Software prototyping is the activity of creating prototypes of software applications, i.e., incomplete versions of the software program being developed. It is an activity that can occur in software development and is comparable to prototyping as known from other fields, such as mechanical engineering or manufacturing.

A prototype typically simulates only a few aspects of, and may be completely different from, the final product.

Prototyping has several benefits: the software designer and implementer can get valuable feedback from the users early in the project. The client and the contractor can compare if the software made matches the software specification, according to which the software program is built. It also allows the software engineer some insight into the accuracy of initial project estimates and whether the deadlines and milestones proposed can be successfully met. The degree of completeness and the techniques used in prototyping have been in development and debate since its proposal in the early 1970s.

Overview

The purpose of a prototype is to allow users of the software to evaluate developers' proposals for the design of the eventual product by actually trying them out, rather than having to interpret and evaluate the design based on descriptions. Software prototyping provides an understanding of the software's functions and potential threats or issues. Prototyping can also be used by end users to describe and prove requirements that have not been considered, and that can be a key factor in the commercial relationship between developers and their clients. Interaction design in particular makes heavy use of prototyping with that goal.

This process is in contrast with the 1960s and 1970s monolithic development cycle of building the entire program first and then working out any inconsistencies between design and implementation, which led to higher software costs and poor estimates of time and cost. The monolithic approach has been dubbed the "Slaying the (software) Dragon" technique, since it assumes that the software designer and developer is a single hero who has to slay the entire dragon alone. Prototyping can also avoid the great expense and difficulty of having to change a finished software product.

The practice of prototyping is one of the points Frederick P. Brooks makes in his 1975 book The Mythical Man-Month and his 10-year anniversary article "No Silver Bullet".

An early example of large-scale software prototyping was the implementation of NYU's Ada/ED translator for the Ada programming language. It was implemented in SETL with the intent of producing an executable semantic model for the Ada language, emphasizing clarity of design and user interface over speed and efficiency. The NYU Ada/ED system was the first validated Ada implementation, certified on April 11, 1983.

Outline

The process of prototyping involves the following steps:

  1. Identify basic requirements
  2. :Determine basic requirements including the input and output information desired. Details, such as security, can typically be ignored.
  3. Develop initial prototype
  4. :The initial prototype is developed that includes only user interfaces. (See Horizontal Prototype, below)
  5. Review
  6. :The customers, including end-users, examine the prototype and provide feedback on potential additions or changes.
  7. Revise and enhance the prototype
  8. : Using the feedback both the specifications and the prototype can be improved. Negotiation about what is within the scope of the contract/product may be necessary. If changes are introduced then a repeat of steps #3 and #4 may be needed.

Dimensions

Nielsen summarizes the various dimensions of prototypes in his book Usability Engineering:

Horizontal prototype

A common term for a user interface prototype is the horizontal prototype. It provides a broad view of an entire system or subsystem, focusing on user interaction more than low-level system functionality, such as database access. Horizontal prototypes are useful for:

  • Confirmation of user interface requirements and system scope,
  • Demonstration version of the system to obtain buy-in from the business,
  • Develop preliminary estimates of development time, cost and effort.

Vertical prototype

A vertical prototype is an enhanced complete elaboration of a single subsystem or function. It is useful for obtaining detailed requirements for a given function, with the following benefits:

  • Refinement database design,
  • Obtain information on data volumes and system interface needs, for network sizing and performance engineering,
  • Clarify complex requirements by drilling down to actual system functionality.

Types

Software prototyping has many variants. However, all of the methods are in some way based on two major forms of prototyping: throwaway prototyping and evolutionary prototyping.

Throwaway prototyping

Also called close-ended prototyping. Throwaway or rapid prototyping refers to the creation of a model that will eventually be discarded rather than becoming part of the final delivered software. After preliminary requirements gathering is accomplished, a simple working model of the system is constructed to visually show the users what their requirements may look like when they are implemented into a finished system.

It is also a form of rapid prototyping.

:Rapid prototyping involves creating a working model of various parts of the system at a very early stage, after a relatively short investigation. The method used in building it is usually quite informal, the most important factor being the speed with which the model is provided. The model then becomes the starting point from which users can re-examine their expectations and clarify their requirements. When this goal has been achieved, the prototype model is 'thrown away', and the system is formally developed based on the identified requirements.

