Conflict Serializability &View Serializability

Conflict Serializability

Conflict serializability deals with detecting the instructions that are conflicting in any way and specifying the order in which the instructions should execute in case there is any conflict.

A conflict serializability arises when both belong to separate transactions.

A conflict serializability arises when At least one of them is "write" operation.

A conflict serializability arises when both accesses the same data item.

A conflict serializability arises when both the schedules contain the same set of Transactions.

A conflict serializability arises when the order of conflicting pairs of operation is maintained in both the schedules.

Note − View equivalent schedules are view serializable and conflict equivalent schedules are conflict serializable. All conflict serializable schedules are view serializable too.

View Serializability

View serializability derived by creating another schedule out of an existing schedule and involves the same set of transactions.

Example: Let us assume two transactions T1 and T2 that are being serialized to create two different schedules SH1 and SH2, where T1 and T2 want to access the same data item. Now there can be three scenarios

1. If in SH1, T1 reads the initial value of data item, then in SH2 , T1 should read the initial value of that same data item.

2. If in SH2, T1 writes a value in the data item which is read by T2, then in SH2, T1 should write the value in the data item before T2 reads it.

3. If in SH1, T1 performs the final write operation on that data item, then in SH2, T1 should perform the final write operation on that data item.

If a concurrent schedule is view equivalent to a serial schedule of same transaction then it is said to be View serializable.