Requirement Is An Atomic Unit Of Product Scope
A Requirement is a type of contractual clause between the user and their system. A Requirement states exactly what each user can do with the system. It is important to note that a system will not perform any functions outside of those stated in the Requirements.
There are Functional and Non-functional Requirements. A Functional Requirement answers the question "what should the system do", and a Non-functional Requirement provides an answer to "how will the system do this"?
Functional Requirements respond when the user makes a 'Request', causing the system to create a 'Reply'. A 'Request' is a formal interaction between the user and system through one of its external interfaces. A 'Reply' is the term for how the system reacts after receiving a Request from the user. Functional Requirements are supported with Non-functional Requirements, which are supplementary specifications in a SRS.
Examples of Functional Requirements and their supplementary specifications include the following:
| # | Requirement | Supplementary |
|---|---|---|
| R1 | User can submit a Request to change the password associated with their account, the system shall generate a Reply by changing the database records for the user. | Web user interface layout, Errors list |
| R2 | Affiliate can submit a Request for an updated report, the system shall generate a Reply with a sorted snapshot of all data for the last week. | XML API Description, Errors list |
| R3 | Administrator can submit a Request for a list of accounts sorted by the date of most recent login, the system shall generate a Reply and sorts the list accordingly. | Web interface layout |
Non-functional requirements can define the following quality characteristics of the system:
- External Interfaces and Error Codes
- Interface Mock-ups and Graphics
- Availability, Reliability, Performance, etc.
Examples of Non-functional Requirements (for the classifications above) include the following:
| # | Definition | Class |
|---|---|---|
| IF1 | System provides XML-based API as defined in the Protocol Description (Protocol-v1.5.pdf) via HTTPS socket. | External Interface |
| SS1 | System works without crashing during 100 hours in 5 consecutive tests with an average load of 1000 concurrent users. | Reliability |
| SS2 | System responds to any type of report request within a 500ms timeframe on the server equipment defined in SRS, and with up to 500 concurrent users, over the course of 100 minutes. | Performance |
Requirements are listed in the SRS, which also includes the Glossary, Use Cases, Functional Requirements, and Non-Functional Requirements.
