非功能性需求 Non-functional requirements
Following are IEEE-ish definitions:
functional requirement--- A system/software requirement that specifies a function that a system/software system or system/software component must be capable of performing. These are software requirements that define behavior. of the system, that is, the fundamental process or transformation that software and hardware components of the system perform. on inputs to produce outputs.
nonfunctional requirement--- in software system engineering,a software requirement that describes not what the software will do, but how the software will do it, for example, software performance requirements, software external interface requirements, software design constraints, and software quality attributes. Nonfunctional requirements are difficult to test; therefore, they are usually evaluated subjectively.
非功能性需求是软件设计与实现的作用力（Force）。然而大多数作用力又相互影响，
所以要应付某种作用力而又不影响其余是很难的。
常见非功能性需求包括：
1.可用性 usability
系统应该是客户友好的，来描述系统对于任何人都是没有意义的。可用性应该从以下几个方面进行说明
1.1 使用速度 speed of use
1.2 用户必须具有的能力 required user ability
1.3 学习能力 learnability
1.4 训练材料 training material
1.5 相关文档 documents
1.6 在线帮助 online help
1.7 一致性 consistence
2.可靠性 reliability
2.1 最大失误率 Maximum Failure Rate
2.2 最大当机时间 Maximum Down Time
2.3 系统恢复时间 Ease of Recovery
2.4 最多程序bug数 Maximum Known Bugs
可用每一千行代码存在的bugs数来衡量
3.性能 performance
3.1 吞吐量 Throughout
3.2 响应时间 Response Time
3.3 资源使用量 Resource Usage
3.4 压力测试 Degradation under Overload Conditions
4.安全性 security
4.1 内部安全 internal security
4.2 外部安全 external security
5.支持性 supportability
5.1 安装 ease of installation
5.2 维护 ease of maintenance
5.3 配置 ease of configuration
5.4 测试 ease of testing
6.基本环境要求 Infrastructure Requirements
6.1 客户端 clients
6.2 服务端 servers
6.3 网络架构 networks
6.4 周边设备 Peripherals
6.5 web service
7.执行期约束 Implementation Constraints
7.1 语言 languages
7.2 操作系统 operation system
7.3 标准 standards
7.4 系统接口 system interfaces
7.5 子系统 legacy system
7.6 数据库 database
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