Three Level Database Architecture

Data is actually stored as bits, or numbers and strings in the database storage. It is very difficult to work with data at this level.  So it is necessary to view data at different levels of abstraction. A schema helps describe data at some layer of visualization of the database. Each level has its own schema.

There are following three levels or layers of DBMS architecture:

  1. External Level: – External view consists of  user specific view of logical records and their relationships. It derives the required objects in the external view from the objects in the conceptual view.
  2. Conceptual Level: – Conceptual schema describes the records and relationship at database level. It derives the objects in the conceptual view from the objects in the internal view.
  3. Physical Level: Physical level describes how the data is internally stored, the relevant data structures and access methods to be used by the database. It gives a unified view of the storage abstracting the individual disks.

database-schema-view

Are these levels relevant in today’s world ?

The three levels hide the internals of their immediate lower level. The physical level gives a single view of storage despite of having many hard disks. The conceptual level gives a logical view of the database despite of the internal complex data structures used in physical level. The external level provides  a controlled secure view to the user, abstracting the logical dependencies of conceptual level.

This theme has been the heart of application driven databases, where many applications interact with the same databases. Without this theme, any change in database or application may have huge impact on the applications.