concurrent access to data. Locks can be obtained on different levels (e.g., row-level, table-level) and can be shared or exclusive. Transactions acquire locks before accessing data and release them after completing the operation.

Concurrency Control Techniques

To ensure isolation and manage concurrent access, various concurrency control techniques are used, such as:

1. Locking: Transactions acquire locks on data items to prevent other transactions from accessing or modifying them concurrently.

2. Timestamp Ordering: Transactions are assigned timestamps, and their execution order is determined based on these timestamps. Conflicts are resolved by aborting and restarting transactions.

3. Multiversion Concurrency Control (MVCC): Each transaction sees a snapshot of the database at the start of its execution. Changes made by other transactions are stored as separate versions, allowing concurrent access without conflicts.

Conclusion

The isolation property in database transactions ensures that concurrent transactions do not interfere with each other, providing the illusion that only one transaction is executed in the system. Concurrency control mechanisms, such as locking, timestamp ordering, and MVCC, are used to manage concurrent access and maintain data consistency. The choice of isolation level depends on the application requirements and trade-offs between isolation and performance.

Atomicity:
Atomicity is one of the ACID properties that ensures that all tasks within a transaction are performed as a single unit. This means that either all tasks are completed successfully or none of them are. There are no partial transactions in Oracle database transactions.

Explanation:
- When a transaction is initiated in Oracle, all the tasks within that transaction must be executed in their entirety.
- If any part of the transaction fails (due to errors, crashes, or other issues), the entire transaction is rolled back, and the database is restored to its state before the transaction started.
- This ensures that the database remains in a consistent state and does not end up with incomplete or partially completed transactions.
- Atomicity guarantees data integrity and prevents the database from being left in a state where only some parts of the transaction have been applied.

Example:
For example, consider a bank transfer transaction where money is being transferred from one account to another. If the debit from one account is successful but the credit to the other account fails, atomicity ensures that the entire transaction is rolled back, and the original state of both accounts is restored.

Conclusion:
In Oracle transactions, atomicity plays a crucial role in maintaining data integrity and ensuring that the database remains consistent. It guarantees that transactions are either fully completed or not at all, thereby preventing any inconsistencies or partial updates in the database.

Answer (B)
Two or more actions are said to be in conflict if:
1) The actions belong to different transactions.
2) At least one of the actions is a write operation.
3) The actions access the same object (read or write).

The schedules S1 and S2 are said to be conflict-equivalent if the following conditions are satisfied:
1) Both schedules S1 and S2 involve the same set of transactions (including ordering of actions within each transaction).
2) The order of each pair of conflicting actions in S1 and S2 are the same.

A schedule is said to be conflict-serializable when the schedule is conflict-equivalent to one or more serial schedules.

In the given scenario, there are two possible serial schedules:
1) T1 followed by T2
2) T2 followed by T1.
In both of the serial schedules, one of the transactions reads the value written by other transaction as a first step. Therefore, any non-serial interleaving of T1 and T2 will not be conflict serializable.