# State-Arity Limit

> A rule or precondition that a property relating two states of content, such as growing but never shrinking, is enforced only by a mechanism holding both readings, in the layer that already spans runs.

Record: `architecture:state-arity-limit`
Kind: constraint
Layer: [Execution Core](https://banes-lab.com/records/layer/execution-core.md)
Severity: recommended
Scope: check, state
Canonical: https://banes-lab.com/ontology#architecture-state-arity-limit

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Fan-In Ceiling](https://banes-lab.com/records/architecture/fan-in-ceiling.md) and before [Carrier and Payload Split](https://banes-lab.com/records/architecture/carrier-and-payload-split.md).

## Repair

- Refactored by: Retain the Prior State in the Spanning Layer, Compare the Two Readings
- Detected by: content removed from a required section while every single-state check passes
- Violated by: enforcing a rule about how content changes with a check that reads one state, or making a checker remember its prior output
- Measured by: change rules held only by single-state checks
- Enforced by: coordination review

## Requires

- [Coordination Surface](https://banes-lab.com/records/lexicon/coordination-surface.md)

## Reinforces

- [Fitness Functions](https://banes-lab.com/records/architecture/fitness-functions.md)

## Enables

- [Traceable Guarantee](https://banes-lab.com/records/lexicon/traceable-guarantee.md)

## Severity

- [recommended](https://banes-lab.com/records/vocabulary/severity-recommended.md)

## Category

- [Coordination Surfaces](https://banes-lab.com/records/architecture-category/coordination-surfaces.md)

## Linked from

- [Architecture Review / Evolution / Governance Artifacts](https://banes-lab.com/ontology/principles/architecture-category-architecture-review-evolution-governance-artifacts.md)
- [Coordination Surfaces](https://banes-lab.com/ontology/lexicon/lexicon-category-coordination-surfaces.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)

## Evidence in the code

- [models/coordination.model.md](https://banes-lab.com/source/coordination/models/coordination.model.md.md)