The most obvious reason for using throwaway prototyping is that it can be done quickly. If the users can get quick feedback on their requirements, they may be able to refine them early in the development of the software. Making changes early in the development lifecycle is extremely cost effective since there is nothing at that point to redo. If a project is changed after a considerable amount of work has been done then small changes could require large efforts to implement since software systems have many dependencies. Speed is crucial in implementing a throwaway prototype, since with a limited budget of time and money little can be expended on a prototype that will be discarded.

Another strength of throwaway prototyping is its ability to construct interfaces that the users can test. The user interface is what the user sees as the system, and by seeing it in front of them, it is much easier to grasp how the system will function.

:…it is asserted that revolutionary rapid prototyping is a more effective manner in which to deal with user requirements-related issues, and therefore a greater enhancement to software productivity overall. Requirements can be identified, simulated, and tested far more quickly and cheaply when issues of evolvability, maintainability, and software structure are ignored. This, in turn, leads to the accurate specification of requirements, and the subsequent construction of a valid and usable system from the user's perspective, via conventional software development models.

Prototypes can be classified according to the fidelity with which they resemble the actual product in terms of appearance, interaction and timing. One method of creating a low fidelity throwaway prototype is paper prototyping. The prototype is implemented using paper and pencil, and thus mimics the function of the actual product, but does not look at all like it. Another method to easily build high fidelity throwaway prototypes is to use a GUI Builder and create a click dummy, a prototype that looks like the goal system, but does not provide any functionality.

The usage of storyboards, animatics or drawings is not exactly the same as throwaway prototyping, but certainly falls within the same family. These are non-functional implementations but show how the system will look.

Summary: In this approach the prototype is constructed with the idea that it will be discarded and the final system will be built from scratch. The steps in this approach are:

  1. Write preliminary requirements
  2. Design the prototype
  3. User experiences/uses the prototype, specifies new requirements
  4. Repeat if necessary
  5. Write the final requirements

Evolutionary prototyping

Evolutionary prototyping (also known as breadboard prototyping) is quite different from throwaway prototyping. The main goal when using evolutionary prototyping is to build a very robust prototype in a structured manner and constantly refine it. The reason for this approach is that the evolutionary prototype, when built, forms the heart of the new system, and the improvements and further requirements will then be built.

When developing a system using evolutionary prototyping, the system is continually refined and rebuilt.

:"…evolutionary prototyping acknowledges that we do not understand all the requirements and builds only those that are well understood."

This technique allows the development team to add features, or make changes that couldn't be conceived during the requirements and design phase.

:For a system to be useful, it must evolve through use in its intended operational environment. A product is never "done;" it is always maturing as the usage environment changes…we often try to define a system using our most familiar frame of reference—where we are now. We make assumptions about the way business will be conducted and the technology base on which the business will be implemented. A plan is enacted to develop the capability, and, sooner or later, something resembling the envisioned system is delivered.

Evolutionary prototypes have an advantage over throwaway prototypes in that they are functional systems. Although they may not have all the features the users have planned, they may be used on an interim basis until the final system is delivered.

:"It is not unusual within a prototyping environment for the user to put an initial prototype to practical use while waiting for a more developed version…The user may decide that a 'flawed' system is better than no system at all."

Incremental prototyping

The final product is built as separate prototypes. At the end, the separate prototypes are merged in an overall design. By the help of incremental prototyping the time gap between user and software developer is reduced.

Extreme prototyping

Extreme prototyping as a development process is used especially for developing web applications. Basically, it breaks down web development into three phases, each one based on the preceding one. The first phase is a static prototype that consists mainly of HTML pages. In the second phase, the screens are programmed and fully functional using a simulated services layer. In the third phase, the services are implemented.

:"The process is called Extreme Prototyping to draw attention to the second phase of the process, where a fully functional UI is developed with very little regard to the services other than their contract."

Advantages

There are many advantages to using prototyping in software development – some tangible, some abstract.

Reduced time and costs: Prototyping can improve the quality of requirements and specifications provided to developers. Because changes cost exponentially more to implement as they are detected later in development, the early determination of what the user really wants can result in faster and less expensive software.

Applicability

It has been argued that prototyping, in some form or another, should be used all the time. However, prototyping is most beneficial in systems that will have many interactions with the users.

