Software Engineering

Software Engineering The Software Process

Why Software Engineering? • Can you approach building software as building a bridge? Why? Why not? • How is software engineering like other engineering disciplines?

Software Process A structured set of activities required to develop a software system. Fundamental Assumption: Good processes lead to good software Good processes reduce risk

Risk Management What can go wrong in a software project? How can the risk be reduced?

The software process • Many different software processes but all involve: • Specification: defining what the system should do; • Design and implementation: defining the organization of the system and implementing it • Validation – checking that it does what the customer wants; • Evolution – changing the system in response to changing customer needs.

Software Process Models A software process model is an abstract representation of a process. • The waterfall model • Separate and distinct phases of specification and development (product focused) • Evolutionary development • Specification, development and validation are interleaved e.g. XP (Beck), increment driven • Component-based software engineering • The system is assembled from existing components. • Spiral (Boehm) • driven by risk analysis and mitigation

1950 ：Code-and-fix model 1956 ： Stagewise model (Bengington ) 1970 ： Waterfall model (Royce) 1971 ： Incremental model(Mills) 1977 ： Prototyping model(Ballyand others) 1988 ： Spiral model(Boehm) Background on software process models

Use of the Models in Practice

The Waterfall Model Requirements Definition System and Software design Programming and Unit Testing Integration and System Testing Operation and Maintenance

perceived need requirements design code test integrate Waterfall Modelmaterial adapted from Steve Easterbrook • View of development: • A process of stepwise refinement • High-level managementview • Problems: • Static view of requirements (ignores volatility) • Lack of user involvement once spec is written • Unrealistic separation of spec from design • Doesn’t accommodate prototyping, reuse

[1] Requirements Analysis and Definition • The system's services, constraints and goals are established by consultation with system users. They are then defined in a manner that is understandable by both users and development staff. • This phase can be divided into: •  Feasibility study (often carried out separately) •  Requirements analysis •  Requirements definition •  Requirements specification

Terminology • A requirement is a technical objective which is imposed upon the software, i.e., anything that might affect the kind of software that is produced • A requirement may be imposed by • the customer • the developer • the operating environment • The requirement must be documented • Requirements fall into two broad categories • functional • non-functional

Functional Requirements • Functional requirements are concerned with what the software must do • capabilities, services, or operations • Functional requirements are not concerned with how the software does things, i.e., they must be free of design considerations

Non-Functional Requirements • Non-functional requirements place restrictions on the range of acceptable solutions • Non-functional requirements cover a broad range of issues • interface constraints • performance constraints • operating constraints • life-cycle constraints • economic constraints • political constraints • manufacturing

Case Study • We participated in a group project where we designed and developed a cell phone game suitable for a cell phone using the programming language Java 2 Micro Edition. • We designed everything from the beginning with no game requirements given to us. • The process consists of defining system requirements, building the system, and testing the system. The goal of the project was to have a working game. We were restricted by time since this was a summer program

Define System Requirements • The system is designed to make the primary persona happy but the other personas should not be unhappy • Define user scenarios • User scenarios describe how someone will interact with the system. • This includes actual scenarios that could happen to a person while playing through the game.

Define System Requirements • Describe the user interface • Describe main interface that the user will interact with. • This includes screen interactions and all options the user will have. • Detailed requirements • Includes all functional, nonfunctional, and constraint requirements that the system must satisfy

[2] System and Software Design System design: Partition the requirements to hardware or software systems. Establishes an overall system architecture Software design: Represent the software system functions in a form that can be transformed into one or more executable programs  Unified Modeling Language (UML)

Case Study • We participated in a group project where we designed and developed a cell phone game suitable for a cell phone using the programming language Java 2 Micro Edition. • We designed everything from the beginning with no game requirements given to us. • The process consists of defining system requirements, building the system, and testing the system. The goal of the project was to have a working game. We were restricted by time since this was a summer program

Build System • Define the system classes • Classes are derived from the nouns in the user scenarios. • Every system activity or primitive function should reside in a class • Define system sequence diagrams • Sequence diagrams show how the classes work together to execute each use-case function

Build System

[3] Programming and Unit Testing The software design is realized as a set of programs or program units. (Written specifically, acquired from elsewhere, or modified.) Individual components are tested against specifications.

