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Software prototyping, refers to the activity of creating "disposable", threw-away version of software applications to test the key ideas of the architecture .
A prototype typically simulates only a key aspects of the system, avoiding bells and whistles and as such can use different, higher level implementation language then the final product. It also can be a virtual machine, not a software implementation in traditional sense (see below).
As Wikipedia article states
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 the prototyping have been in development and debate since its proposal in the early 1970s.
The monolithic "waterfall" approach to software development 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 help in avoiding the great expense and difficulty of changing an almost finished software product due to changes in specification or understanding of the problem (and understanding of the problem often comes too late in the "prototype-less" development of software).
The process of prototyping involves the following steps
Approaches to prototyping
There are several appaches to prototyping. Among them there are two more or less realistic:
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.
- Rapid Prototyping involved 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 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 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 work.
- …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.
Not exactly the same as Throwaway Prototyping, but certainly in the same family, is the usage of storyboards, animatics or drawings. 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:
- Write preliminary requirements
- Design the prototype
- User experiences/uses the prototype, specifies new requirements
- Repeat if necessary
- Write the final requirements
- Develop the real products
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 is that the Evolutionary prototype, when built, forms the heart of the new system, and the improvements and further requirements will 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."
In Evolutionary Prototyping, developers can focus themselves to develop parts of the system that they understand instead of working on developing a whole system.
- To minimize risk, the developer does not implement poorly understood features. The partial system is sent to customer sites. As users work with the system, they detect opportunities for new features and give requests for these features to developers. Developers then take these enhancement requests along with their own and use sound configuration-management practices to change the software-requirements specification, update the design, recode and retest.
Scripting language are ideal for software prototyping. Included among these benefits is the ability to address rapidly changing software requirements. In addition, the efficiency gained from user-centered design, which consists of constant user feedback impacting the software design, has received significant attention recently [Landauer95]. Fast compile-debug-test cycles, and high programming flexibility make dynamically typed languages such as Perl, Python, PHP and Tcl ideal for the rapid creation of prototypes both on compoment and system levels
Ousterhout was the first to argue that a two language approach is both feasible and practical for software development [Ousterhout97] and created the scripting language (TCL) that has a transparent interface with C and can be extended using C modules. Python also provide consistent interface to C to allow developers to move performance critical methods and procedures into C without modification of the calling code.
Supporting prototyping requires a language which can support transformation of components previously written in cscritping langue into low level statically typed compiled language (C or C++) and vise versa. Currently, few scripting languages allow for this type of flexibility.
If developers choose to use a statically typed language (for example Java) they often encounter problems in the early prototyping and development phases.
These limitations often make it difficult to use statically typed languages for rapid prototyping of all or part of an application.
On the other hand, if developers choose to use a single dynamically typed language they often encounter problems in the latter phases of development when performance, scalability, and maintainability become critical issues. They often encounter performance problems when writing computationally intensive code and tend to spend an unordinary amount of time optimizing code. They may also suffer scalability problems related to CPU and memory usage typical for scripting languages. The detection of all coding errors is delayed until the actual line of code containing the error is executed meaning that complex tests must be written in order to check the validity of code. Despite the benefits of a scripting languages, these limitations increase the cost of maintaining a large codebase.
Modern software development demands the use of Rapid Application Prototyping. Professor Luqi at the Naval Postgraduate School coined the term "Computer Aided Prototyping" to describe this work. Her leadership showed its effectiveness in gaining understanding of the requirements, reducing the complexity of the problem and providing an early validation of the system design. For every dollar invested in prototyping, one can expect a $1.40 return within the life cycle of the system development.
Dr. Barry Boehm's experiments showed that prototyping reduces program size and programmer effort by 40%. It is the technology that is the foundation for his Spiral development method. Prototyping is being used successfully to gain an early understanding of system requirements, to simplify software designs, to evaluate user interfaces and to test complex algorithms. It is a best-in-class software approach.
Fully 30 to 40% of system requirements will change without prototyping. Rapid Application Prototyping provides a look at the dynamic states of the system before we build it, whereas most other software engineering focuses on the source code. The special problems of reliability, throughput and response time as well as system features are addressed in the best prototypes. A new field of study, Software Dynamics, will emerge once Rapid Application Prototyping is widely practiced. It will focus on quantitative analysis of how software performs under various loads and include a set of design constraints that will make it possible for us to build components that can be hooked together without exhaustive coverage testing.
Software is hard because it has a weak theoretical foundation. Most of the theory that does exist focuses on the static behavior of the software, analysis of the source listing. There is little theory on its dynamic behavior, how it performs under load. To avoid serious network problems software systems are over-engineered with plenty of bandwidth for two or three times the expected load. Without analysis of the dynamic behavior, application designers have no idea of the resources they will need once their software is operational. Software has the awful propensity to fail with no warning. Even after we find and fix a bug, how do we restore the software to a known state, one where we have tested its operation? For most systems, this is impossible except with lots of custom design that is itself error-prone. Software prototyping has proven its mettle in helping designers avoid these problems in their production systems.
Much has been written about the best way to develop software applications. But there is no "best way." Both prototyping and requirements are necessary. The tried and true process of synthesis and analysis is used to solve Software Engineering problems. Bottom-up is synthesis. Top-down is analysis. Bottom-up is prototyping. Top-down is developing requirements. Prototyping is the best way to encourage synthesis. Prototyping also eases communication with the customer and with the designer. Formal written requirements are needed to establish a clear definition of the job, to control changes and to communicate the system capabilities between the customer and the developer.
So where does this leave us?
Start with an English language written statement of a problem and broadly outline its solution. Now build a prototype for the elements where you need insight. Analyze the prototype using computer aided prototyping technology and synthesize a new solution either by refining the prototype or building a new one. Once you and the customer agree on the workings of the prototype, write requirements that include features, performance goals, product costs, product quality, development costs and schedule estimates.
What do I mean by prototype?
Prototyping is the use of approximately 30% of the ultimate staff to build one or two working versions of various aspects of a system. It is not production code but it may eventually become pre-production code or it may be completely discarded. In the prototyping effort, we are not concerned with the maintainability of the code nor are we concerned with formally documenting it. Code resulting from prototyping is often used to train the programmers. Only after we have written specifications resulting from the experience with the prototype should we start the formal development process. If we are fortunate enough that some of the code that was developed for the prototypes can be carried forward, that's great, if not, there is no loss.
A prototype produces "running" software and the production development produces "working" software.
Recent project experience has led to the widespread acceptance of the concept that early prototyping is fundamental to the success of operations supporting software products.The reasons why prototyping is fundamental include:
- The prototype provides a vehicle for systems engineers to better understand the environment and the requirements problem being addressed.
- A prototype is a demonstration of what's actually feasible with existing technology, and where the technical weak spots still exist.
- A prototype is an efficient mechanism for the transfer of design intent from system engineer to the developer.
- A prototype lets the developer meet earlier schedules for the production version.
- A prototype allows for early customer interaction.
- A prototype demonstrates to the customers what is functionally feasible and stretches their imagination, leading to more creative inputs and a more forward-looking system.
- The prototype provides an analysis test bed and a vehicle to validate and evolve system requirements.
About the Author
Mr. Bernstein is president of the Center of National Software Studies and is a recognized expert in Software Technology. He provides consulting through his firm Have Laptop - Will Travel and is the Executive Technologist with Network Programs, Inc. building software systems for managing telephone services.
Mr. Bernstein was an Executive Director of AT&T Bell Laboratories where he worked for 35 years.
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