The spiral model is a risk-driven software development process model. Based on the unique risk patterns of a given project, the spiral model guides a team to adopt elements of one or more process models, such as incremental, waterfall, or evolutionary prototyping.
History
This model was first described by Barry Boehm in his 1986 paper, "A Spiral Model of Software Development and Enhancement". In 1988 Boehm published a similar paper to a wider audience. These papers introduce a diagram that has been reproduced in many subsequent publications discussing the spiral model.
These early papers use the term "process model" to refer to the spiral model as well as to incremental, waterfall, prototyping, and other approaches. However, the spiral model's characteristic risk-driven blending of other process models' features is already present:
In later publications, Boehm describes the spiral model as a "process model generator", where choices based on a project's risks generate an appropriate process model for the project. Thus, the incremental, waterfall, prototyping, and other process models are special cases of the spiral model that fit the risk patterns of certain projects.
Boehm also identifies a number of misconceptions arising from oversimplifications in the original spiral model diagram. He says the most dangerous of these misconceptions are: that the spiral is simply a sequence of waterfall increments; that all project activities follow a single spiral sequence; that every activity in the diagram must be performed, and in the order shown. While these misconceptions may fit the risk patterns of a few projects, they are not true for most projects.
In a National Research Council report this model was extended to include risks related to human users.
To better distinguish them from "hazardous spiral look-alikes", Boehm lists six characteristics common to all authentic applications of the spiral model.
The six invariants of spiral model
Authentic applications of the spiral model are driven by cycles that always display six characteristics. Boehm illustrates each with an example of a "dangerous spiral look-alike" that violates the invariant.
Define artifacts concurrently
Sequentially defining the key artifacts for a project often increases the possibility of developing a system that meets stakeholder "win conditions" (objectives and constraints).
This invariant excludes “hazardous spiral look-alike” processes that use a sequence of incremental waterfall passes in settings where the underlying assumptions of the waterfall model do not apply. Boehm lists these assumptions as follows: The requirements are known in advance of implementation. The requirements have no unresolved, high-risk implications, such as risks due to cost, schedule, performance, safety, user interfaces, organizational impacts, etc. The nature of the requirements will not change very much during development or evolution. The requirements are compatible with all the key system stakeholders’ expectations, including users, customer, developers, maintainers, and investors. The right architecture for implementing the requirements is well understood. There is enough calendar time to proceed sequentially. In situations where these assumptions do apply, it is a project risk not to specify the requirements and proceed sequentially. The waterfall model thus becomes a risk-driven special case of the spiral model.
Perform four basic activities in every cycle
This invariant identifies the four activities that must occur in each cycle of the spiral model: Consider the win conditions of all success-critical stakeholders. Identify and evaluate alternative approaches for satisfying the win conditions. Identify and resolve risks that stem from the selected approach(es). Obtain approval from all success-critical stakeholders, plus commitment to pursue the next cycle. Project cycles that omit or shortchange any of these activities risk wasting effort by pursuing options that are unacceptable to key stakeholders, or are too risky.
Some "hazardous spiral look-alike" processes violate this invariant by excluding key stakeholders from certain sequential phases or cycles. For example, system maintainers and administrators might not be invited to participate in definition and development of the system. As a result, the system is at risk of failing to satisfy their win conditions.
Risk determines level of effort
For any project activity (e.g., requirements analysis, design, prototyping, testing), the project team must decide how much effort is enough. In authentic spiral process cycles, these decisions are made by minimizing overall risk.