Chapter 2 SW Process Models

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Objectives After completing this chapter you should be able to: Describe various SW process models Identify the advantages and disadvantages of each model Evaluate the applicability of each model for a certain type of software development

Overview What are SW process models? Why SW process models? The Waterfall Model Evolutionary Models The CBSE Model Agile Models Which is best?

A Software Process is A structured set of activities required to develop a software system Requirements Specification Design and Implementation Validation Evolution

Ad hoc Software Development Developing software without planning for each phase, and without specifying tasks, deliverables, or time constraints. Relies entirely on the skills and experience of the individual staff for performing the work. The software process is constantly changed or modified as the work progresses.

Activity Formulate Teams As a team, answer the following question: What do you think are the problems with ad-hoc development.

Ad hoc Software Development Some problems with ad-hoc development Difficult to distinguish between tasks and therefore some important tasks may be ignored Schedules, budgets, functionality, and product quality are generally (inconsistent). If problems are found late, they are more costly to fix. How can we avoid these problems?

We need .. A Software process Model which is “an abstract representation of a process. It presents a description of a process from some particular perspective.” It provides guidelines to organize how software process activities should be performed and in what order.

SW Process Models Waterfall model Evolutionary models Component-based development model Iterative Models

THE Waterfall model

The Waterfall Model Niagara Falls, Canada

The Waterfall Model Oldest model, it’s been around since 1970. Called “Linear Sequential Model”. Most widely used model for SW engineering Documentation is produced at each stage.

Phases Requirements analysis and definition System and software design Implementation and unit testing Integration and system testing Operation and maintenance

Waterfall model diagram

Evolutionary Models

Evolutionary Models Exploratory Model Prototyping Model

The Exploratory Model Objective is to work with customers and evolve a final system from an initial outline specification. Should start with well-understood requirements and add new features as proposed by the customer.

The Exploratory Model

The Prototyping Model When a customer defines a set of general objectives for a software but does not identify detailed input, processing, or output requirement. It consists of the iterating phases: Requirements gathering Design and build SW prototype Evaluate prototype with customer Refine requirements

The Prototyping Model Listen to Customer Build / Revise prototype Customer Test Drives the prototype

Cbse model

Component Based Software Engineering (CBSE) Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems. Process stages Component analysis; Requirements modification; System design with reuse; Development and integration. This approach is becoming increasingly used as component standards have emerged.

Component Based Software Engineering (CBSE)

Iterative models

Iterative Models Incremental Model Spiral Model Agile Model

The Incremental Model 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. Once the development of an increment is started, the requirements are frozen though requirements for later increments can continue to evolve.

The Incremental Model

The Spiral Model Defined by Barry Boehm in his 1988 article A Spiral Model of Software Development and Enhancement. Process is represented as a spiral rather than as a sequence of activities with backtracking. Each loop in the spiral represents a phase in the process. Suitable for large, expensive and complicated projects

The Spiral Model

The Spiral Model Objective setting Risk assessment and reduction Specific objectives for the phase are identified. Risk assessment and reduction Risks are assessed and activities put in place to reduce the key risks. Development and validation A development model for the system is chosen which can be any of the generic models. Planning The project is reviewed and the next phase of the spiral is planned.

The Spiral Model Risk driven process model What is a risk? Different risk patterns can lead to choosing different process models What is a risk? Situations or possible events that may cause a project to fail to meet its goal. Example risks: Experienced staff leave the project Hardware which is essential for the system will not be delivered on schedule (more about risks in Chapter 3)

Agile Models- Individual Reading Individual Homework Assignment , more details later in the chapter.

Evaluation of Models

Activity Formulate teams Each team chooses a slip from the envelope. List the advantages, disadvantages, and when to use a certain software process model. Share your list with the rest of the class.

The Waterfall Model Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements. Only appropriate when the requirements are well-understood and changes will be fairly limited during the design process. The waterfall model is mostly used for large systems engineering projects.

The Exploratory Model Problems Applicability Lack of process visibility; Systems are often poorly structured; Applicability For small or medium-size interactive systems; For parts of large systems (e.g. the user interface); For short-lifetime systems.

The Prototyping Model Advantages Disadvantages Users get a feel for the actual system Developers get to build something immediately Specifications can be developed incrementally Disadvantages The developer may make implementation compromises in order to get a prototype working quickly. The process in not visible (few documents that reflect every version of the system) Systems poorly structured

Component Based Software Engineering (CBSE) Advantages: Reduce amount of software to be developed Reduce costs and risks Faster delivery Disadvantages: Requirements compromises, system does not meet real needs of users Control over system evolution is lost

The Incremental Model 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 Incremental Model Disadvantages: Increments should be relatively small (20,000 lines of code) Can be difficult to map the customer’s requirements onto increments of the right size Hard to identify common functions

The Spiral Model Advantages: Risks are explicitly assessed and resolved throughout the process. Software engineers can start working on the project earlier rather than wading through a lengthy early design process.

The Spiral Model Disadvantages: Requires highly skilled people in risk analysis and planning Requires more time, and is more expensive Estimates of budget and time are harder to judge at the beginning of the project since the requirements evolve through the process

Which model is best? The choice of a model depends on the project circumstances and requirements. A combination of models is used sometimes get the benefits of more than one model. Criteria for evaluating models: Risk management Quality / cost control Visibility of progress Early system functionality Customer involvement and feedback

Assignment 1 Individual Homework Write a short essay about Agile models (history, description, difference from other models, known methods, evaluation and suitability. The essay should not exceed 1000 words. More details can be found on the course blog.