Difference Between Conflict and View Serializability
Last Updated :
03 Oct, 2024
When multiple transactions run at the same time in a database, it's important to ensure that the final outcome is consistent and reliable. Two key concepts that help with this are conflict serializability and view serializability. In this article, we are going to discuss the difference between conflict and view serializability in detail.
What is a Serializable Schedule?
A transaction schedule is serializable if its outcome is equal to the outcome of its transactions executed serially i.e. sequentially without overlapping in time. A serializable schedule always leaves the database in a consistent state. A serial schedule is always a serializable schedule because a new transaction only starts when the older one has finished execution.
Example:
Let us consider the following schedule and see if it is serializable.
| T1 | T2 |
|---|
| | R(X) |
| R(X) | |
| | R(Y) |
| | W(Y) |
| R(Y) | |
| W(X) | |
Now, let us figure out if the above schedule is serializable.
- Swapping R(X) of T1 and R(Y) of T2.
- Swapping R(X) of T1 and W(Y) of T2.
| T1 | T2 |
|---|
| | R(X) |
| | R(Y) |
| | W(Y) |
| R(X) | |
| R(Y) | |
| W(X) | |
Thus, after changing the conflicting operations in the initial schedule we get a serial schedule. Hence, this schedule is serializable.
Types of Serializable Schedules
- Conflict Equivalent Schedule or Conflict Serializability
- View Equivalent Schedule or View Serializability
What is Conflict Serializability?
Conflict serializability checks if a schedule of transactions can be transformed into a sequence where transactions are executed one after another, without overlapping, while keeping the same results. This type of serializability focuses on the order of conflicting operations, meaning those that can affect each other's outcomes.
Advantages of Conflict Serializability
- Data Integrity: Ensures that the final state of the database is consistent, as it prevents conflicting transactions from interfering with each other.
- Clear Rules: The rules for conflict serializability are straightforward and easy to understand, making it easier to implement in database systems.
- Efficient Execution: Many database systems can optimize transaction execution based on conflict serializability, potentially improving performance.
- Detectable Issues: The use of precedence graphs makes it easier to detect cycles and conflicts, allowing for quicker identification of problematic schedules.
- Strong Isolation: Provides a strong level of isolation between transactions, which can be crucial for applications requiring high reliability.
Disadvantages of Conflict Serializability
- Restrictive: The strict nature of conflict serializability can lead to reduced concurrency, as it may unnecessarily block transactions that could otherwise run simultaneously.
- Complexity in Management: Implementing conflict serializability may require additional mechanisms, such as locking, which can complicate transaction management.
- Performance Overhead: The need to check for conflicts and maintain locks can introduce performance overhead, especially in high-load environments.
- Not Always Necessary: In some applications, the strict guarantees of conflict serializability may be more than what is needed, leading to inefficiencies.
- Deadlock Potential: The use of locks to enforce conflict serializability can lead to deadlocks, where two or more transactions are waiting indefinitely for each other to release resources.
Example for Conflict Serializability
Let us consider the following transaction schedule and test it for Conflict Serializability
| T1 | T2 | T3 |
|---|
| | R(X) | |
| | | R(X) |
| W(Y) | | |
| | W(X) | |
| | | R(Y) |
| | W(Y) | |
Now, we will list all the conflicting operations. Further, we will determine whether the schedule is conflict serializable using Precedence Graph.
Two operations are said to be conflicting if the belong to different transaction, operate on same data and at least one of them is a write operation.
- R3(X) and W2(X) [ T3 -> T2 ]
- W1(Y) and R3(Y) [ T1 -> T3 ]
- W1(Y) and W2(Y) [ T1 -> T2 ]
- R3(Y) and W2(Y) [ T3 -> T2 ]
Constructing the precedence graph, we see there are no cycles in the graph. Therefore, the schedule is Conflict Serializable.
The serializable schedule is,
T1 -> T3 -> T2
What is View Serializability?
On the other hand, view serializability is a bit broader. It ensures that even if the transactions overlap, they produce the same final state as some serial execution. This means that as long as the final view of the database is consistent with a serial order, the schedule is considered valid.
Advantages of View Serializability
- Greater Flexibility: View serializability allows transactions to overlap, which can improve system performance and resource utilization compared to stricter methods.
