Database Transaction basic: ACID properties

Database transaction:
A transaction is a unit of program execution that accesses and possibly updates various data items.A transaction has many program segment such as read segment, write segment etc. We combine all segment as a unit and call it database transaction. A transaction require to maintain the following properties. These properties are often called the ACID properties. Given below the ACID properties of a transaction:
Atomicity: All operations of a transaction must be completed otherwise the program must aborted.
Consistency:
Database consistency stats that only valid data will be written to database. Database consistency says that total sum of enter program of a transaction must unchanged. If a transaction violets the database consistency rules, the entire database will be rolled back and the database restored.
Isolation:
Isolation means multiple transactions occurring at same time but not impact each other execution. A transaction X and another transaction Y occur at same time but don't disturb each other it is called isolation in database transaction.
Durability: Durability ensures a transaction committed successfully but will not lost even the database crashes. It says any fault of database cannot remove transaction data.
Let, a transaction transfer $50 from account A to account B. This transaction can be defined as
          1. read(A);
          2. A:=A-50;
          3. write(A);
          4. read(B);
          5. B:=B+50;
          6. write(B);
Now I discuss the ACID properties of the example. Atomicity: We know all operation must completed in a transaction otherwise the system abort. In step 3 if the database system crash what will occur? Atomicity property handle it because transaction not completed fully. Consistency:The sum of A and B be unchanged by the execution of the transaction. If the total sum changed it is say that an error occurred in the system. So database must restored. Isolation: Isolation means two or more program runs concurrently in database without impact each other. In this example the read and write segment runs concurrently without impacting each other. Durability: Think that a transaction completed successfully. When it completed successfully and at the same time the system crashes. Durability handle the situation and the actual data cannot lost.

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