Heuristic Thresholds

Heuristic gaps are patterns that violate established DDD principles. Unlike structural gaps (binary -- exists or doesn't), heuristic gaps are measured against thresholds sourced from the DDD literature.

Threshold Table

Heuristic	Threshold	Source	Rationale
Aggregate command density	6 or fewer commands per aggregate	Vernon, Implementing DDD	High command count signals an aggregate with too many responsibilities. Consider decomposition.
Context term overlap	3 or fewer shared term definitions across contexts	Evans, Domain-Driven Design	Excessive term sharing suggests blurred context boundaries or a missing shared kernel.
Saga step count	5 or fewer steps per saga	Practical experience across production systems	Long sagas are fragile and difficult to reason about. Consider decomposition into smaller coordinated processes.
Aggregates per context	3 or fewer preferred	Context size heuristic	Too many aggregates in a single context may signal a missing boundary.
Policy fan-out	1 event in, 1 command out	Policy simplicity principle	A policy should be a single reaction. Multiple outputs suggest a saga or orchestration pattern.

How Thresholds Work

Thresholds are defaults, not absolutes. When a threshold is exceeded:

1. The gap report flags it as a WARNING (not an error)
2. The architect investigates whether the violation is meaningful
3. Resolution options:
   • Change the model to bring the metric within threshold
   • Override the threshold with documented rationale explaining why the domain justifies the exception

Overriding a threshold is a valid architectural decision. The value of the threshold is not the number -- it's the investigation it triggers.

Threshold Sources

Eric Evans -- Domain-Driven Design (2004)

Evans established the principles of bounded contexts, ubiquitous language, and strategic design. The context term overlap threshold derives from his work on context boundaries and shared kernels.

Vaughn Vernon -- Implementing Domain-Driven Design (2013)

Vernon provided tactical guidance for implementing DDD patterns. The aggregate command density threshold comes from his "small aggregate" heuristic -- aggregates should be small, focused consistency boundaries.

Alberto Brandolini -- EventStorming

Brandolini's collaborative discovery vocabulary informs how we identify missing reactive patterns (policies and sagas). The policy fan-out threshold derives from EventStorming's "event triggers command" pattern.

Vlad Khononov -- Balancing Coupling in Software Design (2024)

Khononov formalized coupling as a measurable heuristic. His work informs how SDD evaluates cross-context dependencies and relationship types.

Adding Custom Thresholds

If your domain has specific heuristics that matter (e.g., regulatory constraints, performance budgets), you can add them to your gap report. Follow the same pattern:

1. Define the heuristic and its threshold
2. Cite the source (domain expertise, regulation, team agreement)
3. Measure against your specification
4. Flag violations as heuristic gaps