DBMS (FYCS) Unit - 1. A database management system stores data in such a way that it becomes easier to retrieve, manipulate, and produce information.

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1 Prof- Neeta Bonde DBMS (FYCS) Unit - 1 DBMS: - Database is a collection of related data and data is a collection of facts and figures that can be processed to produce information. Mostly data represents recordable facts. Data aids in producing information, which is based on facts. For example, if we have data about marks obtained by all students, we can then conclude about toppers and average marks. A database management system stores data in such a way that it becomes easier to retrieve, manipulate, and produce information. Characteristics of DBMS : Traditionally, data was organized in file formats. DBMS was a new concept then, and all the research was done to make it overcome the deficiencies in traditional style of data management. A modern DBMS has the following characteristics: Real-world entity: A modern DBMS is more realistic and uses real-world entities to design its architecture. It uses the behavior and attributes too. For example, a school database may use students as an entity and their age as an attribute Relation-based tables: DBMS allows entities and relations among them to form tables. A user can understand the architecture of a database just by looking at the table names. Isolation of data and application: A database system is entirely different than its data. A database is an active entity, whereas data is said to be passive, on which the database works and organizes. DBMS also stores metadata, which is data about data, to ease its own process. Less redundancy: DBMS follows the rules of normalization, which splits a relation when any of its attributes is having redundancy in values. Normalization is a mathematically rich and scientific process that reduces data redundancy.

2 Consistency: Consistency is a state where every relation in a database remains consistent. There exist methods and techniques, which can detect attempt of leaving database in inconsistent state. A DBMS can provide greater consistency as compared to earlier forms of data storing applications like file-processing systems.. Query Language: DBMS is equipped with query language, which makes it more efficient to retrieve and manipulate data. A user can apply as many and as different filtering options as required to retrieve a set of data. Traditionally it was not possible where file-processing system was used. Multiuser and Concurrent Access: DBMS supports multi-user environment and allows them to access and manipulate data in parallel. Though there are restrictions on transactions when users attempt to handle the same data item, but users are always unaware of them. Multiple views: DBMS offers multiple views for different users. A user who is in the Sales department will have a different view of database than a person working in the Production department. This feature enables the users to have a concentrate view of the database according to their requirements. Security: Features like multiple views offer security to some extent where users are unable to access data of other users and departments. DBMS offers methods to impose constraints while entering data into the database and retrieving the same at a later stage. DBMS offers many different levels of security features, which enables multiple users to have different views with different features. For example, a user in the Sales department cannot see the data that belongs to the Purchase department. Additionally, it can also be managed how much data of the Sales department should be displayed to the user. Since a DBMS is not saved on the disk as traditional file systems, it is very hard for miscreants to break the code. Advantages of DBMS 1. Controlling Redundancy : In file system, each application has its own private files, which cannot be shared between multiple applications. 1:his can often lead to considerable redundancy in the stored data, which results in wastage of storage space. By having centralized database most of this can be avoided. It is not possible that all redundancy should be

3 eliminated. Sometimes there are sound business and technical reasons for maintaining multiple copies of the same data. In a database system, however this redundancy can be controlled. For example: In case of college database, there may be the number of applications like General Office, Library, Account Office, Hostel etc. Each of these applications may maintain the following information into own private file applications: It is clear from the above file systems, that there is some common data of the student which has to be mentioned in each application, like Rollno, Name, Class, Phone_No~ Address etc. This will cause the problem of redundancy which results in wastage of storage space and difficult to maintain, but in case of centralized database, data can be shared by number of applications and the whole college can maintain its computerized data with the following database: It is clear in the above database that Rollno, Name, Class, Father_Name, Address,

