Schedules that are conflict serializable - solved examples

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 the following schedule;

Schedule S: r1(X), r2(X), r3(X), r1(Y), w2(Z), r3(Y), w3(Z), w3(Y)

Here, r1(X) denotes that transaction 1 read data item X, w2(z) denotes that transaction 2 writes data item Z, and so on. Is the given schedule a conflict serializable schedule? If yes, what is the equivalent serial schedule?

 

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.

 

To find whether the given schedule is conflict serializable or not, we can draw precedence graph. 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.

 

Is the schedule a conflict serializable schedule?

YES. The precedence graph of this S does no consist of a cycle.

The precedence graph includes an edge from T2 to T3 due to the conflict between the instructions w2(Z) and w3(Z). There will be another edge from T1 to T3 due to the conflict between instructions r1(Y) and w3(Y).

The precedence graph for S is as follows;

 

What is the equivalent serial schedule?

Equivalent serial schedule to S is (T2, T1, T3). Another equivalent schedule is (T1, T2, T3)

 

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