Architecture Of DBMA
In this Article :- Architecture Of DBMA
THE LOGICAL DBMS ARCHITECTURE
Database Management Systems are very complex, sophisticated software applications that provide reliable management of large amounts of data. To describe general database concepts and the structure and capabilities of a DBMS better, the architecture of a typical database management system should be studied.
There are two different ways to look at the architecture of a DBMS: the logical DBMS architecture and the physical DBMS architecture. The logical architecture deals with the way data is stored and presented to users, while the physical architecture is concerned with the software components that make up a DBMS.
Three Level Architecture of DBMS or Logical DBMS Architecture
The logical architecture describes how data in the database is perceived by users. It is not concerned with how the data is handled and processed by the DBMS, but only with how it looks. The method of data storage on the underlying file system is not revealed, and the users can manipulate the data without worrying about where it is located or how it is actually stored. This results in the database having different levels of abstraction. Architecture Of DBMA
The majority of commercial Database Management Systems available today are based: on the ANSI/SPARC generalised DBMS architecture, as proposed by the ANSI/SPARC Study Group on Data Base Management Systems. Hence this is also called as the ANSI/SPARC model. It divides the system into three levels of abstraction: the internal or physical level, the conceptual level, and the external or view level. The diagram below shows the logical architecture for a typical DBMS. Architecture Of DBMA
The External or View Level
The external or view level is the highest level of abstraction of database. It provides a window on the conceptual view, which allows the user to see only the data of interest to them. The user can be either an application program or an end user. There can be many external views as any number of external schema can be defined and they can overlap each other. It consists of the definition of logical records and relationships in the external view. It also contains the methods for deriving the objects such as entities, attributes and relationships in the external view from the Conceptual view. Architecture Of DBMA
The Conceptual Level or Global level
The conceptual level presents a logical view of the entire database as a unified whole. It allows the user to bring all the data in the database together and see it in a consistent manner. Hence, there is only one conceptual schema per database. The first stage in the design of a database is to define the conceptual view, and a DBMS provides .. data definition language for this purpose. It describes all the records and relationships included in the database.
The data definition language used to create the conceptual level must not specify any physical storage considerations that should be handled by the physical level. It does not provide any storage or access details, but defines the information content only.
The Internal or Physical Level
The collection of files permanently stored on secondary storage devices is known as the physical database. The physical or internal level is the one closest to physical. storage, and it provides a low-level description of the physical database, and an interface between the operating systems file system and the record structures used in higher levels of abstraction. It is at this level that record types and methods of storage are defined, as well as how stored fields are represented, what physical sequence the stored records are in, and what other physical structures exist. Architecture Of DBMA
Mappings between Levels and Data Independenc
The three levels of abstraction in the database do not exist independently of each other. There must be some correspondence, or mapping, between the levels. There are two types of mappings: the conceptual/internal mapping and the external/conceptual mapping Architecture Of DBMA
The conceptual/internal mapping lies between the conceptual and internal levels, and defines the correspondence between the records and the fields of the conceptual view and the files and data structures of the internal view. If the structure of the stored database is changed, then the conceptuall internal mapping must also be changed accordingly so that the view from the conceptual level remains constant. It is this mapping that provides physical data independence for the database.
For example, we may change the internal view of student relation by breaking the student file into two files, one containing enrolment, name and address and other containing enrolment, programme. However, the mapping will make sure that the conceptual view is restored as original. The storage decision is primarily taken for optimisation purposes. Architecture Of DBMA
The external/conceptual view lies between the external and conceptual levels, and defines the correspondence between a particular external view and the conceptual view. Although these two levels are similar, some elements found in a particular external view may be different from the conceptual view. For example, several fields can be combined into a single (virtual) field, which can also have different names from the original fields. Architecture Of DBMA
If the structure of the database at the conceptual level is changed, then the external/conceptual mapping must change accordingly so that the view from the external level remains constant. It is this mapping that provides logical data independence for the database. For example, we may change the student relation to have more fields at conceptual level, yet this will not change the two user views atall.
.It is also possible to have another mapping, where one external view is expressed in terms of other external views (this could be called an external/external mapping). This is useful if several external views are closely related to one another, as it allows you to avoid mapping each of the similar external views directly to the conceptual level.
The need for three level architecture
The objective of the three level architecture is to separate each user’s view of the database from the way the database is physically represented.
- Support of multiple user views: Each user is able to access the same data, but have a different customized view of the data. Each user should be able to change the way he or she views the data and this change should not affect other users. Architecture Of DBMA
- Insulation between user programs and data that does not concern them: Users should not directly deal with physical storage details, such as indexing or hashing. The user’s interactions with the database should be independent of storage considerations. Architecture Of DBMA
Insulation between conceptual and physical structures
It can be defined as:
- The Database Administrator should be able to change the storage structures without affecting users’ views.
- The internal structure of the database should be unaffected by the changes to the physical aspects of the storage, such as changing to a new storage device. Architecture Of DBMA
- The DBA should be able to change the conceptual structure of the database without affecting all users.
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