1. To see why database management systems are necessary, let’s look at a typical “file-processing system” supported by a conventional operating system.The application is a savings bank:
   • Savings account and customer records are kept in permanent system files.
   • Application programs are written to manipulate files to perform the following tasks:
     • Debit or credit an account.
     • Add a new account.
     • Find an account balance.
     • Generate monthly statements.
2. Development of the system proceeds as follows:
   • New application programs must be written as the need arises.
   • New permanent files are created as required.
   • but over a long period of time files may be in different formats, and
   • Application programs may be in different languages.
3. So we can see there are problems with the straight file-processing approach:
   • Data redundancy and inconsistency
     • Same information may be duplicated in several places.
     • All copies may not be updated properly.
   • Difficulty in accessing data
     • May have to write a new application program to satisfy an unusual request.
     • E.g. find all customers with the same postal code.
     • Could generate this data manually, but a long job…
   • Data isolation
     • Data in different files.
     • Data in different formats.
     • Difficult to write new application programs.
   • Multiple users
     • Want concurrency for faster response time.
     • Need protection for concurrent updates.
     • E.g. two customers withdrawing funds from the same account at the same time - account has $500 in it, and they withdraw $100 and $50. The result could be $350, $400 or $450 if no protection.
   • Security problems
     • Every user of the system should be able to access only the data they are permitted to see.
     • E.g. payroll people only handle employee records, and cannot see customer accounts; tellers only access account data and cannot see payroll data.
     • Difficult to enforce this with application programs.
   • Integrity problems
     • Data may be required to satisfy constraints.
     • E.g. no account balance below $25.00.
     • Again, difficult to enforce or to change constraints with the file-processing approach.

   These problems and others led to the development of database management systems.

Data Abstraction