A functional specification (also, ''functional spec'', ''specs'', ''functional specifications document (FSD)'', ''functional requirements specification'') in

systems engineering Systems engineering is an interdisciplinary field of engineering and engineering management that focuses on how to design, integrate, and manage complex systems over their Enterprise life cycle, life cycles. At its core, systems engineering uti ...

and

software development Software development is the process of designing and Implementation, implementing a software solution to Computer user satisfaction, satisfy a User (computing), user. The process is more encompassing than Computer programming, programming, wri ...

is a document that specifies the functions that a system or component must perform (often part of a requirements specification) (ISO/IEC/IEEE 24765-2010).ISO/IEC/IEEE 24765:2010 Systems and software engineering — Vocabulary
/ref> The documentation typically describes what is needed by the system user as well as requested properties of inputs and outputs (e.g. of the

software Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications. The history of software is closely tied to the development of digital comput ...

system). A functional specification is the more technical response to a matching requirements document, e.g. the

product requirements document A product requirements document (PRD) is a document containing all the requirements for a certain product. It is written to allow people to understand ''what'' a product should do. A PRD should, however, generally avoid anticipating or defining ...

"PRD". Thus it picks up the results of the

requirements analysis In systems engineering and software engineering, requirements analysis focuses on the tasks that determine the needs or conditions to meet the new or altered product or project, taking account of the possibly conflicting requirements of the v ...

stage. On more complex systems multiple levels of functional specifications will typically nest to each other, e.g. on the system level, on the module level and on the level of technical details.

Overview

A functional specification does not define the inner workings of the proposed system; it does not include the specification of how the system function will be implemented. A functional requirement in a functional specification might state as follows: :''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 Ancient Egyptian roles * User (ancient Egyptian official), an ancient Egyptian nomarch (governor) of the Eighth Dynasty * Useramen, an ancient Egyptian vizier also called "User" Other uses * User (computing), a person (or software) using an ...

) 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.

Functional specification topics

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 In computing, source code, or simply code or source, is a plain text computer program written in a programming language. A programmer writes the human readable source code to control the behavior of a computer. Since a computer, at base, only ...

and

test case In software engineering, a test case is a specification of the inputs, execution conditions, testing procedure, and expected results that define a single test to be executed to achieve a particular software testing objective, such as to exercise ...

s, followed by a period of

debugging In engineering, debugging is the process of finding the Root cause analysis, root cause, workarounds, and possible fixes for bug (engineering), bugs. For software, debugging tactics can involve interactive debugging, control flow analysis, Logf ...

. 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. # To let the developers know what to build. # To let the testers know what tests to run. # To let stakeholders know what they are getting.

Process

In the ordered industrial software engineering life-cycle (

waterfall model The waterfall model is a breakdown of developmental activities into linear sequential phases, meaning that each phase is passed down onto each other, where each phase depends on the deliverables of the previous one and corresponds to a speciali ...

), functional specification describes ''what'' has to be implemented. The next,

systems architecture A system architecture is the conceptual model that defines the structure, behavior, and view model, views of a system. An architecture description is a formal description and representation of a system, organized in a way that supports reasoning ...

document describes ''how'' the functions will be realized using a chosen software environment. In non industrial, prototypical systems development, 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.

Methods

One popular method of writing a functional specification document involves drawing or rendering either simple wire frames or accurate, graphically designed UI screenshots. After this has been completed, and the screen examples are approved by all stakeholders, graphical elements can be numbered and written instructions can be added for each number on the screen example. For example, a login screen can have the username field labeled '1' and password field labeled '2,' and then each number can be declared in writing, for use by software engineers and later for beta testing purposes to ensure that functionality is as intended. The benefit of this method is that countless additional details can be attached to the screen examples.

Examples of functional specifications

* Advanced Microcontroller Bus Architecture *

Extensible Firmware Interface Unified Extensible Firmware Interface (UEFI, as an acronym) is a specification for the firmware architecture of a computing platform. When a computer is powered on, the UEFI implementation is typically the first that runs, before starting th ...

Multiboot specification The Multiboot specification is an open standard describing how a boot loader can load an x86 operating system kernel. The specification allows any compliant boot-loader implementation to boot any compliant operating-system kernel. Thus, it all ...

* Real-time specification for Java *

Single UNIX Specification The Single UNIX Specification (SUS) is a standard for computer operating systems, compliance with which is required to qualify for using the "UNIX" trademark. The standard specifies programming interfaces for the C language, a command-line shell, ...

Types of software development specifications

* Bit specification (disambiguation) *

Design specification A design specification (or product design specification) is a document which details exactly what criteria a product or a process should comply with. If the product or its design are being created on behalf of a customer, the specification should ...

* Diagnostic design specification * Product design specification *

Software requirements specification A software requirements specification (SRS) is a description of a software system to be developed. It is modeled after the business requirements specification (CONOPS). The software requirements specification lays out functional and non-fun ...

References

External links

Painless Functional Specifications, 4-part series by Joel Spolsky
{{DEFAULTSORT:Functional Specification Systems engineering Software documentation Software design bs:Specifikacija programa de:Pflichtenheft hr:Specifikacija programa pt:Especificação de programa