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Explain the precedence graph for testing conflict serializability(DBMS)
Conflict serializability orders any conflicting operations in the same way as some serial execution. A pair of operations is said to conflict if they operate on the same data item and one of them is a write operation.
That means,
Readi(x) readj(x) - non conflict read-read operation
Readi(x) writej(x) - conflict read-write operation.
Writei(x) readj(x) - conflict write-read operation.
Writei(x) writej(x) - conflict write-write operation.
Where, I and j denote two different transactions Ti and Tj.
Precedence graph
Now consider a precedence graph for testing conflict serializability. It is used to check conflict serializability.
Steps for precedence graph
For each transaction T, put a node or vertex in the graph.
For each conflicting pair, put an edge from Ti to Tj.
If there is a cycle in the graph then schedule is not conflict serializable else schedule is conflict serializable.
Let us consider some examples.
Example 1
Cycle is present so it is not conflict serializable.
Example 2
Cycle is not present, so it is conflict serializable.
Example 3
Cycle is not present, so it is conflict serializable.
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