Build System • Implement the system • The class skeletons are coded based on the code generated from the class diagrams. • All coding takes place here.

[4] Integration and System Testing The individual program units are:  integrated and tested as a complete system  tested against the requirements as specified  delivered to the client

[5] Operation and Maintenance  Operation: The system is put into practical use. Maintenance: Errors and problems are identified and fixed.  Evolution: The system evolves over time as requirements change, to add new functions or adapt the technical environment.  Phase out: The system is withdrawn from service.

Discussion of the Waterfall Model Advantages:  Process visibility  Dependence on individuals  Quality control  Cost control

The Classical Software Lifecycle The classical software lifecycle models the software development as a step-by-step “waterfall” between the various development phases. Requirements Collection Analysis Design Implementation Testing Maintenance The waterfall model is unrealistic for many reasons: • requirements must be frozen too early in the life-cycle • requirements are validated too late

Problems with the Waterfall Lifecycle • “Real projects rarely follow the sequential flow that the model proposes. Iteration always occurs and creates problems in the application of the paradigm” • “It is often difficult for the customer to state all requirements explicitly. The classic life cycle requires this and has difficulty accommodating the natural uncertainty that exists at the beginning of many projects.” • Each stage in the process reveals new understanding of the previous stages, that requires the earlier stages to be revised.

Cons • Not a good model for complex and object-oriented projects. • High amounts of risk and uncertainty. • Not suitable for the projects where requirements are at a moderate to high risk of changing. So risk and uncertainty is high with this process model.

Problem Case Study • Publisher of books want to develop a product that will carry out all the accounting functions of the company and provide online information to the head office staff regarding orders and inventory • Terminals are required for 15 accounting clerks, 32 order clerks, and 15 managers need access to the data. The publisher will pay 30000 and wants the complete product in 4 weeks • What do you tell him?

Case Study: Sensor Display • Consider a small system that displays the status of several sensors (e.g., pressure, temperature, rate of change, etc.) on a limited-size screen • What are some of its functional requirements? • What are some of its non-functional requirements?

Iterative Development In practice, development is always iterative, and all activities progress in parallel. Requirements Testing based on requirements Collection Testing Maintenance through iteration Analysis Testing throughout implementation Validation through prototyping If the waterfall model is pure fiction, why is it still the dominant software process? Implementation Design Design through refactoring

Iterative Refinement Requirements Evaluation Implementation (prototype) Design

design design design design code code code code test test test test integrate integrate integrate integrate O&M O&M O&M O&M Release 1 Incremental development (each release adds more functionality) Requirements release 2 release 3 release 4 version 1 reqts reqts design design code code test test integrate integrate O&M O&M lessons learnt version 2 lessons learnt Evolutionary development (each version incorporates new requirements) version 3 reqts design code test integrate Phased Modelsmaterial adapted from Steve Easterbrook

Incremental Development Plan to incrementally develop (i.e., prototype) the system. • If possible, always have a running version of the system, even if most functionality is yet to be implemented. • Integrate new functionality as soon as possible. • Validate incremental versions against user requirements.

Incremental development

Incremental delivery • Rather than deliver the system as a single delivery, the development and delivery is broken down into increments with each increment delivering part of the required functionality. • User requirements are prioritised and the highest priority requirements are included in early increments. • It is easier to get customer feedback on the development work that has been done.

Incremental development 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. • The cost of accommodating changing customer requirements is reduced.

When to use the Incremental model • This model can be used when the requirements of the complete system are clearly defined and understood. • Major requirements must be defined; however, some details can evolve with time. • There is a need to get a product to the market early. • A new technology is being used • Resources with needed skill set are not available • There are some high risk features and goals.

Evolutionary Process Model

Software Engineering