Writing a Software Requirement Specification

Question

I have a few questions about writing a specification and they are:

1. When we write a software specification, under the topic "User requirements definition" we have to specify the "Functions" and "Constraints" only ?

2. Does "User Interface" fall into "functions" or "constraints" ?

3. What are the major key areas (requirements) a software can be broken into (e.g. UI ) ?

Answer 1

I recently wrote a blog post about Characteristics of a Good Software Requirements Specification (SRS). There is also an IEEE standard for SRS (830-1998) and in the standard there are templates how to write good SRS document. Easy to follow and very informative.

Answer 2

While I am not a big fan of gathering all requirements in detail up front (as they are subject to so much change over the course of a non trivial project), if you are writing requirements documents, the Volere requirements specification template is an excellent guide.

While it may be overkill for some projects, it provides a great checklist of things to think about, even if it's just to mentally check off the list that you don't need that item for this requirement.

Here's a link to more information about the template:

http://www.volere.co.uk/template.htm

The template itself (and the book Mastering the Requirements Process - which is actually slightly less expensive than the template and contains the full template text) contains a lot of information, examples and advice within the various sections as to what should go in each section.

Here's a summary of the sections in it (quoted from the above link):

1. The Purpose of the Project

2. The Stakeholders

3. Mandated Constraints

4. Naming Conventions and Definitions

5. Relevant Facts and Assumptions

6. The Scope of the Work

7. Business Data Model and Data Dictionary

8. The Scope of the Product

9. Functional and Data Requirements

10. Look and Feel Requirements

11. Usability and Humanity Requirements

12. Performance Requirements

13. Operational and Environmental Requirements

14. Maintainability and Support Requirements

15. Security Requirements

16. Cultural and Political Requirements

17. Legal Requirements

18. Open Issues

19. Off-the-Shelf Solutions

20. New Problems

21. Tasks

22. Migration to the New Product

23. Risks

24. Costs

25. User Documentation and Training

26. Waiting Room

27. Ideas for Solutions

Answer 3

When we write a software specification, under the topic "User requirements definition" we have to specify the "Functions" and "Constraints" only ?

A requirement is a combination of two things...

1. What the thing does. Functional requirement.
2. How well it does it. Non Functional requirement or "Constraint"

Does "User Interface" fall into "functions" or "constraints" ?

I would say "User Interface" would be category of requirements as you have identified in your last question.

What are the major key areas (requirements) a software can be broken into (e.g. UI ) ?

It depends on the software. You can group requirements based on parts of the system or you can group them based on use case or the business requirement that the functions are fulfilling.

Of course all of this is secondary to your actual goal which is to determine a clear, unambiguous and testable description of the software system.

Answer 4

I recommend reading Joel on software. I'm not sure if it answers your specific questions, but he has an excellent overview of what it means to write functional specifications. http://www.joelonsoftware.com/articles/fog0000000036.html

Answer 5

The main requirement for a requirement is that it is testable. If you can't figure out how to test a requirement, the odds are that it won't be implemented the way the writer intended.

I've never seen a requirements document limited to Functions and Constraints only, but I can see some value in having a structure like this - it forces the writer to categorize the requirements into "things the software needs to do", and "rules the software needs to follow."

I think a user interface has requirements in both categories

Constraints:

• "the startup screen shall display two buttons: "Start", and "Stop"
• "The display font shall be no smaller than 10 point."

Functions:

• "When the Start key is pressed, the software shall establish a TCP/IP connection to WOPR"