Given schedule is conflict serializable or not - exercise

Given schedule is conflict serializable or not exercise, Conflict serializability exercise, how to find whether a schedule is conflict serializable? conflict serializability example

 

Conflict Serializable Schedule - Solved Exercise

Question:

Consider a schedule S with two transactions T₁, T₂, T₃ and T₄ as follows;

S: R₂(x); W₃(x); W₁(x); W₂(y); R₂(y); R₄(x); R₄(y);

Is the schedule S conflict serializable?

Solution:

Conflict serializable schedule: A schedule is conflict serializable if it can be transformed into an equivalent serial schedule by swapping pairs of non-conflicting instructions. Two instructions conflict if they involve the same data item and at least one of them is a WRITE.

The schedule S given in questions is as follows;

Instruction	T1	T2	T3	T4
1 2 3 4 5 6 7	W(x)	R(x) W(y) R(Z)	W(x)	R(x) R(y)

We can construct a precedence graph to decide whether the given schedule is conflict serializable or not.

Precedence graph can be constructed by carefully analyzing the conflicting instructions. For every conflicting instruction, an edge can be inserted into the precedence graph. At the end, if the graph has not formed a cycle, then we would say that the schedule S is conflict serializable, otherwise not.

Conflicting instruction and the edge to be inserted		Precedence graph
R2(x) and W3(x); T2 → T3	➔
R2(X) and W1(X); T2 → T1	➔
W3(X) and W1(X); T3 → T1	➔
W3(X) and R4(X); T3 → T4	➔
W1(X) and R4(X); T1 → T4	➔
W2(Y) and R4(Y); T2 → T4	➔

From the precedence graph, it is clear that no cycle has been formed through the conflicting instructions. Hence, the schedule is a conflict serializable schedule.

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how to find whether a schedule is conflict serializable or not?

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How to check whether a schedule is conflict serializable?

How to check whether a schedule is conflict serializable?

Question:

Consider a schedule S with two transactions T₁ and T₂ as follows;

S: R1(x);R2(x);W1(y);W2(y);commit1;commit2;

Is the schedule S conflict serializable?

Solution:

The given schedule S is as follows;

Instruction	T1	T2
1 2 3 4 5 6	R(x) W(y) Commit	R(x) W(y) Commit

A schedule is conflict serializable if it can be transformed into an equivalent serial schedule by swapping pairs of non-conflicting instructions. Two instructions conflict if they involve the same data item and at least one of them is a WRITE.

Let us construct a precedence graph;

Ins. 1 and 2: R(x) of T₁ does not conflict with R(x) of T₂ because they both read x and hence not conflict.

Ins. 2 and 3: R(x) of T₂ does not conflict with W(y) of T₁. Both are not conflicting instructions because T₂ and T₁ are trying on different data items x and y not same data items.

Ins. 3 and 4: W(y) of T₁ conflicts with W(y) of T₁. They both try on same data item with conflicting instructions (write-write conflict). Hence we need an edge to be inserted in our precedence graph as follows;

Precedence graph

There are no more instructions (either read or write). And our graph does not contain a cycle. Hence, we can conclude that the given schedule S is conflict serializable and the serialized schedule will be T₁;T₂, that is, all instructions from T₁ first followed by all instructions from T₂.

If we swap all non-conflicting instructions (swap instruction 2 with 3) in our schedule, we shall end up in the following serial schedule S’;

Instruction	T1	T2
1 2 3 4 5 6	R(x) W(y) Commit	R(x) W(y) Commit

***********

Go to Define Serializability page

Go to Conflict Serializability explained with example page

Go to Solved exercises in DBMS page

Conflict serializability solved exercise

conflict serializable schedule example

how to find whether a schedule is conflict serializable or not?

how to check a schedule is serializable or not?

use precedence graph for checking serializability

serializability solved exercise

concurrency and serializability