DBMS 3-tier Architecture

DBMS 3-tier Architecture

DBMS 3-tier architecture divides the complete system into three inter-related but independent modules as shown below:

1.      Physical Level: At the physical level, the information about the location of database objects in the data store is kept. Various users of DBMS are unaware of the locations of these objects.

2.      Conceptual Level: At conceptual level, data is represented in the form of various database tables. For Example, STUDENT database may contain STUDENT and COURSE tables which will be visible to users but users are unaware of their storage.

3.      External Level:  An external level specifies a view of the data in terms of conceptual level tables.  Each external level view is used to cater to the needs of a particular category of users.

Data Independence

Data independence means a change of data at one level should not affect another level. Two types of data independence are present in this architecture:

1.      Physical Data Independence: Any change in the physical location of tables and indexes should not affect the conceptual level or external view of data. This data independence is easy to achieve and implemented by most of the DBMS.

2.      Conceptual Data Independence: The data at conceptual level schema and external level schema must be independent. This means a change in conceptual schema should not affect external schema. e.g.; Adding or deleting attributes of a table should not affect the user’s view of the table. But this type of independence is difficult to achieve as compared to physical data independence because the changes in conceptual schema are reflected in the user’s view.