Chapter 2: Requirements and Assumptions¶
Purpose¶
Chapter 2 asks: what must the toaster do, and what do we assume about the conditions under which it operates? After completing this chapter, the model has a requirement definition, two named usages of Toaster, and the first engineering judgment record.
A judgment record is a written, checkable record of an engineering judgment call, and following Hawkins et al. (2011) there are three kinds. An asserted_context records a context or assumption that is asserted to be appropriate for the argument elements it applies to. An asserted_solution records evidence cited as a solution that is asserted to be sufficient to support a claim. An asserted_inference records a claim said to be supported by other claims, with the inference asserted to be appropriate and sufficient. Notebook 03 builds the first of these, an asserted_context; a record states what is being asserted and does not settle the question, so its disposition stays pending.
Ingredients¶
| Notebook | Construct / operation | Concept |
|---|---|---|
| 01: requirement def | requirement def + subject + require constraint | A formal statement of what the system must satisfy |
| 02: attribute override | attribute :>> override | A named usage that redeclares an inherited attribute with a deliberately faulty value |
| 03: asserted context | asserted_context record | An assumption underlying a cycle-time estimate, not an evaluation of the requirement |
Equipment¶
See setup. Chapter 2 also uses toaster.evidence.ReviewRecord; run uv sync --locked to ensure the package is installed.
Method¶
Notebook 01 adds the requirement definition to the cumulative model from Chapter 1, along with nominal, a bare usage of Toaster with no attribute values set. Notebook 02 adds slow, overriding cycleTime with a deliberately injected fault value that exceeds the requirement’s bound: a fixture for the requirement’s failing branch, not a design variant or an operating condition. Notebook 03 introduces the first judgment record: an asserted_context that records an assumed estimate of nominal cycle time before any comparison against the requirement is reported.
The judgment record is the first example of Hawkins et al. (2011) §3.2 in the tutorial. It does not assert that the design is correct. It asserts that the assumption is appropriate for the context.
Expected result¶
After notebook 03:
TimelyToastis aRequirementDefinitionin the modelnominalandslowarePartUsageinstances ofToasternominal’scycleTimecarries no value (Chapter 1 removed the default)slowhascycleTime = 200.0viaattribute :>>, a fixed, deliberately injected fault valuecontext_recordis a PythonReviewRecordwithkind="asserted_context"anddisposition="pending"
Experiment¶
The chapter exercise asks you to add a temperature requirement to your coffee maker model and write the first context record for the brew-temperature assumption.