4 Phone_No, Date_of_birth which are stored repeatedly in file system in each application, need not be stored repeatedly in case of database, because every other application can access this information by joining of relations on the basis of common column i.e. Rollno. Suppose any user of Library system need the Name, Address of any particular student and by joining of Library and General Office relations on the basis of column Rollno he/she can easily retrieve this information. Thus, we can say that centralized system of DBMS reduces the redundancy of data to great extent but cannot eliminate the redundancy because RollNo is still repeated in all the relations. 2. Inconsistency can be avoided : When the same data is duplicated and changes are made at one site, which is not propagated to the other site, it gives rise to inconsistency and the two entries regarding the same data will not agree. At such times the data is said to be inconsistent. So, if the redundancy is removed chances of having inconsistent data is also removed. Let us again, consider the college system and suppose that in case of General_Office file it is indicated that Roll_Number 5 lives in Amritsar but in library file it is indicated that Roll_Number 5 lives in Jalandhar. Then, this is a state at which tile two entries of the same object do not agree with each other (that is one is updated and other is not). At such time the database is said to be inconsistent. An inconsistent database is capable of supplying incorrect or conflicting information. So there should be no inconsistency in database. It can be clearly shown that inconsistency can be avoided in centralized system very well as compared to file system.. Let us consider again, the example of college system and suppose that RollNo 5 is.shifted from Amritsar to Jalandhar, then address information of Roll Number 5 must be updated, whenever Roll number and address occurs in the system. In case of file system, the information must be updated separately in each application, but if we make updation only at three places and forget to make updation at fourth application, then the whole system show the inconsistent results about Roll Number 5. In case of DBMS, Roll number and address occurs together only single time in General _Office table. So, it needs single updation and then another application retrieve the address information from General Office which is updated so, all application will get the current and latest information by providing single update operation and this single update operation is propagated to the whole database or

5 all other application automatically, this property is called as Propagation of Update. We can say the redundancy of data greatly affect the consistency of data. If redundancy is less, it is easy to implement consistency of data. Thus, DBMS system can avoid inconsistency to great extent. 3. Data can be shared : As explained earlier, the data about Name, Class, Father name etc. of General_Office is shared by multiple applications in centralized DBMS as compared to file system so now applications can be developed to operate against the same stored data. The applications may be developed without having to create any new stored files. 4 Standards can be enforced : Since DBMS is a central system, so standard can be enforced easily may be at Company level, Department level, National level or International level. The standardized data is very helpful during migration or interchanging of data. The file system is an independent system so standard cannot be easily enforced on multiple independent applications. Advantage of DBMS over file system There are several advantages of Database management system over file system. Few of them are as follows: No redundant data Redundancy removed by data normalization Data Consistency and Integrity data normalization takes care of it too Secure Each user has a different set of access Privacy Limited access Easy access to data Easy recovery Flexible Data Abstraction in DBMS Database systems are made-up of complex data structures. 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.

6 We have three levels of abstraction: Physical level: This is the lowest level of data abstraction. It describes how data is actually stored in database. You can get the complex data structure details at this level. Logical level: This is the middle level of 3-level data abstraction architecture. It describes what data is stored in database. View level: Highest level of data abstraction. This level describes the user interaction with database system.

7 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. Architecture of Database Database architecture can be 2-tier or 3 tier architecture based on how users are connected to the database to get their request done. They can either directly connect to the database or their request is received by intermediary layer, which synthesizes the request and then it sends to database. 2-tier architecture In 2-tier architecture, application program directly interacts with the database. There will not be any user interface or the user involved with database interaction. Imagine a front end application of School, where we need to display the reports of all the students who are opted for different subjects. In this case, the application will directly interact with the database and retrieve all required data. Here no inputs from the user are required. This involves 2-tier architecture of the database. Let us consider another example of two tier architecture. Consider a railway ticket reservation system. How does this work? Imagine a person is reserving the ticket from Delhi to Goa on particular day. At the same time another person in some other place of Delhi is also reserving the ticket to Goa on the same day for the same train. Now there is a requirement for two tickets, but for different persons. What will reservation system do? It takes the request from both of them, and queues the requests entered by each of them. Here the request entered to

8 application layer and request is sent to database layer. Once the request is processed in database, the result is sent back to application layer for the user. Advantages of 2-tier Easy to understand as it directly communicates with the database. Requested data can be retrieved very quickly, when there is less number of users. Easy to modify any changes required, directly requests can be sent to database Easy to maintain When there are multiple requests, it will be handled in a queue and there will not be any chaos. Disadvantages of 2-tier It would be time consuming, when there is huge number of users. All the requests will be queued and handed one after another. Hence it will not respond to multiple users at the same time. This architecture would little cost effective. 3-tier architecture 3-tier architecture is the most widely used database architecture. It can be viewed as below. Presentation layer / User layer is the layer where user uses the database. He does not have any knowledge about underlying database. He simply interacts with the database as though he has all data in front of him. You can imagine this layer as a registration form where you will be inputting

