Keep calculation and explanation revisions in the same contract family
A quote calculated with reseller C revision 2 displays revision 1 explanation text, which omits the cap. Separate registries allow a valid calculator to be paired with an incompatible explainer.
- Focused work estimate
- 3h 30m + prerequisites
- Priority in the scenario
- High
- Engineering practice
- Abstract factories · Version compatibility · Invariant testing
Estimated field mix
- Backend60%
- System design40%
Field percentages are editorial estimates of the ticket's engineering focus. They total 100%; they are not measured time, proficiency scores, or ownership evidence.
Pattern topics
- Abstract FactoryCompare
Keep the calculator and explainer revision-compatible as a family while evaluating whether a factory interface adds value beyond an immutable record.
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Project context
A fictional equipment-rental service supports direct customers and two reseller contracts. Pricing now lives in a long conditional with subclasses left over from a discontinued campaign. Build a local TypeScript quoting module and synthetic fixtures; no starter repository, real charges, tax advice, or payment connection is supplied. Amounts use integer cents and the exercise supports USD only.
Setup prerequisites
- Pure functions
- Object composition
- Integer arithmetic
Preceding work
Complete these dependencies, or supply their agreed outputs before taking this ticket.
Acceptance criteria
- Resolve calculator and structured explainer as one contract family with a shared revision identity.
- Reject a family whose members declare different revisions before serving a quote.
- Compare an Abstract Factory with one immutable family record; preserve independent tests for calculation and explanation.
Implementation constraints
- Explanation output must use the actual calculation breakdown rather than recalculating the discount or parsing a display string.
Verification to include
- Generate explanations for capped and uncapped quotes and reconcile every reported amount.
- Deliberately pair revision 2 calculation with revision 1 explanation and assert assembly rejection.
Deliverables
- Contract-family resolver, mismatch fixture, and design comparison
Rollout and recovery
Run family validation at local startup; keep the prior complete family available rather than rolling back individual members.
Value of the work
For the engineer: Practice choosing, testing, and removing object-design abstractions around versioned business rules and tenant isolation.
For the team: Review changes that let a team introduce a contract without silently changing existing quotes, together with the maintenance costs of the chosen design.
Evidence boundaries
Outcome Evidence: Tests, patches, and runbooks are requested deliverables. They become Outcome Evidence only through a qualified Mission and immutable Evidence IDs.
Ownership Evidence: Independent adaptation must be observed under a declared verification policy and cite immutable Evidence IDs. Completing a planning ticket establishes no Ownership Evidence.