View Serializability is the process of determining whether a given schedule is view serializable or not. If a schedule is a view equivalent to a serial schedule, it is view serializable.
Why do We Require View Serializability in DBMS?
Because the concept of Conflict-Serializability becomes limited when a schedule’s Precedence Graph contains a loop/cycle, there may be some schedules that are not Conflict-Serializable but still produce consistent results. In such a case, We cannot predict whether a schedule will be consistent or inconsistent. According to the concept of Conflict-Serializability, a schedule is Conflict-Serializable if its corresponding precedence graph lacks any loop/cycle.
Because no concurrent transactions are executed, we know that a serial schedule never leaves the database in an inconsistent state. A non-serial schedule, on the other hand, can leave the database in an inconsistent state because multiple transactions are running concurrently. We ensure that a given non-serial schedule is consistent by ensuring that it is view serializable.
Let’s see how to determine whether the two schedules are view equivalent.
S1 and S2 are said to view equivalent if they meet the following conditions:
The initial read of each data item in transactions must match both schedules. For example, if transaction T1 reads data item Y before transaction T2 in schedule S1, then in schedule S2, T1 should read Y before T2.
In both schedules, the final write operations on each data item must match. For example, if a data item Y was last written by Transaction T1 in schedule S1, the last write operation on Y should be performed by Transaction T1 in schedule S2.
If transaction T1 in schedule S1 is reading a data item updated by T2, then in schedule S2, T1 should read the value after T2’s write operation on the same data item. For example, if in schedule S1, T1 performs a read operation on X after T2 performs a write operation on Y, then in schedule S2, T1 should read the Y after T2 performs a write operation on Y.
When a schedule is view equivalent to its serial schedule, it is said to be View Serializable ( view serializability in DBMS).
Let’s understand the concept of the view serializable with the help of the example.
Let’s look at the three view serializability conditions:
First, we will see that transaction T1 in schedule S1 reads data item A first. Transaction T1 in S2 also reads data item A first. Then look for B. Transaction T1 in schedule S1 reads data item B first. T1 also performs the first read operation on B in S2. We checked both data items A and B, and the initial read condition is met in S1 and S2.
The second method is about update read operation. Transaction T2 in S1 reads the value of A written by T1. In the same transaction, T2 reads the A after it is written by T1 in S2. Transaction T2 in S1 reads the value of B written by T1. The same transaction T2 reads the value of B after it has been updated by T1 in S2. Both schedules also satisfy the update read condition.
In schedule S1, the final write operation on A is done by transaction T2. Transaction T2 also performs the final write on A in S2. Let’s look for B. Transaction T2 performs the final write operation on B in schedule S1. T2 completes the final write on B in schedule S2. We checked both data items A and B, and the final write condition is met in S1 and S2.
In conclusion, this article will help you to understand what is view serializability in DBMS and why we require view serializability in DBMS. In addition, you will also learn how to check whether the two schedules are view equivalent or not. In the end, one important example is given to understand the view serializability in DBMS.
FAQs Related to View Serializability
1. How is View Serializability different from Conflict Serializability?
View serializability is a weaker form of serializability than conflict serializability. Conflict serializability requires that transactions do not have conflicting accesses to the same data item, while view serializability only requires that transactions produce the same final result as a serial schedule. As a result, some schedules that are viewed as serializable may not be conflict serializable.
2. How is View Serializability tested?
View serializability can be tested using the notion of equivalent histories. Two schedules are said to be equivalent if they produce the same final database state. To test view serializability, we can check if the original schedule is equivalent to some serial schedule by comparing their final database states.
3. Can a schedule be view serializable without being conflict serializable?
Yes, a schedule can be view serializable without being conflict serializable. This can happen when transactions do not conflict with each other but the order in which they are executed leads to a final state that is different from what would have been obtained if the transactions had been executed serially.
4. How does View Serializability affect database performance?
View serializability can have a significant impact on database performance, as it may require the use of locking mechanisms and other concurrency control techniques to ensure that transactions do not interfere with each other. These techniques can lead to decreased concurrency and increased overhead, which can impact the performance of the system.
5. What are some advantages of View Serializability over other forms of transaction isolation?
View serializability provides a stronger guarantee of transaction isolation than other forms of isolation, such as Read Committed or Repeatable Read, as it ensures that the final state of the database is the same as if the transactions had been executed serially. This can lead to greater consistency and reliability of the data.
6. What are some limitations of View Serializability?
View serializability assumes that all transactions are independent of each other, which may not be the case in some real-world scenarios. It also does not consider the possibility of deadlocks, which can occur when two transactions are waiting for each other to release a lock.