Limits of a file system

Keeping data in separate files (a file system) still has limitations:

• Difficulty in access: data can be read only through application programs written in advance; a new kind of question needs a new program.
• Data redundancy: the same data is stored in more than one place. Student names appear in both STUDENT and ATTENDANCE. This wastes space and can cause inconsistency.
• Data inconsistency: copies of the same data disagree. If a student's name is corrected in one file but not the other, the two files give different answers.
• Data isolation: the files have no link or mapping between them, may be kept in different formats, and need separate programs, which makes writing new programs across them hard.
• Data dependence: data is stored in a specific structure; if that structure changes, every program that uses the file must be changed too, or it may stop working.
• Controlled data sharing: different users need different rights (for example, guardians should only read attendance, while only the teacher may change it). This is hard to enforce with plain files.

These problems led to Database Management Systems.

Example (new): if a shop keeps customer addresses both in its billing file and in its delivery file, and a customer's new address is updated only in billing, the next parcel goes to the old house: redundancy has turned into inconsistency.