Conflict Serializable Schedule in DBMS

Conflict Serializable Schedule in DBMS is an important concept in concurrency control. When multiple transactions execute simultaneously, their operations may be interleaved. This improves system performance, but it can also cause data inconsistencies if transactions interfere with each other.

Conflict serializability ensures that a concurrent schedule produces the same result as some serial schedule. In other words, a schedule is conflict serializable if it can be transformed into a serial schedule by swapping operations that do not conflict.

What is Conflict Serializability in DBMS?

A schedule is called conflict serializable if it is conflict equivalent to a serial schedule.

A serial schedule executes transactions one after another without interleaving their operations. A conflict-serializable schedule may execute transactions concurrently, but its final result is equivalent to some serial execution.

Conflict serializability is useful because it allows DBMSs to obtain the performance benefits of concurrent execution while maintaining database consistency.

Conflicting Operations

Two operations are said to be conflicting operations when all of the following conditions are satisfied:

They belong to different transactions.
They operate on the same data item.
At least one of the operations is a write operation.

There are three common types of conflicting operation pairs:

Read-Write (R-W): One transaction reads an item while another writes it.
Write-Read (W-R): One transaction writes an item while another reads it.
Write-Write (W-W): Two transactions write the same data item.

Two read operations on the same data item do not conflict because neither operation changes the data.

Examples of Conflicting and Non-Conflicting Operations

Operation Pair	Conflict?	Reason
R1(A), R2(A)	No	Both operations only read A.
R1(A), W2(A)	Yes	Same item and one operation writes.
W1(A), R2(A)	Yes	Same item and one operation writes.
W1(A), W2(A)	Yes	Both operations write the same item.
R1(A), W2(B)	No	They operate on different data items.

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Conflict Equivalent Schedules

Two schedules are conflict equivalent if they contain the same transactions and preserve the relative order of every pair of conflicting operations.

Non-conflicting operations can be swapped without changing the conflict relationships between transactions.

Therefore, if a schedule can be transformed into a serial schedule by repeatedly swapping adjacent non-conflicting operations, the original schedule is conflict serializable.

How to Check Conflict Serializability?

The easiest and most commonly used method is the precedence graph, also called a serialization graph.

The procedure is:

Create one node for each transaction.
Identify every pair of conflicting operations.
If an operation of T1 occurs before a conflicting operation of T2, draw an edge from T1 → T2.
Check the resulting graph for cycles.

The result is determined as follows:

No cycle: The schedule is conflict serializable.
Cycle present: The schedule is not conflict serializable.

Example of Conflict Serializable Schedule

Consider the following schedule:

S1:
R1(A)
W1(A)
R2(B)
W2(B)

Here, T1 operates on data item A, while T2 operates on data item B. Since the transactions operate on different data items, their operations do not conflict.

Therefore, their operations can be rearranged to produce a serial schedule:

S2:
R1(A)
W1(A)
R2(B)
W2(B)

or:

S3:
R2(B)
W2(B)
R1(A)
W1(A)

Since the operations can be reordered without changing the result, S1 is conflict serializable.

Example Using a Precedence Graph

Consider the following schedule:

S:
R1(A)
W1(A)
R2(A)
W2(A)

Here, T1 performs W1(A) before T2 performs R2(A). These operations conflict, so we create the edge:

T1 → T2

There is no edge from T2 back to T1. Therefore, the precedence graph contains no cycle.

Hence, the schedule is conflict serializable and is equivalent to the serial order:

T1 → T2

Example of a Non-Conflict-Serializable Schedule

Consider:

S:
W1(A)
R2(A)
W2(A)
R1(A)

From W1(A) followed by R2(A), we obtain:

T1 → T2

From W2(A) followed by R1(A), we obtain:

T2 → T1

The resulting precedence graph contains a cycle:

T1 → T2
↑     ↓
└─────┘

Because the graph contains a cycle, the schedule is not conflict serializable.

Conflict Serializability vs View Serializability

Feature	Conflict Serializability	View Serializability
Basis	Ordering of conflicting operations	Preservation of read-from and final-write relationships
Testing	Can be checked using a precedence graph	Generally more difficult to test
Condition	Precedence graph must be acyclic	Schedule must be view equivalent to a serial schedule
Relationship	Every conflict-serializable schedule is view serializable	Some view-serializable schedules are not conflict serializable

Advantages of Conflict Serializability

Maintains database consistency during concurrent execution.
Allows transactions to execute concurrently.
Provides a systematic way to test schedules using precedence graphs.
Helps prevent problems caused by conflicting transaction operations.
Provides results equivalent to a safe serial execution.

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Frequently Asked Questions

Q1. What is a conflict serializable schedule?

A conflict serializable schedule is a schedule that can be transformed into a serial schedule by swapping non-conflicting operations.

Q2. How do you check conflict serializability?

Conflict serializability is commonly checked using a precedence graph. If the graph has no cycle, the schedule is conflict serializable. If it contains a cycle, the schedule is not conflict serializable.

Q3. Which operations conflict in DBMS?

Read-write, write-read, and write-write operations on the same data item belonging to different transactions are conflicting operations.

Q4. Do two read operations conflict?

No. Two read operations on the same data item do not conflict because neither operation modifies the data.

Q5. What is the difference between conflict and view serializability?

Conflict serializability is based on the ordering of conflicting operations, while view serializability is based on whether transactions preserve the same read-from relationships and final writes as a serial schedule.

Key Takeaway

Conflict serializability allows transactions to execute concurrently while ensuring that their execution is equivalent to a serial schedule. The precedence graph provides a simple way to determine whether a schedule is conflict serializable: an acyclic graph means the schedule is conflict serializable, while a cycle means it is not.

Conclusion

Conflict Serializable Schedule in DBMS is a fundamental concept in concurrency control. It provides a balance between transaction concurrency and database consistency by ensuring that a concurrent schedule behaves like a valid serial execution.

Understanding conflicting operations, conflict equivalence, precedence graphs, and cycles makes it easier to analyze transaction schedules and determine whether they are conflict serializable.

Keywords

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Conflict Serializable Schedule in DBMS