1. CS223: Software Engineering
Lecture 11: Software Design

2. Recap
UML Diagram
Sequence Diagram
Activity Diagram

3. Objective After completing this lecture students will be able to
Model software design using Data Flow Diagram

4. Structure and Architecture
Software design
Design Fundamentals
Concepts
Context
Process
Principles
Key Issues
Concurrency
Events
Data
Components
Errors and Exceptions
Interaction
Security
Structure and Architecture
Structure
View-point DP
Programs
UI Design
Issues
Modality
Presentation
Localization
Metaphor
Modeling QA
Attributes
Measure
Strategies FO OO DS
CBD
Notations
Static view
Dynamic view
Tools

5. Introduction Definition:
The process of defining the architecture, components, interfaces, and other characteristics of a system or component
The result of [that] process
Creates the blueprint of the solution to be implemented

6. Standards
Architectural design
Detailed design
Software design
Software life cycle process standard: ISO/IEC/IEEE Std
Top-level structure and organization
Specify components in sufficient detail

7. Data Flow Modeling
Widely used; focuses on functions performed in the system
Views a system as a network of data transforms through which the data flows
Uses data flow diagrams (DFDs) and functional decomposition in modeling
The SSAD methodology uses DFD to organize information, and guide analysis

8. Data flow diagrams A DFD shows flow of data through the system
Views system as transforming inputs to outputs
Transformation done through transforms
DFD captures how transformation occurs from input to output as data moves through the transforms
Not limited to software

9. Data flow diagrams… DFD
Transforms represented by named circles/bubbles
Bubbles connected by arrows on which named data travels
A rectangle represents a source or sink and is originator/ consumer of data (often outside the system)

10. DFDs: Graphical Notation
Symbols to represent external entities i

11. DFDs: Graphical Notation
Data flow: A path of a particular type of data

12. DFDs: Graphical Notation
Functions/Processes: Transform an input data flow to an output data flow

13. DFDs: Graphical Notation
Data store: Data/information store

14. DFD Example

15. DFD Conventions External files shown as labeled straight lines
Need for multiple data flows by a process represented by * (means and)
OR relationship represented by +
All processes and arrows should be named
Processes should represent transforms
Arrows should represent some data

16. Data flow diagrams
Focus on what transforms happen , how they are done is not important
Usually major inputs/outputs shown, minor are ignored in this modeling
No loops , conditional thinking , …
DFD is NOT a control chart, no algorithmic design/ thinking
Sink/ Source , external files

17. Drawing a DFD for a system
Identify inputs, outputs, sources, sinks for the system
Work your way consistently from inputs to outputs, and identify a few high-level transforms to capture full transformation
If get stuck, reverse direction
When high-level transforms defined, then refine each transform with more detailed transformations

18. Drawing a DFD for a system..
Never show control logic; if thinking in terms of loops/decisions, stop & restart
Label each arrows and bubbles; carefully identify inputs and outputs of each transform
Make use of + & *
Try drawing alternate DFDs

19. Leveled DFDs DFD of a system may be very large
Can organize it hierarchically
Start with a top level DFD with a few bubbles
Then draw DFD for each bubble
Preserve I/O when “ exploding” a bubble so consistency preserved
Makes drawing the leveled DFD a top-down refinement process, and allows modeling of large and complex systems

20. DFDs: Levels DFDs can be drawn at multiple levels:
Topmost level of DFD is called the Context diagram.
Context diagram:
Level 1 DFD:

21. DFDs: Levels Level 2 DFD:
Each process in a Level 2 DFD can be further exploded to form a Level 3 DFD and so on..
Top-down expansion:
Usually not advisable to go beyond Level 3 DFD.

22. Data Dictionary In a DFD arrows are labeled with data items
Data dictionary defines data flows in a DFD
Shows structure of data; structure becomes more visible when exploding
Can use regular expressions to express the structure of data

23. DFD drawing – common errors
Unlabeled data flows
Missing data flows
Extraneous data flows
Consistency not maintained during refinement
Missing processes
Too detailed or too abstract
Contains some control information

24. DFD: Course Registration System
Consider a course registration system. When a student provides a prioritized list of courses and other information to the system, this information is transformed into a list of preferences. The list of preferences is used to verify the eligibility of the students using the student records and the course prerequisites. If the student is eligible to register for the courses he/she desires, then the student is enrolled in those courses, and the class schedule is communicated back to the student. In addition, the system also compiles the list of students enrolled in each class using the registration information for each student. This list is then given to the faculty. The list is also forwarded to the registrar so that a classroom of an appropriate capacity can be allocated depending on the number of students enrolled.

25. DFD: Course Registration System
Consider a course registration system. When a student provides a prioritized list of courses and other information to the system, this information is transformed into a list of preferences. The list of preferences is used to verify the eligibility of the students using the student records and the course prerequisites. If the student is eligible to register for the courses he/she desires, then the student is enrolled in those courses, and the class schedule is communicated back to the student. In addition, the system also compiles the list of students enrolled in each class using the registration information for each student. This list is then given to the faculty. The list is also forwarded to the registrar so that a classroom of an appropriate capacity can be allocated depending on the number of students enrolled.

26. DFD: Course Registration System
Context Diagram for Course Registration System
Courses & other info.
Faculty
Students
Registration
Process
Class schedule
Class list
Registrar

27. DFD: Course Registration System
Level 1 DFD
Students
Individual Course
Registration
Information
Courses &
Other info.
2. Compile &
Distribute
Information
1. Enroll
Students
Registrar
Schedules
Class Lists
Students
Faculty
Note: External entity Students is replicated to avoid crossing lines

28. DFD: Course Registration System
Level 2 DFD
Courses &
Other info.
Student Records
Course Prereqs
1.1 Obtain
Student
Preferences
1.2 Check
Eligibility
List of
Preferences
Eligible Students
1.3 Enroll Students
in Classes
Individual course registration
information

29. Structured Analysis Method
Structured system analysis and design (SSAD) – we will focus only on analysis
Was used a lot when automating existing manual systems
Main steps
Draw a context diagram
Draw DFD of the existing system
Draw DFD of the proposed system and identify the man- machine boundary

30. Context Diagram
Views the entire system as a transform and identifies the context
Is a DFD with one transform (system), with all inputs, outputs, sources, sinks for the system identified

31. DFD of the current system
The current system is modeled as-is as a DFD to understand the working
The context diagram is refined
Each bubble represents a logical transformation of some data
Leveled DFD may be used
Generally obtained after understanding and interaction with users
Validate the DFD by walking through with users

32. Modeling the Proposed System
No general rules for drawing the DFD of the future system
Use existing system understanding
DFD should model the entire proposed system - process may be automated or manual
Validate with the user
Then establish man-machine boundary
what processes will be automated and which remains manual
Show clearly interaction between automated and manual processes

33. Example – context diagram

34. Example – DFD of existing sys

35. Example – DFD of proposed system

36. Thank you
Next Lecture: System Modeling