Functional Design: Definition, Process & Example

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  • 0:03 What Is Functional Design?
  • 2:15 Functional Design…
  • 3:10 Functional vs Nonfunctional
  • 4:19 Functional Design Example
  • 6:05 Lesson Summary
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Lesson Transcript
Instructor: Ronald Price

Ron has held a variety of positions in higher ed and business, including 25+ years as an instructor and 20+ years as a corporate senior manager, and consultant.

A functional design provides a specification of the functions of a product or service on which the designers and end-users have agreed. This lesson looks at the steps of a functional design in the context of software development.

What Is a Functional Design?

The functional design (FD) phase of a development project focuses on the actions of a new or revised product, program, service, or process. The functional design specification (FDS) identifies what its design object is to do and is more concerned with what is to be done and less with how it happens.

Design and development projects go through several phases, which are typically requirements analysis, functional design, detail design, unit and system testing, documentation, and implementation. The functional design phase provides a translation between the requirements analysis and the detail design.

In a software development project, the functional design focuses on the general definition of the whole system or application. The FDS specifies the logical flow of the system, its inputs and outputs, its data organization, the applicable business and processing rules, and how it should appear to users. At this point in a design project, the underlying hardware and software (operating system, virtualization, platform, etc.) are not important. The overriding purpose of the functional design is to specify a system's actions in a form that allows developers, users, stakeholders, and sponsors to reach a consensus.

Depending on the complexity of the design object, prototyping may be included as a demonstrable part of the functional design. The demonstration of a complicated activity or condition may be easier for stakeholders to understand than a written or oral description. A prototype can be very useful in translating technical design language into an understandable action.

Commonly, the functional design is confused with a technical design. Where a functional design provides the detail of what a system is to do, a technical design (TD) provides the specifics of how the actions are to be carried out. A technical design translates the generalities of a functional design into the specifics of the system's database, programming, and hardware and software requirements.

The Functional Design Specification

The document produced by the functional design phase of a project is an FDS, which spells out the functions of a proposed system, commonly expressed as what the system shall perform. The FDS also marks the point in a development project after which the nature and content of the documentation changes from user-oriented to technician-oriented.

The FDS document should include descriptions of the important systems elements, including:

  • The input data and who can enter it
  • The operations of each action (such as different web pages or screen displays)
  • Any manual or automated workflows required by the system
  • The format of output displays or reports
  • If applicable, how the system conforms to any regulatory requirements

The FDS should be written to a non-technical general audience so that anyone reading it can understand the functions of the system.

Functional vs Non-Functional

The requirements we have discussed are actually functional requirements. In other words, these are the features, actions, and outputs that a system must include to function as desired. On the other hand, we have what are called non-functional requirements, which aren't really non-functional. In fact, these requirements determine how functional the functional requirements really are.

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