Software Requirements Engineering

Types of Requirements

IEEE Std 830 – 1998 http://ieeexplore.ieee.org/xpl/standardstoc.jsp?isnumber=15571&isYear=1998

Defines the following kinds of requirements:

1. Functional

2. External interfaces

3. Performance

4. Logical database

5. Design constraints: standards compliance; software systems attributes

6. Software system attributes: reliability; availability; security; maintainability; portability

External Interfaces

Requirements Specification for Real-Time Systems

Specification methods: formal, informal, semiformal

Formal Methods in Software Specification

There are three general uses for formal methods:

•Consistency checking

•Model Checking

•Theorem Proving

Formal methods also provide opportunities for reusing.

Example from the nuclear monitoring system:

1.1. If interrupt A occurs then task B stops executing

1.2. Task A begins executing upon arrival of interrupt A

1.3. Either task A is executing and task B is not, or task B is executing and task A is not, or both are not executing

qp rp

)()()( qrqrqr

p – interrupt A arrives; q – task B is executing; r – task A is executing

Finite State Machine

Finite State Machine

Mealy Automaton

Statecharts

Statecharts

Chain reaction:

Orthogonal states: if state Y consists of and components A and D, Y is called an orthogonal product of A and D. If Y is entered from outside, both states A and D are entered simultaneously. Communication between the and states can be achieved via global memory, whereas synchronization can be achieved through broadcast communication.

Broadcast communication is depicted by the transition of orthogonal states based on the same event.

Broadcast communication can describe a chain reaction.

Petri Nets

Petri Nets

Petri Nets

Requirements Analysis with Petri Nets

They can be used for race condition and deadlock identification

Structured Analysis and Design

Structured Analysis and Design

Object-Oriented Analysis and the Unified Modeling Language

UML Class Diagrams

Requirements Document

Software Requirements Organization

Requirements to Software Requirements

1. Correct

2. Unambiguous (not subject to different interpretations)

3. Complete

4. Consistent (no contradicting requirements)

5. Ranked for importance

6. Verifiable

7. Modifiable (information hiding)

8. Traceable

Language Issues

Requirements Validation

Software System DesignSoftware Properties

1. Reliability

1.1. r(t) – probability that time T of failure is greater than t:

)()( tTPtr 1.2. Failure function

1.3. Mean time to first failure (MTFF) and Mean time between failures (MTBF)

Software Properties

2. Correctness (close to reliability)

3. Performance

4. Usability

5. Interoperabililty (ability of coexist and cooperate with other systems. Can be measured in terms of compliance with open system standards)

6. Maintainability - a system in which changes are are easy to implement

6.1. Evolvability (how easy to incorporate new)

6.2. Repairability (how easy to fix bugs)

7. Portability

8. Verifiability

Basic Software Engineering Principles

1. Rigor and Formality – use mathematical and algorithmic descriptions

2. Separation of Concerns – Divide-and-Conquer

3. Modularity

Cohesion and Coupling

Basic Software Engineering Principles

4. Anticipation of Change

5. Generality

6. Incrementality – increment provides additional functionality, brings the product closer to the final one

7. Traceability – a high level of traceability ensures that the software requirements flow down through the design and code and then can be traced back up at every stage of the process. Traceability can be obtained by providing links between all documentation and the software code

The Design Activity

Procedural-Oriented Design

Top-down or bottom-up approaches. Parnas partitioning uses principle of information hiding. A list of difficult decisions of things which are likely to change is prepared. Modules are then designated to hide the eventual implementation of of each design decision or feature from the rest of the system. Thus, only the function of the module is visible to other modules, not the method of implementation. Changes in these modules are not likely to affect the rest of the system.

Structured Design and Analysis

Structured Design and Analysis

Data Dictionary is supported:

•Entry type (data flow, data store, terminator, process)

•Name

•Alias

•Description

•Found in

Real-Time Extensions of SASD

Dashed lines are used to show control flow and solid bars show “stored” control commands (control stores)

Relationship between Data and Control Flow Diagrams

Design in Procedural Form Using FSM

Object-Oriented Design

OO languages are characterized by data abstraction, inheritance, polymorphism and messaging.

•Open-Closed Principle – classes should be open to extensions but closed to modifications

•Once and Only Once – any aspect of the software system should exist in only one copy

•Dependency Inversion Principle – high-level modules should not depend on low-level modules

OO Design Using UML

UML Diagrams

1. Activity diagrams – close to flow charts but can model parallel activities

2. Class diagrams

3. Collaboration diagrams – show messages passed between objects

4. Component diagrams – are made of components, interfaces and relationships

5. Deployment diagrams – show real-world nodes and deployment of components in them

6. Object diagrams are related to class diagrams

7. Sequence diagrams are related to collaboration diagrams

8. Statechart Diagrams

UML Sequence Diagrams

UML Collaboration Diagrams

UML Statechart Diagrams

UML Activity Diagrams

UML Deployment Diagrams