A cross-aggregate constraint is usually a requirements question before it is a technical question.
Workflow
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Restate the rule in business language and ask whether it must be immediately true.
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If it is a true invariant, consider merging the aggregate boundary or changing the command owner.
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If it is a policy, use events, reservations, or compensating actions; use a saga only for multi-step orchestration and compensation, not for synchronous existence checks.
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In CQRS/ES, avoid command decisions based on stale read models unless the business accepts that staleness.
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Document the inconsistency window and repair mechanism.
Decision Options
- Same aggregate: use when the rule is a true invariant.
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Reservation aggregate: use when scarce resources need temporary holds.
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Policy or saga: use when the reaction can be eventual; reject saga for rules that must block the first command synchronously.
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Read-model check: use only when stale decisions are acceptable.
Detailed Reference
For non-trivial implementation, review, or refactoring work, read references/details.md before giving final guidance. It contains the detailed rules, examples, smells, and migration notes that do not belong in the short invocation body.
Output
Return the business interpretation, chosen consistency model, rejected options, and exact failure/retry behavior.