Serializability in DBMS

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Definition : It is sequences of actions (read,write, commit,abort) from set of transactions (which obey the sequence of operations with in the transactions ).



Serial schedules : The schedules which gets execute one after other is serial schedules.


Interleaved schedules : The schedules which allows the parts of multiple transactions to get execute.

Basic Assumption :  Each  transactions preserve database consistency. Thus serial execution of set of transactions preserves database consistency.

Difference forms of scheduling gives rise to the notations of :

1.Conflict Serializability

2.View Serializability

1. Conflict Serializability

                                 There are 4 cases to consider 

>>>1<<<<

                  I(1) = read(Q)

                  I(2) = read(Q)

                                        The order of I(1) & I(2) doesn't matter , since the same value of Q is read by T(1) & T(2).

>>>2<<<<  

                  I(1) = read(Q)

                  I(2) = write(Q)    

                                        If the I(1) comes before I(2),then T(1) doesnt read the value of Q that is  written by T(2) in Instruction I(2).

                                        If the I(2) comes before I(1) then T(1) reads the value of the Q i.e. written by T(2).Thus the order of I(1) & I(2) matters.

       

>>>3<<<<

                  I(1) = write(Q)

                  I(2) = read(Q)

                                        The order of I(1) & I(2) matters (reason is same as previous one.)

                                  >>>4<<<                                        

                  

                  I(1) = write(Q)

                  I(2) = write(Q)

                                       Since both instructions are write operations, the order of their instructions doesnt affect either T(1) or T(2) .

Thus  I(1) = read(Q),I(2) = read(Q).I(1) & I(2)  don't conflict.

        I(1) = read(Q),I(2) = write(Q).I(1) & I(2)  are conflict.

        I(1) = write(Q),I(2) = read(Q).I(1) & I(2) are conflict.

        I(1) = write(Q),I(2) = write(Q).I(1) & I(2) are conflict.

NOTE:

         1.different transactions in same data items.

         2.at least one of these instructions is write operation.