9 your details. Did you ever guessed, after pressing submit button where the data goes? No right? You just know that your details are saved. This is the presentation layer where all the details from the user are taken, sent to the next layer for processing. Application layer is the underlying program which is responsible for saving the details that you have entered, and retrieving your details to show up in the page. This layer has all the business logics like validation, calculations and manipulations of data, and then sends the requests to database to get the actual data. If this layer sees that the request is invalid, it sends back the message to presentation layer. It will not hit the database layer at all. Data layer or Database layer is the layer where actual database resides. In this layer, all the tables, their mappings and the actual data present. When you save you details from the front end, it will be inserted into the respective tables in the database layer, by using the programs in the application layer. When you want to view your details in the web browser, a request is sent to database layer by application layer. The database layer fires queries and gets the data. These data are then transferred to the browser (presentation layer) by the programs in the application layer. DBMS Component Modules Figure 2.3 illustrates, in a simplified form, the typical DBMS components. The figure is divided into two parts. The top part of the figure refers to the various users of the database environment and their interfaces. The lower part shows the internals of the DBMS responsible for storage of data and processing of transactions. The database and the DBMS catalog are usually stored on disk. Access to the disk is Controlled primarily by the operating system (OS), which schedules disk read/write. Many DBMSs have their own buffer management module to schedule Disk read/write, because this has a considerable effect on performance. Reducing disk read/write improves performance considerably. A higher-level stored data manager module of the DBMS controls access to DBMS information that is stored on disk, whether it is part of the database or the catalog.

10 Let us consider the top part of Figure 2.3 first. It shows interfaces for the DBA staff, casual users who work with interactive interfaces to formulate queries, application programmers who create programs using some host programming languages, and parametric users who do data entry work by supplying parameters to predefined transactions. The DBA staff works on defining the database and tuning it by making changes to its definition using the DDL and other privileged commands. The DDL compiler processes schema definitions, specified in the DDL, and stores descriptions of the schemas (meta-data) in the DBMS catalog. The catalog includes information such as the names and sizes of files, names and data types of data items, storage details of each file, mapping information among schemas, and constraints. In addition, the catalog stores many other types of information that are needed by the DBMS modules, which can then look up the catalog information as needed. Casual users and persons with occasional need for information from the database interact using some form of interface, which we call the interactive query interface in Figure 2.3. We have not explicitly shown any menu-based or form-based interaction that may be used to generate the interactive query automatically. These queries are parsed and validated for correctness of the query syntax, the names of files and data elements, and so on by a query compiler that compiles them into an internal form. This internal query is subjected to query optimization.

11 Figure 2.3 Component modules of a DBMS and their interactions.. Basic Client/Server Architectures First, we discuss client/server architecture in general, then we see how it is applied to DBMSs. The client/server architecture was developed to deal with computing environments in which a large number of PCs, workstations, file servers, printers, data

12 base servers, Web servers, servers, and other software and equipment are connected via a network. The idea is to define specialized servers with specific functionalities. For example, it is possible to connect a number of PCs or small workstations as clients to a file server that maintains the files of the client machines. Another machine can be designated as a printer server by being connected to various printers; all print requests by the clients are forwarded to this machine. Web servers or servers also fall into the specialized server category. The resources provided by specialized servers can be accessed by many client machines. The client machines provide the user with the appropriate interfaces to utilize these servers, as well as with local processing power to run local applications. This concept can be carried over to other software packages, with specialized programs such as a CAD (computer-aided design) package being stored on specific server machines and being made accessible to multiple clients. Figure 2.5 illustrates client/server architecture at the logical level; Figure 2.6 is a simplified diagram that shows the physical architecture. Some machines would be client sites only (for example, diskless workstations or workstations/pcs with disks that have only client software installed).

