📚 System Development Life Cycle (SDLC) Phases Explained | 系统开发生命周期各阶段详解
The Systems Development Life Cycle (SDLC) is a structured process used to plan, create, test, and deploy an information system. For A-Level Computer Science, understanding each phase is essential for answering exam questions on system development methodologies.
系统开发生命周期（SDLC）是一种用于规划、创建、测试和部署信息系统的结构化流程。对于 A-Level 计算机科学而言，理解每个阶段对于回答系统开发方法相关的考题至关重要。
1. Overview of the SDLC | 系统开发生命周期概述
The SDLC provides a framework that breaks down the entire system development process into manageable stages. Each stage has a clear objective, set of inputs, outputs, and deliverables. This helps developers, clients, and stakeholders monitor progress and control quality.
SDLC 提供了一个框架，将整个系统开发过程分解为可管理的阶段。每个阶段都有明确的目标、输入、输出和交付物，这有助于开发者、客户和利益相关者监控进度并控制质量。
- English: The most common phases are planning, analysis, design, implementation, testing, deployment, and maintenance.
- 中文：最常见的阶段包括：规划、分析、设计、实现、测试、部署和维护。
Different process models, such as the waterfall model and agile methodologies, organise these phases in different ways, but the underlying principles remain consistent.
不同的过程模型，如瀑布模型和敏捷方法，以不同方式组织这些阶段，但其基本原则是一致的。
2. Planning and Feasibility Study | 规划与可行性研究
Planning is the first and arguably most critical phase. It defines the scope of the project, its objectives, and the resources required. A feasibility study is carried out to evaluate whether the project is technically, economically, and operationally viable.
规划是第一个阶段，也可以说是最关键的阶段。它定义项目的范围、目标和所需资源。通过可行性研究来评估项目在技术上、经济上和操作上是否可行。
- English: The feasibility study examines technical feasibility (can we build it?), economic feasibility (will it cost too much?), and social/organisational feasibility (will users accept it?).
- 中文：可行性研究考察技术可行性（我们能构建吗？）、经济可行性（成本是否过高？）以及社会/组织可行性（用户会接受吗？）。
| Feasibility Type | Question Asked |
| Technical | Is the hardware/software available? |
| Economic | Is the cost justified by benefits? |
| Operational | Will the system fit the organisation? |
A project charter or project plan is often produced at the end of this phase, outlining the timeline and milestones.
在此阶段结束时，通常会生成项目章程或项目计划，概述时间线和里程碑。
3. Requirements Analysis | 需求分析
During this phase, analysts work with end users and stakeholders to identify and document the detailed functional and non-functional requirements of the proposed system. The goal is to understand exactly what the system must do.
在此阶段，分析师与最终用户和利益相关者合作，识别并记录所提议系统的详细功能性和非功能性需求。目标是准确理解系统必须做什么。
- English: Functional requirements describe what the system should do, such as “the system must calculate VAT.”
- 中文：功能性需求描述系统应做什么，例如“系统必须计算增值税”。
- English: Non-functional requirements describe how the system behaves, such as performance, security, and usability standards.
- 中文：非功能性需求描述系统如何表现，例如性能、安全性和可用性标准。
Common fact-finding methods include interviews, questionnaires, observation, and document analysis. The output is usually a software requirements specification (SRS) document.
常见的事实调查方法包括访谈、问卷调查、观察和文档分析。输出通常是软件需求规格说明书（SRS）文档。
4. System Design | 系统设计
In the design phase, the requirements are translated into a detailed blueprint for the system. This includes the overall architecture, data structures, databases, user interfaces, and hardware/software specifications.
在设计阶段，需求被转化为系统的详细蓝图，包括整体架构、数据结构、数据库、用户界面以及硬件/软件规格。
- English: High-level design describes the system architecture and modules.
- 中文：概要设计描述系统架构和模块。
- English: Low-level design provides the detailed logic of each module, including algorithms and database schemas.
- 中文：详细设计提供每个模块的具体逻辑，包括算法和数据库模式。
Design tools such as entity-relationship diagrams (ERDs), data flow diagrams (DFDs), and Unified Modeling Language (UML) diagrams are commonly used.
常用的设计工具包括实体关系图（ERD）、数据流图（DFD）和统一建模语言（UML）图。
5. Development and Implementation | 开发与实现
This phase is where the actual code is written. Programmers convert the design documentation into a working system using programming languages, databases, and other tools. It is often the longest phase in the SDLC.
这个阶段是实际编写代码的阶段。程序员使用编程语言、数据库和其他工具将设计文档转换为可运行的系统。这通常是 SDLC 中最长的阶段。
- English: The development phase should follow programming standards and code reviews to ensure quality.
