DATABASE SYSTEM AND ARCHITECTURE

The recent advance in the amount of data requiring storage has led to database systems with distributed architectures which composed of lots of computers that manage the data stores. This evolution reflects the trends in computing, where large centralized mainframe computers are replaced by hundreds of distributed workstations and personal computers connected via communications networks to various types of server machines such as Web servers, database servers, file servers, application servers, and so on. The current cloud computing environments consist of thousands of large servers managing so-called big data for users on the Web.

DATA MODELS AND SCHEMAS

Data model:- a collection of concepts that can be used to describe the structure of a database which provides the necessary means to achieve this abstraction.

High-level or conceptual data models provide concepts that are close to the way many users perceive data, whereas low-level or physical data models provide concepts that describe the details of how data is stored on the computer storage media, typically magnetic disks.

Concepts provided by physical data models are generally meant for computer specialists, not for end users.

Between these two extremes is a class of representational data models,4 which provide concepts that may be easily understood by end users but that are not too far removed from the way data is organized in computer storage.

The description of a database is called the database schema, which is specified during database design and is not expected to change frequently.

THREE SCHEMA ARCHITECTURE

The internal level has an internal schema, which describes the physical storage structure of the database. The internal schema uses a physical data model and describes the complete details of data storage and access paths for the database

The conceptual level has a conceptual schema, which describes the structure of the whole database for a community of users. The conceptual schema hides the details of physical storage structures and concentrates on describing entities, data types, relationships, user operations, and constraints.

The external or view level includes a number of external schemas or user views. Each external schema describes the part of the database that a particular user group is interested in and hides the rest of the database from that user group.

DATA INDEPENDENCE

Logical data independence is the capacity to change the conceptual schema without having to change external schemas or application programs.

Physical data independence is the capacity to change the internal schema without having to change the conceptual schema.

DATABASE LANGUAGES

Data definition language (DDL), is used by the DBA and by database designers to define both schemas.

Storage definition language (SDL), is used to specify the internal schema.

View definition language (VDL), to specify user views and their mappings to the conceptual schema.

The DBMS provides a set of operations or a language called the data manipulation language (DML) .

DBMS INTERFACES

Menu-based Interfaces for Web Clients or Browsing:- These interfaces present the user with lists of options (called menus) that lead the user through the formulation of a request.

Apps for Mobile Devices:- These interfaces present mobile users with access to data.

Forms-based Interfaces. A forms-based interface displays a form to each user. users can fill out all of the form entries to insert new data, or they can fill out only certain entries, in which case the DBMS will retrieve matching data for the remaining entries.

Graphical User Interfaces:- A GUI typically displays a schema to the user in diagrammatic form.

Natural Language Interfaces:- These interfaces accept requests written in English or some other language and attempt to understand them.

Keyword-based Database Search:- These are somewhat similar to Web search engines, which accept strings of natural language and match them with documents at specific sites or Web pages on the Web at large .

Speech Input and Output:-Limited use of speech as an input query and speech as an answer to a question or result of a request is becoming commonplace.

Interfaces for Parametric Users:-Parametric users, such as bank tellers, often have a small set of operations that they must perform repeatedly.

Interfaces for the DBA:- Most database systems contain privileged commands that can be used only by the DBA staff.

CENTRALIZED CLIENT-SERVER STRUCTURE

Firstly, database systems used these computers similarly to how they had used display terminals, so that the DBMS itself was still a centralized DBMS in which all the DBMS functionality, application program execution, and user interface processing were carried out on one machine.In relational database management systems (RDBMSs), many of which started as centralized systems, the system components that were first moved to the client side were the user interface and application programs. Because SQL provided a standard language for RDBMSs, this created a logical dividing point between client and server. Hence, the query and transaction functionality related to SQL processing remained on the server side. In such an architecture, the server is often called a query server or transaction server because it provides these two functionalities. In an RDBMS, the server is also often called an SQL server. The architectures described here are called two-tier architectures because the software components are distributed over two systems: client and server.

Many Web applications use an architecture called the three-tier architecture, which adds an intermediate layer between the client and the database server