Approaches to software development
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# An overall process model

A process model (or life cycle) is a description of all the events and activities in the life of a software system or product and the sequence in which they happen. So you can choose how to connect the activities together to form a process model, which you can then use to elaborate a process for developing software. For example, if you arrange the five activities of requirements, design, implementation, testing and maintenance into a single sequence, you have the classic waterfall model.

However, in practice it is not usually possible to complete each activity correctly in one attempt. In addition, as development proceeds, the products of earlier stages become dated, as your understanding of both the software and its environment evolves.

If projects use a strictly sequential process model, a working version of the software system will not be available until late in the testing activity. This will represent a long wait for both customer and users, who won’t be able to see a working response to their requirements until the final product is finished. In addition, any errors detected in the working version of the software could be disastrous as it would be too late to correct them. Real projects rarely follow a purely sequential process model. The act of reviewing is an important activity when testing the quality of any development process and its resulting products.

An alternative process model is to iterate around one or more of the activities. Iteration allows the developers to improve the outputs from a given set of activities and get feedback before moving on to the next activity. In particular, iteration allows a group of people, usually developers, to perform a review of a sequence of activities, or of an activity and its outputs.

In general, reviewing a proposed solution provides the feedback necessary to modify it and improve the solution. Think what happens when you need to tune a guitar or violin. You pluck a string, a note sounds, and you adjust the tension on that string. You repeat the process until you get the desired pitch.

It is often difficult to identify all requirements and state them explicitly at the outset of a development project. It is a good idea to start with a subset of the requirements and incrementally grow the system with feedback from each iteration. This approach is known as iterative and incremental development. As shown in Figure 6, each iteration is a complete, small project, with a short, fixed timeframe (timeboxed), consisting of requirements, design, implementation, testing and integration, and resulting in a partially working system. Each of these repeated short iterations adds complexity until the final system is produced.

Figure 6 Iterative and incremental development process (Larman 2005, p. 20)
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In an iterative and incremental development, users obtain useful and usable software quickly. This method also enables the developers to take on board feedback from users as the software develops – an increment may simply be an enhancement of the previous version. Increments can be developed sequentially or in parallel, depending on circumstances. For example, a small team might choose to develop increments sequentially, according to a priority agreed with the users.

## Activity 9 Development process

Allow approximately 20 minutes.

1. What is a development process?
2. What should the role of project management be with respect to the deliverables of a development project?
3. What is the difference between a customer and a user?
4. Suggest a reason why maintenance is a core activity in the development of a good software system.
5. What additional task is needed when the development of a software system is partitioned into a number of increments?
6. What are the assumptions on which the waterfall model is based?

## Answer

1. A development process is a set of rules that defines how a software development project should be carried out. It incorporates a number of activities, and a process model (or life cycle) that indicates how these activities are ordered.
2. A good software system must be affordable and available within an appropriate timeframe. Each deliverable uses resources, such as developers’ time, that add to its cost. Project management involves the identification of the appropriate deliverables for a given set of requirements and controlling the cost of producing them. Project management also involves ensuring that deliverables are produced on time and taking steps to cope with any delays.
3. A customer is the person who pays for a software system, whereas a user is someone who will use that system on a day-to-day basis. A customer will also be a user when the proposed software system is intended to support their job.
4. Maintenance allows a software system to evolve over its operational lifetime so that it continues to be useful.
5. A task devoted to the integration of the increments to form the final software system will be needed.
6. There is an assumption that once a particular activity or phase is finished, it is not re-entered, and that the activities do not overlap but follow each other in a sequential life cycle. There is no need for reviewing or reworking. All this presupposes that an end point for each activity can be identified.

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