The document discusses different database architectures including personal computer databases, file server architectures, file sharing architectures, client/server architectures, two-tier architectures, and three-tier architectures. A personal computer database has the database and applications on one PC. A file server architecture stores database files on a file server but applications run on client PCs. A file sharing architecture allows multiple users to access the same database files stored on a file server. Client/server architectures improve on file sharing by having the database server directly answer queries. Two-tier architectures have clients do input/output and some logic while the server performs storage and access. Three-tier architectures move more logic and business rules to an application server layer.

2. Personal Computer - Stand-Alone Database Database (or files) reside on a PC - on the hard disk. Applications run on the same PC and directly access the database. In such cases, the application is the DBMS. Business rules are enforced in the applications running on the PC. A single user accesses the applications.

3. File Server Architecture All processing is done at the PC that requested the data Entire files are transferred from the server to the client for processing Problems : Huge amount of data transfer on the network Each client must contain full DBMS Heavy resource demand on clients Client DBMSs must recognize shared locks, integrity checks, etc. FAT CLIENT

4. File Server Architecture FAT CLIENT

5. File Sharing Architecture PCs are connected to a local area network (LAN). A single file server stores a single copy of the database files. Applications run on each PC on the LAN and access the same set of files on the file server. The application is also the DBMS. Business rules are enforced in the applications - Also, the applications must handle concurrency control. Possibly by file locking. Each user runs a copy of the same application and accesses the same files. Example: Sharing MS Access files on a file server.

6. Advantages & Disadvantages of File Sharing Architecture Advantages: (limited) Ability to share data among several users Costs of storage spread out among users Most components are now commodity items - prices falling Disadvantages: Limited data sharing ability - a few users at most

7. Client/server Architecture Client-server architecture developed as a response to the limitations of file-sharing architectures, which require tons of bandwidth and can often stall or jam a network causing it to crash. They require low shared usage and low volume of data to be transferred. In client-server architecture, the database server replaced the file server. Relational data base management systems (DBMSs) answered user queries directly. Since only specific queries were being answered, only that data was transferred instead of entire files that slow down networks. It also improved consistency in data between users, since all users had access to the same updated information.

8. Client / Server Architecture clients Server

9. Client/Server Systems Networked computing model Processes distributed between clients and servers Client – Workstation (usually a PC) that requests and uses a service Server – Computer (PC/mini/mainframe) that provides a service For DBMS, server is a database server

10. Application Logic in Client/Sever Systems Procedures, functions, programs Processing Logic I/O processing Business rules Data management DBMS activities Storage Logic Data storage/retrieval GUI Interface Presentation Logic Input – keyboard/mouse Output – monitor/printer

11. Client/Server Architectures File Server Architecture Database Server Architecture Three-tier Architecture Client does extensive processing Client does little processing

12. Two-tier Architecture Two tier architectures consist of three components: user system interfaces, processing management, and database management. User system interface (USI) is a component of an organization’s decision support system, which includes human decision-makers. It provides a user friendly layer of communication to make requests of the server and offers multiple forms of input and output. USIs include features like display management services, sessions, text input, and dialog. Processing management includes process development, process implementation, process monitoring, and process resources services. Database management includes database and file services. Two tier client-server design derives its name from how it distributes work between client and server. Clients access databases through the user system interface. Database management, on the server side, distributes processing between both client and server. Both tiers, the client and the server, are responsible for some of the processing management. Simply put, the client uses the user interface to make requests through database management on the server side.

13. Two-tier database server architecture Thinner clients DBMS only on server

14. Two-Tier Database Server Architectures Client is responsible for I/O processing logic Some business rules logic Server performs all data storage and access processing  DBMS is only on server

15. Advantages of Two-Tier Approach Clients do not have to be as powerful Greatly reduces data traffic on the network Improved data integrity since it is all processed centrally Improve usability though user-friendly, form-based interfaces. Improve scalability because two tiered systems can hold up to 100 users, whereas file server architectures can only accommodate 12. best suited to homogeneous environments for processing non-complex, non-time sensitive information. Stored procedures  some business rules done on server

16. Advantages of Stored Procedures Compiled SQL statements Reduced network traffic Improved security Improved data integrity Thinner clients

17. Three-tier Client/Server Architecture

18. Three-tier Architecture Thinnest clients Business rules on application server DBMS only on Database server

19. Three-Tier Architectures Thin Client PC just for user interface and a little application processing. Limited or no data storage (sometimes no hard drive) GUI interface (I/O processing) Browser Business rules Web Server Data storage DBMS Client Application server Database server

20. Advantages of Three-Tier Architectures Scalability Centralize applications logic (one place to make changes) Relieves clients from having to load up on applications logic (the "fat client" approach). Frees up DBMS server to efficiently process transactions Technological flexibility Long-term cost reduction Better match of systems to business needs Improved customer service Competitive advantage Reduced risk

21. Challenges/Disadvantages of Three-tier Architectures System complexity - extremely complex to program and debug Security issues High short-term costs Tools and training Experience Incompatible standards Lack of compatible end-user tools

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