Let T1, T2, T3 be transactions that operate on the same data items A, B, C. Let r1(A) means that T1 reads A, w1(A) means that T1 writes A and so on for T2 and T3. Consider the following schedule: S1: r2(C), r2(B), w2(B), r3(C), r1(A), w1(A), w3(B), w3(C), r2(A), r1(B), w1(B), w2(A). Is the schedule serializable and why?
A precedence graph, also named conflict graph and serializability graph, is used in the context of concurrency control in databases.
An arc from Ti to Tj if an action of Ti precedes and conflicts with one of Tj's actions.
The drawing sequence for the precedence graph:-
The schedule S is serializable if the precedence graph has no cycles. As T1 and T2 constitute a cycle, then we cannot declare S as serializable.
The Precedence Graph is cyclic, so, this is not serializable.
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