- Increased Concurrency: By permitting non-conflicting transactions to run simultaneously, view serializability can enhance throughput in high-transaction environments.
- Maintains Consistency: It ensures that the final state of the database is consistent with some serial execution, which is essential for data integrity.
- Broader Applicability: It can be used in scenarios where the strict order of operations is not necessary, making it suitable for many real-world applications.
- Simpler Management: Since it allows more overlapping operations, the management of transactions can sometimes be less complex compared to conflict serializability.
Disadvantages of View Serializability
- Complexity of Validation: Determining whether a schedule is view serializable can be more complex than checking for conflict serializability, requiring detailed analysis of transaction outcomes.
- Potential for Inconsistency: While it aims to maintain a consistent final state, the overlapping nature of transactions can lead to challenges in ensuring that all intermediate states are valid.
- Less Strong Isolation: It does not provide as strong a level of isolation as conflict serializability, which might be a concern for applications requiring high reliability.
- Performance Issues: In some cases, allowing too much overlap can lead to performance bottlenecks or resource contention, particularly if transactions are not carefully managed.
- Not Always Enforced: Some database systems may not fully support view serializability, limiting its practical application in certain environments
Example For View Serializability
Let us consider the following transaction schedule and test it for View Serializability.
| T1 | T2 | T3 |
|---|
| R(A) | | |
| | W(A) | |
| | | R(A) |
| W(A) | | |
| | | W(A) |
As we know that if a schedule is Conflict Serializable, then it is View Serializable also. So first let us check for Conflict Serializability.
The conflicting operations for this schedule are -
- R1(A) and W2(A) [ T1 -> T2 ]
- R1(A) and W3(A) [ T1 -> T3 ]
- W2(A) and R3(A) [ T2 -> T3 ]
- W2(A) and W1(A) [ T2 -> T1 ]
- W2(A) and W3(A) [ T2 -> T3 ]
- R3(A) and W1(A) [ T3 -> T1 ]
- W3(A) and W1(A) [ T1 -> T3 ]
Constructing the precedence graph for conflicting operations in the schedule.
As we can see that there is a cycle in the precedence graph, it means that the given schedule is not Conflict Serializable. Now, on checking for blind write we get that there exists a blind write W2(A) in the given schedule. Thus, the schedule may or may not be View Serializable.
In order to check for View Serializability, we will draw a Dependency Graph of the schedule. From the given schedule we gather the following points :
- T1 reads A before T2 updates A thus, T1 must execute before T2.
- T3 does the final update on A thus, it must execute in the end.
Constructing the dependency graph.
As there exists no cycle in the graph, we can say that the given schedule is View Serializable.
The serializable schedule is T1 -> T2 -> T3.
Difference Between Conflict and View Serializability
Conflict Serializability | View Serializability |
|---|
Two schedules are said to be conflict equivalent if all the conflicting operations in both the schedule get executed in the same order. If a schedule is a conflict equivalent to its serial schedule then it is called Conflict Serializable Schedule. | Two schedules are said to be view equivalent if the order of initial read, final write and update operations is the same in both the schedules. If a schedule is view equivalent to its serial schedule then it is called View Serializable Schedule. |
If a schedule is view serializable then it may or may not be conflict serializable. | If a schedule is conflict serializable then it is also view serializable schedule. |
Conflict equivalence can be easily achieved by reordering the operations of two transactions therefore, Conflict Serializability is easy to achieve. | View equivalence is rather difficult to achieve as both transactions should perform similar actions in a similar manner. Thus, View Serializability is difficult to achieve. |
For a transaction T1 writing a value A that no one else reads but later some other transactions say T2 write its own value of A, W(A) cannot be placed under positions where it is never read. | If a transaction T1 writes a value A that no other transaction reads (because later some other transactions say T2 writes its own value of A) W(A) can be placed in positions of the schedule where it is never read. |
Conclusion
In conclusion, conflict serializability and view serializability are two important ways to ensure that database transactions are processed correctly. Conflict serializability is more strict, focusing on the order of operations that can interfere with each other. It requires that transactions can be rearranged into a sequence without any overlapping effects. View serializability, on the other hand, offers more flexibility. It allows transactions to overlap as long as the final result is the same as if the transactions were executed one after another in some order. Both methods help maintain data consistency, but they do so in different ways, balancing between strict rules and more relaxed approaches to transaction processing.
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