ソフトウェア開発ライフサイクル

The airport that opened sixteen months late

• Denver's new airport was due to open in October 1993 with the most ambitious baggage system ever built: 4,000 automatic carts, 35 km of track, no human hands. It opened in February 1995, $560 million over budget, with the carts shredding suitcases in test runs.
• The schedule had been fixed before anyone analysed what the system had to do. Requirements kept changing while it was being built. Testing started when there was no time left to fix what it found.
• Every one of those failures has a name in this lesson. A development life cycle 开发生命周期 exists so that a project is planned, managed and controlled instead of discovered.
• This lesson is what a life cycle is for, the three models the syllabus names, and the five stages every program goes through.

Why a life cycle is needed

• A development life cycle is the set of stages from idea to finished, maintained software. Its purpose is to plan, manage and control a project: the right product, on time, with good quality.
• It manages complexity by breaking a big program into phases, coordinates a team, and tracks progress against milestones.
• It builds in testing rather than leaving it to the end, records design decisions for whoever maintains the program later, and manages risk.

Why there is more than one

• No single life cycle fits every project, so several exist and the exam asks you to choose.
• The choice depends on the size and complexity of the program, how clear the requirements 需求 are at the start, how much change is expected, the risk if it fails, the team, and the deadline.
• A payroll system with fixed legal rules and a game whose players do not yet know what they want are different projects. They deserve different cycles.

Waterfall

• The waterfall 瀑布模型 model is a linear sequence: analysis, design, coding, testing, maintenance. Each stage is finished, documented and signed off before the next begins.
• Benefits: clear structure, easy to manage and schedule, thorough documentation at every stage, well suited to stable requirements.
• Drawbacks: a change of requirements mid-project means going back up the waterfall; the customer sees nothing working until the end; errors in analysis surface only at testing.

Water does not flow back uphill

Iterative

• The iterative model 迭代模型 makes repeated passes. Each pass produces a partial working version that is reviewed by the user and refined in the next pass.
• Benefits: problems are caught earlier; the user sees working software early and often; good when requirements are discovered over time.
• Drawbacks: harder to estimate the total cost and time; repeated reviews need the user's attention; the design can drift without discipline.

Build, show, refine, repeat

Rapid application development

• Rapid application development 快速应用开发 (RAD) builds a prototype 原型 quickly, puts it in front of the users, and rebuilds it from their feedback until it becomes the product. Teams often work on parts in parallel.
• Benefits: a very fast first delivery; requirements that change are absorbed as they appear; users shape the product directly.
• Drawbacks: it depends on users being available throughout; the prototype's shortcuts can survive into the product; it suits smaller systems better than large safety-critical ones. Agile 敏捷 methods take the same idea further, with short sprints and constant testing.

Prototype, feedback, prototype again

Worked example: choose a life cycle and justify it

The five stages

• Analysis: find out what the program must do; gather and document the requirements from the users.
• Design: decide how: data structures, algorithms, modules, the user interface, file layouts, expressed in structure charts, flowcharts and pseudocode.
• Coding (implementation 实现): write the source code following the design. Testing: run it against planned test data and correct the errors found.
• Maintenance 维护: after release, keep it working and useful, correcting faults, adapting to change and improving it.

Worked example: what happens in analysis and design

Marks that slip away

You've got it