THREE LEVEL LOGICAL ARCHITECTURE OF DATABASE MANAGEMENT SYSTEM|DBMS
THE THREE LEVEL LOGICAL ARCHITECTURE OF DBMS CAN BE DESCRIBED IN TERMS OF LEVEL OF DATA ABSTRACTION.
LEVELS OF DATA ABSTRACTION
DATA ABSTRACTION IS A MECHANISM THAT ENABLES THE DESIGNERS IN PROVIDING ONLY AN ABSTRACT VIEW OF DATA ENVIRONMENT TO THE THE END URES i.e, THE IMPLEMENTATION COMPLEXITIES ARE HIDDEN FROM THE USERS.ANSI/SPARC ARCHITECTURE DEFINES THREE LEVEL OF DATA ABSTRACTION USING WHICH IT IS VERY EASY FOR A DESIGNER TO INTEGRATE MULTIPLE VIEW OF DATA PRESENT AT DIFFERENT LEVELS OF ORGANIZATION. THE THREE LEVELS OF DATA ABSTRACTION ARE AS FOLLOWS,
1. EXTERNAL LEVEL
2. CONCEPTUAL LEVEL
3. INTERNAL LEVEL
LATER, ANS/SPARC EXPANDED THEIR ARCHITECTURE TO INCLUDE ANOTHER LEVEL OF ABSTRACTION CALLED PHYSICAL LEVEL WHICH SPECIFIES THE PHYSICAL LEVEL IMPLEMENTATION DETAILS OF INTERNAL MODEL.
THE LOWEST LEVEL OF ABSTRACTION IS THE PHYSICAL LEVEL WHICH SPECIFIES THE WAY OF STORING DATA ON STORING DATA ON STORAGE DEVICES. BASICALLY, PHYSICAL MODEL THAT OPERATORS AT PHYSICAL LEVEL REQUIRES THE INFORMATION ABOUT THE PHYSICAL STORAGE DEVICES AND ASLO THE ACCESS METHODS. THE STORAGE STRUCTURES WHICH ARE USED FOR STORING THE DATA ARE GENERALLY DEPENDENT ON THE SOFTWARE AND ASLO ON THE TYPE OF STORAGE DEVICES. IN ORDER TO ACCURATELY DEFINE PHYSICAL MODEL, IT IS NECESSARY FOR DATABASE DESIGNER PHYSICAL TO HAVE DETAILED KNOWLEDGE REGARDING THE HARDWARE AND SOFTWARE THAT ARE USED FOR IMPLEMENTING THE DATABASE DESIGN.
EXTERNAL LEVEL
EXTERNAL LEVEL IS THE HIGHEST AND THE OUTERMOST LEVEL OF DATA ENVIRONMENT WHICH REPRESENTS THE END USER'S VIEW, WHO USUALLY USE THE APPLICATION PROGRAM FOR MANIPULATING DATA AND GENERATING INFORMATION. IT IS POSSIBLE THAT DIFFERENT EXTERNAL VIEWS CAN USE THE SAME CONCEPTUAL VIEW IN ORDER TO SATISFY THE REQUIREMENTS OF VARIOUS USERS. EXTERNAL LEVEL OF ABSTRACTION IS OPERATED USING EXTERNAL MODEL.
EXTERNAL VIEW OF DATA CAN BE REPRESENTED USING ER-DIGRAMS. THIS REPRESENTATION OF EXTERNAL VIEW IS REFERRED TO AS "EXTERNAL SCHEM", WHICH INCLUDES APPROPRIATE BUILDING BLOCK OF DATA MODEL IMPOSED BY BUSINESS UNIT.
CONCEPTUAL LEVEL
CONCEPTUAL VIEW REPRESENTS THE GLOBAL OF DATA ENVIRONMENT WHERE IN ALL EXTERNAL VIEWS ARE INTEGRATED IN ORDER TO FROM A SINGLE GLOBAL VIEW OF DATA. THIS VIEW IS GRAPHICALLY REPRESENTED USING ER-DIGRAM AND RESULTING REPRESENTING IS KNOW AS CONCEPTUAL SCHEMA. THIS LEVEL OF ABSTRACTION IS USED BY DBA, WHO MAKES A DECISION REGARDING WHAT INFORMATION IS TO KEPT IN THE DATABASE. A DATABASE CONSISTS ON ONLY ONE CONCEPTUAL SCHEMA, WHICH IS CONNECTED TO BOTH EXTERNAL SCHEMA AND INTERNAL SCHEMA.
INTERNAL LEVEL
THE INTERNAL LEVEL IS THE LEVEL OF ABSTRACTION PRESENT BETWEEN THE CONCEPTUAL LEVEL AND PHYSICAL LEVEL. INTERNAL MODEL WITH THE SELECTED DBMS VIEW OF DATA ENVIRONMENT. IT ENABLE THE DESIGNER TO MATCH THE CHARACTERISTICS OF IMPLEMENTATION MODEL. INTERNAL MODEL. THIS REPRESENTING INTERNAL MODEL. THIS REPRESENTATION IS DONE USING DIFFERENT BATA CONSTRAINTS THAT ARE SUPPORT BY SELECTED DBMS.
INTERNAL MODEL IS SOFTWARE DEPENDENT, WHICH IMPLIES THAT CHANGES MADE IS DBMS SOFTWARE MUST ASLO BE REFLECTED IN THE INTERNAL MODEL. ON THE OTHER HAND, THE MODEL IS HARDWARE INDEPENDENT BECAUSE CHANGES MADE IN-STORE DEVICES DOESN'T HAVE ANY IMPACT ON THE INTERNAL MODEL DOESN'T HAVE ANY EFFECT ON CONCEPTUAL MODEL THEREBY ENSURING LOGICAL INDEPENDENCE.
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