- 中文：开发阶段应遵循编程标准和代码审查以保证质量。
- English: Version control systems are used to track changes and facilitate collaboration.
- 中文：使用版本控制系统来跟踪变更并促进协作。
During implementation, individual modules may be developed separately and then integrated. This phase is closely linked with testing because code is often tested by developers before handover.
在实现过程中，各个模块可以单独开发，然后再集成。此阶段与测试紧密相关，因为代码在移交前通常由开发者进行测试。
6. Testing | 测试
Testing is a separate but critical phase that verifies the system meets the specified requirements and is free of bugs. It can be performed at multiple levels, including unit, integration, system, and acceptance testing.
测试是一个独立但关键的阶段，用于验证系统是否满足指定需求且没有错误。测试可以在多个层面执行，包括单元测试、集成测试、系统测试和验收测试。
- English: Unit testing checks individual components.
- 中文：单元测试检查单个组件。
- English: Integration testing checks that modules work together correctly.
- 中文：集成测试检查模块能否正确协同工作。
- English: System testing checks the whole system against the requirements.
- 中文：系统测试根据需求检查整个系统。
- English: Acceptance testing is carried out with the user to confirm the system is ready for use.
- 中文：验收测试与用户共同进行，以确认系统可以投入使用。
Test data should include normal, boundary, and invalid values to ensure the system handles edge cases correctly.
测试数据应包含正常值、边界值和无效值，以确保系统能正确处理边界情况。
7. Deployment | 部署
After testing is complete, the system is deployed to the live environment. This could be a new installation, an upgrade, or a replacement of an existing system. Several implementation strategies exist.
测试完成后，系统将被部署到实际运行环境中。这可以是全新安装、升级或替换现有系统。存在多种实施策略。
| Strategy | Description |
| Direct changeover | Old system is switched off, new system on. |
| Parallel running | Both systems run together for a while. |
| Phased implementation | System is introduced in stages. |
| Pilot implementation | System is used by one small group first. |
Each strategy has trade-offs between risk, cost, and disruption. Direct changeover is risky but cheapest; parallel running is safer but more expensive.
每种策略在风险、成本和干扰之间都有权衡。直接切换风险大但成本最低；并行运行更安全但成本更高。
8. Maintenance | 维护
Once deployed, the system enters the maintenance phase, which continues for the rest of the system’s life. Maintenance is often the most expensive stage because ongoing changes are needed to fix errors, adapt to new environments, and add features.
一旦部署，系统进入维护阶段，该阶段持续到系统生命周期结束。维护通常是成本最高的阶段，因为需要持续进行更改以修复错误、适应新环境并添加功能。
- English: Corrective maintenance fixes bugs and defects discovered after release.
- 中文：更正性维护修复发布后发现的错误和缺陷。
- English: Adaptive maintenance changes the system to cope with new hardware or software environments.
- 中文：适应性维护使系统适应新的硬件或软件环境。
- English: Perfective maintenance improves performance or adds new features.
- 中文：完善性维护改进性能或添加新功能。
- English: Preventive maintenance is carried out to prevent future errors.
- 中文：预防性维护用于防止将来发生错误。
Maintenance requires careful change control and regression testing to ensure that updates do not introduce new problems.
维护需要仔细的变更控制和回归测试，以确保更新不会引入新问题。
9. Documentation and User Training | 文档与用户培训
Although documentation and training are not always listed as separate phases, they are vital throughout the SDLC. Good documentation supports maintenance, and user training ensures the system is used effectively.
虽然文档和培训并不总被列为独立阶段，但它们在 SDLC 中至关重要。良好的文档支持维护，用户培训确保系统被有效使用。
- English: Technical documentation describes how the system is built, including architecture, code, and data structures.
- 中文：技术文档描述系统的构建方式，包括架构、代码和数据结构。
- English: User documentation provides instructions for end users, such as manuals and help files.
- 中文：用户文档为最终用户提供指导，如使用手册和帮助文件。
- English: Training may use hands-on sessions, tutorials, or online resources.
- 中文：培训可以使用实操课程、教程或在线资源。
A well-documented system is easier to maintain and upgrade, and it reduces dependency on the original developers.
文档完善的系统更容易维护和升级，并减少对原始开发者的依赖。
Understanding the SDLC phases helps you analyse real-world system development scenarios and answer exam questions about methodologies, deliverables, and stakeholder roles. Each phase produces specific outputs, and skipping any stage can lead to project failure.
理解 SDLC 各阶段有助于你分析现实世界的系统开发场景，并回答有关方法、交付物和利益相关者角色的考试问题。每个阶段都会产生特定的输出，跳过任何阶段都可能导致项目失败。
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