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Software enginneering

A quick guide through software engineering

Use Cases

Show the external vew of the system. Are scenario based technique in the UML which identify the actors in an interaction. A task which the actor needs to perform.

• Use -> How you use the system
• Case -> An example
• Actor -> is a user if the system (Can be human or non-human)

The details of each use case should also be documented by use case description

Template

• ID
  • Short ID
• Name
  • Full Name
• Description
  • Full Description
• Pre-condition
  • What must be true before the use case can proceed
• Event flow
  • Flow of behavior that makes up this use case
• Post condition
  • What should be true if the use case successfully completes
• Includes
  • What other use cases are used
• Extensions
  • Optional behavior
• Triggers
  • What Makes this use case happen

Example

Actors

• Can be human
  • Customer
  • Player
  • Driver
• Can be non-human
  • Sensor
  • Payment Service
  • Geo location
  • Robotic arm

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Software Processes

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What is a Process?

Series of activities when providing a service.

All process have the following characteristics:

• It prescribes all of the major activities
• It uses resources and produces intermediate and final products
• It may include sub-processes and has entry and exit criteria
• The activities are organized in a sequence
• Constraints or controls may apply to activities (budget constraints, availability of resources)

Software Processes

When the process involves the building of some product we refer to the process as a life cycle

Coherent sets of activities for software systems

• Specifying
• Designing
• Implementing
• Testing
• Evolution

Software Process Models

The Waterfall Model

Separate and distinct phases of specification and development.

Problems:

• Inflexible partitioning of the project into distinct stages making it difficult to respond to changing customer requirements.
• Describes a process of stepwise refinement based on hardware engineering models, widely used in military and aerospace industries.

Evolutionary Development

Specification and development are interleaved

Based upon the idea of developing an initial implementation, exposing it to the user and refining it based upon their feedback.

Problems:

• Lack of process visibility
• System are sometimes poorly structured

Agile and Scrum

Use widely in industry today. Lightweight approach to software development

Focused:

• Code development as code activity
• Test driven development
• Often use pair programming
• Iterative development
• Self organised teams

Scrum

• Rather than deliver the system as a single delivery, the development and delivery is broken down in to increments (Scrum Sprints) with each increment delivering part of the required functionality.
• User requirements are prioritised and the highest priority requirements are included in early increments.
• Once the development of an increment is started the requirements are frozen though requirements for later increments can continue to evolve.

Advantages

• Customer Value can be delivered with each increment so system functionality is available earlier.
• Early increments act as a prototype to help elicit requirements for later increments.
• Lower risk of overall project failure.
• The highest priority system services tend to receive the most testing.

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Requirements

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• Functional - What the system does.
• Non-Functional - Constrains on how the functions are provided

Objectives

• To introduce the concepts of user and system requirements
• To describe functional / non-functional requirements
• To explain two techniques for describing system requirements

Requirements engineering

Is the process of establishing :

• The services that the customer requires from a system
• The constraints under which it operates and is developed

Why do we need requirements?

To ensure software solution correctly solves a particular problem we must:

• Fully understand the problem that needs to be solved
• Discover why the problem needs to be solved
• Determine who

What is a requirement?

It may range from a high-level abstract statement of a service or of a system constraint to be detailed mathematical functional specification

Types

• User requirements
• System requirements
• Software specification

Functional Requirements

Statements of services the system should provide, how the system should react to particular inputs and how the system should behave in particular situations

Example

• All users will access the system using a user id and a password

Non-functional Requirements

Constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards.

Define system properties and constraints:

• Reliability
• Response time
• Storage requirements.

Process requirements may also be specified, mandating a particular CASE system, programming language or development method.

Classifications

• Product requirements
  • Requirements which specify that the delivered product must behave in a particular way.
• Organisational requirements
  • Requirements which are ……
• External requirements
  • Requirements which arise from factors which are external to the system and its development process e.g. interoperability requirements, legislative requirements

Example:

• All encryption should use the advanced encryption standard
• The system development process and deliverable documents shall conform to the process
• The system shall not disclose any personal information about customers apart from their name.
• The system shall not disclose any personal information about customers apart from their name and reference number to the operators of the system
• The system should respond to a user’s request for information in less than 0.1 seconds during “peak-time” and 0.01 seconds during “normal time”.

Domain Requirements

Requirements that come from the application domain of the system and that reflect characteristics of the domain.

Requirements Imprecision

Problems arise when requirements are not precisely stated.

Requirements Completeness and Consistency

They should include description of all facilities required and there should be no contradictions in the description.

Requirements Measures