Different Models of Software Development

• The Waterfall Model

Making something can be thought of as a linear sequence of events. You start at A, you do B and then go to C and eventually end up at Z. This is extremely simplistic but it does allow you to visualise the series of events in the simplest way and it emphasises the importance of delivery with steps being taken towards a conclusion.
Below is the “Waterfall Model” which shows typical development tasks flowing into each other. Early in the history of software development it was adapted from engineering models to be a blueprint for software development.

Requirement -> Design -> Implementation -> Verification -> Maintenance
WaterFall Model

The five steps outlined are :

• Analyse the requirements of the project and decide what it is supposed to do
• Design a solution to meet these requirements
• Implement the design into a working product
• Verify the finished product against the design (and requirements)
• Maintain the project as necessary

The Waterfall Model was widely adopted in the early days of software development and a lot of blame has been laid at its door.

Critics argue that many problems in software development stem from this model. Early development projects that followed this model ran over budget and over schedule and the blame was attributed to the linear, step-wise nature of the model.

It is very rare that requirements analysis can be entirely completed before design and design before development and so on. It is far more likely that each phase will have interaction with each of the other phases.

In a small project this is not a problem since the span from “analyse” to “implement” may be a period of weeks or even days. For a large scale project which span months or even years the gap becomes significant. The more time that passes between analysis and implementation, the more a gap exists between the delivered project and the requirements of end-users.

Think about a finance system which is ‘analysed’ one year, designed the next year and developed and implemented the following year. That’s three years between the point at which the requirements are captured and the system actually reaches its end users. In three years its likely that the business, if not the whole industry, will have moved considerably and the requirements will no longer be valid. The developers will be developing the wrong system! Software of this scale is not uncommon either.

A definition of requirements may be accurate at the time of capture but decays with frightening speed. In the modern business world, the chance of your requirements analysis being valid a couple of months after it has been conducted is very slim indeed. Other versions of the waterfall model have been developed to alleviate this. One, the Iterative Waterfall Model, includes a loop as shown below. This model attempts to overcome the limitations of the original model by adding an "iterative" loop to the end of the cycle. That is, in order to keep up with changing requirements the "analysis" phase is revisited at the end of the cycle and the process starts over again.

Iterative Waterfall Model
Fig 24.1 Iterative Waterfall Model

This alleviates the situation somewhat but still introduces a considerable lag between analysis and implementation. The waterfall model implies you have to complete all the steps before you start the process again. If requirements change during the life of the project the waterfall model requires the completion of a full cycle before they can be revisited.

• Iterative Development

Iterative Development Model is also known as Rapid Development Model. In iterative development, the same waterfall process is used to deliver smaller chunks of functionality in a step-by-step manner. This reduces the management and overheads in delivering software and reduces the risk inherent in the project.

One of the major reasons cited for software project failure (and a common sources of defects) is poor quality requirements. That is a failure to correctly specify what to build. By delivering small chunks and validating them, the project can self correct and hopefully converge on the desired outcome. This contrasts with the long lag times in the waterfall model.

A variation on this theme is "Rapid Applications Development" or RAD.

Rapid Application Development Model
Fig 24.2 Rapid Application Development Model

The phases are similar to waterfall but the 'chunks' are smaller. The emphasis in this model is on fast iterations through the cycle. Prototypes are designed, developed and evaluated with users, involving them in the process and correcting the design. The model is particularly suited to projects in rapidly changing environments where the team needs to adapt to different situations.

• Incremental Development

Note that not all iterations need be complete, fully functional software. Nor should they necessarily focus on the same areas of functionality. It is better to deliver separate "increments" of functionality and assemble the whole project only at the conclusion of the production phase. This way you can take partial steps towards your completed goal. Each unit can be individually developed, tested and then bolted together to form the overall product or system.

Incremental Development Model
Fig 24.3 Incremental Development Model

The diagram above indicates progress through the development life-cycle in an iterative/incremental development. Early on the iterations focus on 'design tasks' and the emphasis is on making design decisions and prototypes. As the project progresses tasks shift to development where the bulk of the coding is done. Finally, the emphasis is on testing or evaluation to ensure that what has been developed meets requirements and is solid, stable and bug free.
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