Saturday, 16 April 2016

Concurrency Anomalies - Lost Update Problem



Need for concurrency control, Concurrency Anomalies, Concurrent Execution Problems, Example for lost update problem, what does cause lost update problem in concurrent execution of transactions?


Lost Update Problem

Lost update problem occurs when two or more transactions are updating same data items simultaneously. The transaction (among the concurrent transactions) that commits at last (lately) will decide the final value of that particular data item.

Example:

To understand lost update problem, let us use the schedule 1 given in Table 2.
Let us assume that the initial value of data items A and B are 1000 and 1000 respectively. Transaction T1 transfers 50 from A to B. Transaction T2 withdraws 10% of amount from A. If T1 and T2 are executed in serial order then the final values will be as given in the Table 1;

If T1 then T2
If T2 then T1
Final values are,
A = 855
B = 1050
Final values are,
A = 850
B = 1050
Table 1: Final values of A and B if T1 and T2 are executed in serial order

Let us suppose that we go for concurrent execution of T1 and T2, ie., the instructions of both transactions are interleaved, for example, as given in the table 2. As given in the table 2, the execution goes as follows;

  • T1 reads A (current value of A = 1000) then it calculates the new value of A (new value of A = 950).

  • Now the control goes to T2 and T2 read A (current value of A = 1000). After that T2 calculates the new value of A as per its instruction (new value of A = 900).

  • Then again control is transferred to T1 and T1 writes new value of A as it has calculated by T1. That means, T1 writes 950 as the new value of A and then reads the current value of B as 1000.

  • Now, the control is transferred to T2 and T2 writes the new value of A as 900. Actually T2 overwrites the value that has been written by T1. That means T1 lost its update due to the action of T2.

  • T1 takes the control and calculates the new B value, ie., B = 1050.

The final value of A and B are 900 and 1050 respectively as produced by schedule 1, which is wrong. This is because, T2 overwrites a value which was written by T1.

Transaction T1
Transaction T2
A = 1000, B = 1000
read(A);
A := A – 50;



write(A);
read(B);


B := B + 50;
write(B);
commit;


read(A);
temp := A * 0.1;
A := A – temp;


write(A);
commit;
T1 read: A = 1000

T2 read: A = 1000


T1 write: A = 950
T1 read: B = 1000
T2 write: A = 900 (Overwrite)


T1 write: B = 1050
Table 1: Schedule 1
Discussion:


  • Overwriting an uncommitted data leads the database to an inconsistent state.

  • Lost update problem is caused due to Write-Write conflict between transactions.


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1 comment:

  1. Sir please review the value of A and B in table 1. I think that those values are incorrect but I might be wrong. Thanks.

    ReplyDelete

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