:It has been found that prototyping is very effective in the analysis and design of on-line systems, especially for transaction processing, where the use of screen dialogs is much more in evidence. The greater the interaction between the computer and the user, the greater the benefit is that can be obtained from building a quick system and letting the user play with it. is a framework for delivering business solutions that relies heavily upon prototyping as a core technique, and is itself ISO 9001 approved. It expands upon most understood definitions of a prototype. According to DSDM the prototype may be a diagram, a business process, or even a system placed into production. DSDM prototypes are intended to be incremental, evolving from simple forms into more comprehensive ones.

DSDM prototypes can sometimes be throwaway or evolutionary. Evolutionary prototypes may be evolved horizontally (breadth then depth) or vertically (each section is built in detail with additional iterations detailing subsequent sections). Evolutionary prototypes can eventually evolve into final systems.

The four categories of prototypes as recommended by DSDM are:

  • Business prototypes – used to design and demonstrates the business processes being automated.
  • Usability prototypes – used to define, refine, and demonstrate user interface design usability, accessibility, look and feel.
  • Performance and capacity prototypes – used to define, demonstrate, and predict how systems will perform under peak loads as well as to demonstrate and evaluate other non-functional aspects of the system (transaction rates, data storage volume, response time, etc.)
  • Capability/technique prototypes – used to develop, demonstrate, and evaluate a design approach or concept.

The DSDM lifecycle of a prototype is to:

  1. Identify prototype
  2. Agree to a plan
  3. Create the prototype
  4. Review the prototype

Operational prototyping

Operational prototyping was proposed by Alan Davis as a way to integrate throwaway and evolutionary prototyping with conventional system development. "It offers the best of both the quick-and-dirty and conventional-development worlds in a sensible manner. Designers develop only well-understood features in building the evolutionary baseline, while using throwaway prototyping to experiment with the poorly understood features." was developed by the Software Productivity Consortium, a technology development and integration agent for the Information Technology Office of the Defense Advanced Research Projects Agency (DARPA).

:Fundamental to ERD is the concept of composing software systems based on the reuse of components, the use of software templates and on an architectural template. Continuous evolution of system capabilities in rapid response to changing user needs and technology is highlighted by the evolvable architecture, representing a class of solutions. The process focuses on the use of small artisan-based teams integrating software and systems engineering disciplines working multiple, often parallel short-duration timeboxes with frequent customer interaction.

:Key to the success of the ERD-based projects is parallel exploratory analysis and development of features, infrastructures, and components with and adoption of leading edge technologies enabling the quick reaction to changes in technologies, the marketplace, or customer requirements.

To simulate applications one can also use software that simulates real-world software programs for computer-based training, demonstration, and customer support, such as screencasting software as those areas are closely related.

Requirements Engineering Environment

"The Requirements Engineering Environment (REE), under development at Rome Laboratory since 1985, provides an integrated toolset for rapidly representing, building, and executing models of critical aspects of complex systems."

Requirements Engineering Environment is currently used by the United States Air Force to develop systems. It is:

:an integrated set of tools that allows systems analysts to rapidly build functional, user interface, and performance prototype models of system components. These modeling activities are performed to gain a greater understanding of complex systems and lessen the impact that inaccurate requirement specifications have on cost and scheduling during the system development process. Models can be constructed easily, and at varying levels of abstraction or granularity, depending on the specific behavioral aspects of the model being exercised. This system is named the Advanced Requirements Engineering Workstation or AREW.

Non-relational environments

Non-relational definition of data (e.g. using Caché or associative models) can help make end-user prototyping more productive by delaying or avoiding the need to normalize data at every iteration of a simulation. This may yield earlier/greater clarity of business requirements, though it does not specifically confirm that requirements are technically and economically feasible in the target production system.

PSDL

PSDL is a prototype description language to describe real-time software.

The associated tool set is CAPS (Computer Aided Prototyping System).

Prototyping software systems with hard real-time requirements is challenging because timing constraints introduce implementation and hardware dependencies.

PSDL addresses these issues by introducing control abstractions that include declarative timing constraints. CAPS uses this information to automatically generate code and associated real-time schedules, monitor timing constraints during prototype execution, and simulate execution in proportional real time relative to a set of parameterized hardware models. It also provides default assumptions that enable execution of incomplete prototype descriptions, integrates prototype construction with a software reuse repository for rapidly realizing efficient implementations, and provides support for rapid evolution of requirements and designs.

References