Architecture of DBMS

Architecture of DBMS:

The Architecture of most of commercial dbms are available today is mostly based on this ANSI-SPARC database architecture. In this architecture the database schemas can be defined at three levels. The 3-tier architecture separates its tiers from each other based on the complexity of the users and how they use the data present in the database. The 3-tier provides data abstraction; which hides the low level complexities from end users.

There are following three levels or layers of DBMS architecture:

·        External Level

·        Conceptual Level

·        Internal Level

The above diagram describes three-tier architecture. Mapping is the process of transforming request response between various database levels of architecture. In External / Conceptual mapping, DBMS transforms a request on an external schema against the conceptual schema. In Conceptual / Internal mapping, it is necessary to transform the request from the conceptual to internal levels. The objective of three-level architecture is to separate each user's view of the database from the way the database is physically represented.

·        External Level or View level:

At the external level, a database contains several schemas that sometimes called as subschema. The subschema is used to describe the different user’s view of the database. External level is the one which is closest to the end users. This level deals with the way in which individual users view data. Individual users are given different views according to the user's requirement. Each view describes the database part that a particular user group is interested and hides the remaining database from that user group.

For Example, FACULTY of a college is interested in looking course details of students, STUDENTS are interested in looking all details related to academics, accounts, courses and hostel details as well.

·        Conceptual Level or Logical level:

This level describes what data is stored in the database and the relationships among the data. The middle level in the three level architecture is the conceptual level. This level contains the logical structure of the entire database as seen by the DBA. It is a complete view of the data requirements of the organization that is independent of any storage considerations. The conceptual level represents: All entities, their attributes, and their relationships.

For example, in student database the entity is student. An attribute is a characteristic of interest about an entity. For example, in case of student database Roll No, Name, Class, Address etc. are attributes of entity student.

·        Internal level or Storage level:

It is the physical representation of the database on the computer. This level describes how the data is stored in the database. The internal level is the one that concerns the way the data are physically stored on the hardware. It covers the data structures and file organizations used to store data on storage devices. It interfaces with the operating system access methods to place the data on the storage devices, build the indexes, retrieve the data, and so· on.

The internal level is concerned with storage space allocation for data and indexes, record descriptions for storage (with stored sizes for data items), record placement, data compression and data encryption techniques.

Advantages of Three-tier Architecture:

· The main objective of it is to provide data abstraction.

·  Same data can be accessed by different users with different customized views.

· The user is not concerned about the physical data storage details.

· Physical storage structure can be changed without requiring changes in internal structure of the database as well as users view.

· Conceptual structure of the database can be changed without affecting end users.

-profshardulp.patil@gmail.com
x

Data Abstraction

Data Abstraction:

Database systems are made-up of complex data structures. Database is full of data and records. What we see in rows and columns is quite different when it reaches the memory. What we see is the actual data. But when they are stored in the memory like disks or tapes, they are stored in the form of bits. But any users will not understand these bits. User needs to see the actual data to understand. But all the details about the data stored in the memory are not necessary for the users. User needs only little information that he is interested or wants to work with. To ease the user interaction with database, the developers hide internal irrelevant details from users. This process of hiding irrelevant details from user is called data abstraction.

Data abstraction is a process of representing the essential features without including implementation details. Many database-systems users are not computer trained, developers hide the complexity from users through several levels of abstraction, to simplify users’ interactions with the system.

There are mainly three levels of data abstraction:

Physical Level / Internal Level:

Logical Level / Conceptual Level:

View Level / External Level:

Physical Level / Internal Level:

This is the lowest level in data abstraction. This level describes how the data is actually stored in the physical memory like magnetic tapes, hard disks etc. In this level the file organization methods like hashing, sequential, B+ tree and access methods like sequential or random access are comes into picture. Usability, size of memory, and the number of times the records are factors which we need to know while designing the database.

Logical Level / Conceptual Level:

This is the middle level of 3-level data abstraction. It describes the actual data stored in the database in the form of tables and relates them by means of mapping. This level will not have any information on what a user views at external level. This level will have all the data in the database.

View Level / External Level:

The highest level of abstraction describes only part of the entire database. The variety of information stored in a large database. Many users of the database system do not need all this information; instead, they need to access only a part of the database. The view level of abstraction exists to simplify their interaction with the system. The users will have different views here, based on their levels of access rights. For example, student will not have access to see Lecturers salary details, one employee will not have access to see other employees details, unless he is a manager.

Example: Let’s say we are storing customer information in a customer table. At physical level these records can be described as blocks of storage (bytes, gigabytes, terabytes etc.) in memory. These details are often hidden from the programmers.

At the logical level these records can be described as fields and attributes along with their data types, their relationship among each other can be logically implemented. The programmers generally work at this level because they are aware of such things about database systems.

At view level, user just interact with system with the help of GUI and enter the details at the screen, they are not aware of how the data is stored and what data is stored; such details are hidden from them.

-profshardulp.patil@gmail.com