1. Requirements Specification
Requirements Specification (CS340 John Knight 2005)

2. Requirements Specification
A Critical Document
Focuses Attention On Asking What The System Should Do
Provides Communication:
User To Developer
Between Developers
Developers To Management
Developers To Software Tools
Managers To Regulators
Requirements Specification (CS340 John Knight 2005)

3. Requirements Specification
Important Goals For A Specification Document:
Completeness
Freedom From Ambiguity
To The Extent Reasonable, Specify External System Behavior Only
Specify Constraints On Implementation
Easy To Change
Reference Tool Throughout Lifecycle
Record Forethought About System Lifecycle
Define Acceptable Responses To Unexpected/Unusual Events
Be As Formal As Possible
Requirements Specification (CS340 John Knight 2005)

4. Why Are Specifications Important?
Requirements Specification (CS340 John Knight 2005)

5. Problems With Specifications
Conventional Specifications:
Tend To Be Novels
Tend To Be Wrong
Tend To Say “How”
Do Not Facilitate Communication
Generally Take The Wrong View
Ignore The Possibility Of Change
Are Not Updated During Remainder Of Lifecycle
No Workable Development Process
Requirements Specification (CS340 John Knight 2005)

6. What Notation Can Be Used?
Key Problem Is Communication Failure:
Analyst And User Do Not Talk
User Does Not See Specification
Delivered System First Source Of Feedback
Does This Seem Like A Good Situation?
Many Approaches:
Natural Language
Structured Natural Language (Semi-Formal)
Formal Notations
Requirements Specification (CS340 John Knight 2005)

7. Natural Language Great Expressive Power “Understood” By All
Many words and phrases have multiple meanings.
Used Routinely In Software Development, BUT:
Assumes Everybody Shares The Same Meaning
Ambiguous Even If Meanings Are Shared
No Inherent Structure
Too Expressive: Same Thing Can Be Said Many Ways
Unsystematic: Hard To Effect Change
Can Be A Disaster Waiting To Happen
What Are The Alternatives?
“Fix” Natural Language
Develop Special-Purpose Formal Notations
Requirements Specification (CS340 John Knight 2005)

8. Fixing Natural Language
Could Natural Language Be Fixed?
There Have Been Many Efforts To Do So
Ideas:
Restrict Features Used
Add Supplementary But Familiar Notation(s)
Use Rigid Document Structures
Be Very Careful
Build Natural Language Analyzers
Be As Formal As Possible Within Natural Language
In This Class:
For Project, We Will Use A Combination Of:
Structured Natural Language
Simple But Powerful Formalisms
Study Formal Specification Notation (Later)
Requirements Specification (CS340 John Knight 2005)

9. Formal Specification Languages
Semi-Formal Techniques Turn Out To Be Really Helpful
Much Better Than Straight Natural Language
Can We Do Better?
Formal Languages:
Languages With Precise Semantics Based On Mathematics
Facilitate Accurate Communication Between Engineers
Many Such Languages Developed Usually Based On:
Propositional And Predicate Calculus
Finite Set Theory
Simple Syntax For Composition And Structure
Example Formal Specification Languages:
Z , VDM, ASLAN, Statecharts, PVS
Relatively Unpopular In USA, Relatively Popular In Europe
Requirements Specification (CS340 John Knight 2005)

10. Simple But Very Useful Formalisms
Data Flow Diagrams
Regular Expressions
BNF
Finite State Machines
Petri Nets
Simple Tabular Notations—E.g. Decision Tables:
Requirements Specification (CS340 John Knight 2005)

11. Requirements Specification (CS340 John Knight 2005)
Data Flow Diagrams
Model Of The Data Flow In The System
Documented As A Simple Graph
Arcs Show How Data Moves
Nodes Show Transformations/Actions On Data
Data Sinks And Sources Used For Input And Output
Data Dictionary Explains What Data Is For
Hierarchic Structure—Refinement And Abstraction
Various Simple Analysis Possible
Can Be Used To Develop Useful Designs For Simple Systems
Core Of A Technique Called Structured Analysis
Requirements Specification (CS340 John Knight 2005)

12. Regular Expressions And BNF
Useful For Many Purposes, E.g.:
Format Of Inputs/Outputs
Formats Of Files
Definition Of Command Languages
Note:
Only Defines Context-Free Syntax
Context-Sensitive Syntax Defined Informally
Actions Also Defined By Different Mechanism, Often Informal
Recognizers Usually Developed Automatically:
LEX For Lexical Analysis
YACC For Syntax Analysis
YACC And LEX Provide Mechanisms For Composition Of All Necessary Elements
Use Regular Expressions And BNF For Things That They Support, You Will Get It Wrong Otherwise
Requirements Specification (CS340 John Knight 2005)

13. Finite-State Machines
Software Usually Implements A Finite-State Machine
Describing Required Actions With FSM Is Very Powerful
Approach:
Define States
Define Transitions
Remember That Actions Are Associated With Transitions
Remember That Which Transition Is Taken Depends On:
Current Input
Current State
Carefully Check Initial State And Final State(s)
Sometimes Subtle Mistakes Remain
Much Of The Specification Can Be Tabular
Use FSMs Carefully
Requirements Specification (CS340 John Knight 2005)

14. Project Specification Document
Follow General Outline In Text
Add:
Cover Page
Signature Page
Table Of Contents
Other Sections Dictated By Selected Techniques
Develop Document In English Supplemented By Parts Of A7E Technique
Use Appropriate Techniques Discussed In Class
Take Care With:
Requirements Analysis - Scheduling
Identification Of Risk Areas - Configuration Management
Integration Of Document Parts - Apportioning Of Work
Validation Of Document
Requirements Specification (CS340 John Knight 2005)