Concurrency Control deals with interleaved execution of more than one transaction. In
the next article, we will see what is serializability and how to find whether a schedule is
serializable or not.
What is Transaction?
A set of logically related operations is known as transaction. The main operations of a
transaction are:
Read(A): Read operations Read(A) or R(A) reads the value of A from the database and
stores it in a buffer in main memory.
Write (A): Write operation Write(A) or W(A) writes the value back to the database from
buffer.
Let us take a debit transaction from an account which consists of following operations:
1. R(A);
2. A=A-1000;
3. W(A);
Assume A’s value before starting of transaction is 5000.
•The first operation reads the value of A from database and stores it in a buffer.
•Second operation will decrease its value by 1000. So buffer will contain 4000.
•Third operation will write the value from buffer to database. So A’s final value will
be 4000.
But it may also be possible that transaction may fail after executing some of its
operations. The failure can be because of hardware, software or power etc. For
example, if debit transaction discussed above fails after executing operation 2, the value
of A will remain 5000 in the database which is not acceptable by the bank. To avoid this,
Database has two important operations:
Commit: After all instructions of a transaction are successfully executed, the changes
made by transaction are made permanent in the database.
Rollback: If a transaction is not able to execute all operations successfully, all the
changes made by transaction are undone.
Properties of a transaction
Atomicity: As a transaction is set of logically related operations, either all of them
should be executed or none. A debit transaction discussed above should either
execute all three operations or none.If debit transaction fails after executing operation 1
and 2 then its new value 4000 will not be updated in the database which leads to
inconsistency.
Consistency: If operations of debit and credit transactions on same account are
executed concurrently, it may leave database in an inconsistent state.
•For Example, T1 (debit of Rs. 1000 from A) and T2 (credit of 500 to A) executing
concurrently, the database reaches inconsistent state.
•Let us assume Account balance of A is Rs. 5000. T1 reads A(5000) and stores
the value in its local buffer space. Then T2 reads A(5000) and also stores the
value in its local buffer space.
