0. Ch5: Software Specification

1. Data Flow Diagrams

2. DFDs: Structured Analysis/Structured Design
Structured Analysis/Structured Design (SA/SD):
Supported by many CASE tools.
Documented in many popular books.
Influenced many other design methods.

3. DFDs: Objectives
Avoiding the cost of:

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

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

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

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

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

9. 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.

10. DFDs: Levels
When to stop expanding?

11. DFDs: Illegal construction

12. DFDs: Hints for construction

13. DFDs: Construction Method
Start from the context diagram
Input
Output 1 1
Input
information
Output 2 2
...
...
system
Input n
Output m

14. DFDs: Construction Method
Proceed with refinements until elementary functions are reached.
Preserve balance I A O A3 H J I A1 A4 A6 K P Q M A2 S A5 N R A7 O K K2 B2 M B1 K3 K1 N B4 T B3 Ag K4

15. 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.

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

17. 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

18. 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