13 Other machines would be dedicated servers, and others would have both client and server functionality. The concept of client/server architecture assumes an underlying framework that consists of many PCs and workstations as well as a smaller number of mainframe machines, connected via LANs and other types of computer networks. A client in this framework is typically a user machine that provides user interface capabilities and local processing. When a client requires access to additional functionality such as database access that does not exist at that machine, it connects to a server that provides the needed functionality. A server is a system containing both hardware and software that can provide services to the client machines, such as file access, printing, archiving, or database access. In general, some machines install only client software, others only server software, and still others may include both client and server software, as illustrated in Figure 2.6. However, it is more common that client and server software usually run on separate machines.

14 Two main types of basic DBMS architectures were created on this underlying client/server framework: Two-tier and three-tier Two-Tier Client/Server Architectures for DBMS 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. between client and server. Hence, the query and transaction functionality related to SQL processing remained on the server side. In such 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 user interface programs and application programs can run on the client side. When DBMS access is required, the program establishes a connection to the DBMS(Which is on the server side); once the connection is created, the client program can communicate with the DBMS. A standard called Open Database Connectivity (ODBC) provides an application programming interface (API), which allows client-side programs to call the DBMS, as long as both client and server machines have the necessary software installed. Most DBMS vendors provide ODBC drivers for their systems. A client program can actually connect to several RDBMSs and send query and transaction requests using the ODBC API, which are then processed at the server sites. Any query results are sent back to the client program, which can process and display the results as needed. A related standard for the Java programming Language, called JDBC, has also been defined. This allows Java client programs to access one or more DBMSs through a standard interface. The different approach to two-tier client/server architecture was taken by some object-oriented DBMSs, where the software modules of the DBMS were divided between client and server in a more integrated way. For example, the server level may include the part of the DBMS software responsible for handling data storage on disk pages, local concurrency control and recovery, buffering and caching of disk pages, and other such functions. Meanwhile, the client level may handle the user interface; data dictionary functions; DBMS interactions with programming language compilers; global query optimization, concurrency control, and recovery across multiple servers; structuring of complex objects from the data in the buffers; and other such functions. In this approach, the client/server interaction is more tightly coupled and is done internally by the DBMS modules some of which

15 reside on the client and some on the server rather than by the users/programmers. The exact division of functionality can vary from system to system. In such a client/server architecture, the server has been called a data server because it provides data in disk pages to the client. This data can then be structured into objects for the client programs by the clientside DBMS software. The architectures described here are called two-tier architectures because the software components are distributed over two systems: client and server. The advantages of this architecture are its simplicity and seamless compatibility with existing systems. The emergence of the Web changed the roles of clients and servers, leading to the three-tier architecture. Three-Tier and n-tier Architectures Many Web applications use an architecture called the three-tier architecture, which adds an intermediate layer between the client and the database server, as illustrated in Figure 2.7 This intermediate layer or middle tier is called the application server or the Web server, depending on the application. This server plays an intermediary role by

16 running application programs and storing business rules (procedures or constraints) that are used to access data from the database server. It can also improve database security by checking a client s credentials before forwarding a request to the database server. Clients contain GUI interfaces and some additional application-specific business rules. The intermediate server accepts requests from the client, processes the request and sends database queries and commands to the database server, and then acts as a conduit for passing (partially) processed data from the database server to the clients, where it may be processed further and filtered to be presented to users in GUI format. Thus, the user interface, application rules, and data access act as the three tiers. Figure 2.7(b) shows another architecture used by database and other application package vendors. The presentation layer displays information to the user and allows data entry. The business logic layer handles intermediate rules and constraints before data is passed up to the user or down to the DBMS. The bottom layer includes all data management services. The middle layer can also act as a Web server, which retrieves query results from the database server and formats them into dynamic Web pages that are viewed by the Web browser at the client side. Other architectures have also been proposed. It is possible to divide the layers between the user and the stored data further into finer components, thereby giving rise to n- tier architectures; where n may be four or five tiers. Typically, the business logic layer is divided into multiple layers. Besides distributing programming and data throughout a network, n-tier applications