System Analysis and Designs

1. CS 313 System Analysis and Designs Lecture 02 Somchai Thangsathityangkul

2. Process Modeling • A process model is a formal way of representing how a business system operates • It illustrates the processes or activities that are performed and the data that moves among them • Data flow diagrams are used to graphically represent the flow of data in a business system • It describes the processes that are involved in an information system to transfer data from the input to the file or database to the report generation

3. Process Modeling: A System Concept

4. Data Flow Diagrams Logical and Physical Data Flow Diagrams • Data flow diagrams can be divided into logical and physical diagrams. • The logical data flow diagram describes flow of data through a system to perform certain functionality of a business. • The physical data flow diagram describes the implementation of the logical data flow through specific methods.

5. Data Flow Diagrams

6. Data Flow Diagrams

7. Data Flow Diagrams Four basic symbols of a data-flow diagram • There are four basic symbols of a data flow diagram: • Process • Data Flow • Data Store • External Entity

8. Data Flow Diagram Symbols

9. Data Flow Rules

10. Data Flow Diagrams Data Store Symbol • A data store or data repository is used to represent a situation when the system must retain data because one or more processes need to use the stored data in a later time • For example, transaction report of a customer bank account is printed at the end of a monthly cycle. Thus a record of each customer account transaction must be kept in the system. • A data store must be connected to a process with a data-flow:Each data store must have at least one input data-flow and at least one output data-flow (even if the output data-flow is a control or confirmation message).

11. Data Store Rules

12. Data Flow Diagrams External Entity Symbol • An external entity is a person, department, outside organization, or other information system that provides data to the system or receives outputs from the system. • External entities are components outside of the boundaries of the information systems. They represent how the information system interacts with the outside world. (Compare this with an Entity in the E-R diagram.) • External entities also are called terminators because they are data origins or final destinations • An external entity must be connected to a process through a data-flow

13. External Entity Rules

14. DFD Incorrect Rule

15. Developing Data Flow Diagrams

16. Data Flow Diagrams Context Diagram The context diagram is the highest level in a data flow diagram It contains only one process (labeled 0) representing the entire business system All external entities are shown on the context diagram as well as major data flow to and from them The diagram does not contain any data storage

17. Context Diagram

18. Data Flow Diagrams Level 0 Diagram • The context diagram is exploded into a level 0 diagram that contains all major processes of the business system • In addition to all external entities, major processes, and major data flows, it contains major data stores • Normally nine processes are included (to limit the number of processes in a page) • The processes are numbered as 1 through 9

19. Context Diagram Level 0 Diagram Look at balancing of Data Flows between the two diagrams

20. Data Flow Diagrams Another way of Creating Level 0 Diagram • First create DFD fragments from the Use Cases of the system – without including details of the processing • Then combine all DFD fragments to draw a 0-level diagram

21. DFD Fragment from a Use Case

22. Data Flow Diagrams Level 1 Diagram • Each of the processes in the level 0 diagram is exploded further to include the detail functionality of the process • Thus there can be nine (9) level 1 diagrams • The processes are numbered as 1.1, 1.2, 1.3, …2.1, 2.2, 2.3, …3.1, 3.2, 3.3, … and so on

23. Level 0 Diagram Level 1 Diagram Explosion of FILL ORDER Process Look at balancing of Data Flows between the two diagrams

24. Level 0 Diagram Level 1 Diagram Explosion of APPLY PAYMENT Process Look at balancing of Data Flows between the two diagrams

25. Data Flow Diagrams Conventions for Data-Flow Diagram • The context diagram must fit in one page • The process name in the context diagram should be the name of the information system. • For example, Grading System, Order Processing System, Registration System • Use unique names within each set of symbols • For example, there can be only one entity CUSTOMER in all levels of the data-flow diagrams. There can be only one process name CALCULATE OVERTIME among all levels of data-flow diagrams

26. Data Flow Diagrams Conventions for Data-Flow Diagram (contd..) • On lower-level data-flow diagrams with multiple processes, one should not have more than nine (9) process symbols • Do not cross lines: • One way to achieve this is to restrict the number of processes in a data-flow diagram • Another way to avoid crossing lines is to duplicate an external entity or data store. Use a special notation such as an asterisk, to denote the duplicate symbol

27. Data Flow Diagrams Conventions for Data-Flow Diagram (contd..) • Use abbreviated identification for external entities and data-flows. For example, C for entity CUSTOMER, and D1 for data store STUDENT • Use a unique reference number for each process symbol • The process number in the context-level diagram is 0 (zero) • Other process numbers are in the hierarchy of (1, 2, 3,…); (1.1, 1.2, 1.3, …., 2.1, 2.2, 2.3,….); (1.1.1, 1.1.2, 1.1.3,…………………………………)

28. Data Flow Diagrams Balancing and Leveling Data-Flow Diagrams: • Balancing refers to the preservation of input and output data flows of the parent diagram on the child diagrams • Leveling means that the information system is first displayed as a single process, and then shows more detail in subsequent child diagrams until all processes become functional primitives

29. Balancing Data Flow Diagrams

30. Mistakes in Data Flow Diagrams