Spiral model of software development

The spiral model of software development in those or other versions used in set of concrete applied techniques, is constructed on a following pattern. First of all during dialogue with the customer the set of the most important and critical opportunities of the future system is defined. Further joint efforts define desirable terms for realization of this base functionality. The plan is formed, works begin and their performance - for example, by means of technique C/SCSC is traced.

By numerous researches it is confirmed, as the customer and the executor usually too optimistically concern to terms and the budget, even at use of good techniques of an estimation of amount of works. Therefore results of such estimations are offered to be increased (to worsen) seriously enough - approximately by 50 %. Besides the customer usually poorly represents architecture of the future system, therefore it should be projected, being pawned on the open technologies and as much as possible flexible opportunities of expansion and escalating of functionality. Specification of concrete requirements is carried out iteratively, thus on each coil of a design spiral more and more exact version corresponding wishes of the customer is created.

Six steps of spiral model

1. During dialogue with the customer the general vision of the project is formed, and also functionalities which are necessary for realizing in the certain terms with the necessary quality are described. 2. The priorities setting the order of realization of the basic functionalities are placed. 3. Time frameworks of the project are coordinated. Often techniques of cost forecasting of type COCOMO II are applied for this purpose . Further the executor solves, how many functionalities according to their priorities will be possible to realize in stipulated time.

4. At the given stage the architecture and a kernel of the future system are defined. It is the most responsible moment as here it is necessary to consider for the present the requirements not detailed completely to the project - and they can quite be inconsistent.

5. The plan of works prepares. It is focused on the terms certain at the third stage, and aimed at the prompt realization of a kernel of system. Cooperating with working prototype, the customer develops more quickly and more precisely and specifies the further requirements and corrects priorities. More often such plan is made on a method of a critical way. 6. System engineering according to the plan. The given approach, certainly, does not guarantee observance of terms - they can be broken, for example, in case of sharp reduction of the budget or serious change of requirements. But it is checked well up by practice on projects of the most different scale and can be easily adapted for needs of the concrete organization.

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