Functional specifiation
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In software development, a functional specification (also, functional spec or specs) is the set of documentation that describes the behavior of a computer program or larger software system. The documentation typically describes various inputs that can be provided to the software system and how the system responds to those inputs.
[edit] Interactions between outside agents and the system
Unlike source code or a technical design document, a functional specification does not typically define the inner workings of the proposed system. Instead, it focuses on what various outside agents (people using the program, computer peripherals, or other computers, for example) might "observe" when interacting with the system.
A typical functional specification might state the following:
When the user clicks the OK button, the dialog is closed and the focus is returned to the main window in the state it was in before this dialog was displayed.
Such a requirement describes an interaction between an external agent (the user) and the software system. When the user provides input to the system by clicking the OK button, the program responds (or should respond) by closing the dialog window containing the OK button.
[edit] Purpose
There are many purposes for functional specifications. One of the primary purposes on team projects is to achieve some form of team consensus on what the program is to achieve before making the more time-consuming effort of writing source code and test cases, followed by a period of debugging. Typically, such consensus is reached after one or more reviews by the stakeholders on the project at hand after having negotiated a cost-effective way to achieve the requirements the software needs to fulfill.
[edit] Process
In traditional (waterfall model) software development methodologies, functional specifications are typically written after or as part of requirements analysis.
When the team agrees that functional specification consensus is reached, the functional spec is typically declared "complete" or "signed off". After this, typically the software development and testing team write source code and test cases using the functional specification as the reference. While testing is performed the behavior of the program is compared against the expected behavior as defined in the functional specification.
