Three architecture
Proposed to support DBMS characteristics of:
•Program-data independence.
•Support of multiple views of the data.
Schemas versus Instances
•Database Schema: The description of a database. Includes descriptions of the database structure and the constraints that should hold on the database.
•Schema Diagram: A diagrammatic display of (some aspects of) a database schema.
•Schema Construct: A component of the schema or an object within the schema, e.g., STUDENT, COURSE.
•Database Instance: The actual data stored in a database at a particular moment in time. Also called database state (or occurrence).
Three architecture
•Defines DBMS schemas at three levels:
•Internal schema at the internal level to describe physical storage structures and access paths. Typically uses a physical data model.
•Conceptual schema at the conceptual level to describe the structure and constraints for the whole database for a community of users. Uses a conceptual or an implementation data model.
•External schemas at the external level to describe the various user views. Usually uses the same data model as the conceptual level.
Data Independence
•Logical Data Independence: The capacity to change the conceptual schema without having to change the external schemas and their application programs.
•Physical Data Independence: The capacity to change the internal schema without having to change the conceptual schema.
Data Independence•When a schema at a lower level is changed, only the mappings between this schema and higher-level schemas need to be changed in a DBMS that fully supports data independence. The higher-level schemas themselves are unchanged. Hence, the application programs need not be changed since they refer to the external schemas.
2 tier client server architecture
User interface and application programs runs on the client side.
•Through the ODBC (Open Database Connectivity ) interface allows client side programs to call the DBMS.