Intent specifications

Intent specifications are based on systems theory, system engineering principles, and psychological research on human problem solving and how to enhance it. The goal is to assist humans in dealing with complexity. While commercial tools exist that implement intent specifications directly, any specification languages and tools can be used that allow implementing the properties of an intent specification. 

An intent specification differs from a standard specification primarily in its structure, not its content no extra information is involved that is not commonly found 

Intent specifications are organized along three dimensions intent abstraction, part-whole abstraction, and refinement. Hiese dimensions constitute the problem space in which the human navigates. Part-whole abstraction (along the horizontal dimension) and refinement (within each level) allow users to change then-focus of attention to more or less detailed views within each level or model. The vertical dimension specifies the level of intent at which the problem is being considered. 

Each intent level contains information about the characteristics of the environment, human operators or users, the physical and functional system components, and requirements for and results of verification and validation activities for that level. The safety information is embedded in each level, instead of being maintained in a separate safety log, but linked together so that it can easily be located and reviewed. 

The vertical intent dimension has seven levels. Each level represents a different model of the system from a different perspective and supports a different type of reasoning about it. Refinement and decomposition occurs within each level of the specification, rather than between levels. Each level provides information not just about what and how, but why, that is, the design rationale and reasons behind the design decisions, including safety considerations. 

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The structure of an intent specification is based on the fundamental concept of hierarchy in systems theory (see chapter 3) where complex systems are modeled in terms of a hierarchy of levels of organization, each level imposing constraints on the degree of freedom of the components at the lower level. Different description languages may be appropriate at the different levels. Figure 10.1 shows the seven levels of an intent specification. 

Figure 10.2 shows an example of the information that might be contained in each level of the intent specification. 

The top level (level 0) provides a project management view and insight into the relationship between the plans and the project development status through links to the other parts of the intent specification. This level might contain the project management plans, the safety plan, status information, and so on. 

The structure of an intent specification does not imply that the development must proceed from the top levels down to the bottom levels in that order, only that at the end of the development process, all levels are complete. Almost all development involves work at all of the levels at the same time. 

When the system changes, the environment in which the system operates changes, or components are reused in a different system, a new or updated safety analysis is required. Intent specifications can make that process feasible and practical. 

Examples of intent specifications are available [121,151] as are commercial tools to support them. But most of the principles can be implemented without special tools beyond a text editor and hyperlinking facilities. The rest of this chapter assumes only these very limited facilities are available. 

What has been presented so far in this chapter would appear in level 1 of an intent specification. The second level of an intent specification contains System Design Principles—the basic system design and scientific and engineering principles needed to achieve the behavior specified in the top level, as well as any derived requirements and design features not related to the level 1 requirements. 

For TCAS, this level of the intent specification includes such general principles as the basic tau concept, which is related to all the high-level alerting goals and... 

Information about the allocation of these design decisions to individual system components and the logic involved is located in level 3, which in turn has links to the implementation of the logic in lower levels. If a change has to be made to a system component (such as a change to a software module), it is possible to trace the function computed by that module upward in the intent specification levels to determine whether the module is safety critical and if (and how) the change might affect system safety. 

A SpecTRM-RL model of TCAS was created by the author and her students Jon Reese, Mats Heim-dahl, and Holly Hildreth to assist in the certification of TCAS II. Later, as an experiment to show the feasibility of creating intent specifications, the author created the level 1 and level 2 intent specification for TCAS. Jon Reese rewrote the level 3 collision avoidance system logic from the early version of the language into SpecTRM-RL. 

The rest of the development process, involving the implementation of the component requirements and constraints and documented at levels 4 and 5 of intent specifications, is straightforward and differs little from what is normally done today. 

Limitations should be included in level 1 of the intent specification, because they properly belong in the customer view of the system and will affect both acceptance and certification. 

In a traditional System Safety program, much of this information is found in or can be derived from the hazard log, but it needs to be pulled out and provided in a form that makes it easy to locate and use in operations. Recording design rationale and assumptions in intent specifications allows using that information both as the criteria under which enforcement of the safety constraints is predicated and in the inevitable upgrades and changes that will need to be made during operations. Chapter 10 shows how to identify and record the necessary information. 

Before any planned changes are made, including organizational and safety control structure changes, their impact on safety must be evaluated. Whether this process is expensive depends on how the original hazard analysis was performed and particularly how it was documented. Part of the rationale behind the design of intent specifications was to make it possible to retrieve the information needed. 

Leveson, Nancy G. (January 2000). Leveson intent specifications An approach to building human-centered specifications. IEEE Transactions on Software Engineering SE-26 (l) 15-35. 

Leveson, Nancy, and Jon Reese. TCAS intent specification, http //sunnyday.mit.edu/papers/tcas-intent.pdf. 

Leveson NG (2000) Intent specifications an approach to building human-centered specifications. IEEE Trans Softw Eng 26(1) 15-26. http //sunnyday.mitedu/paptas/intent-tse.pdf. Accessed 16 June 2014... 

The book Engineering a Safer World by Nancy Leweson mentions a suggestion that describes the structure for various views of architecture and the allocation of requirements. All coherences are described with the term Intent Specification (Fig. 3.5). 

The Courts have focused on the phrase reasonably related within 271(e)(1) in that this phrase appears to demonstrate congressional intent. Specifically, the Courts have held that ... 

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