# Invariant

> A rule or precondition that holds for an entity or aggregate in every state it can reach.

Record: `architecture:invariant`
Kind: constraint
Layer: [Contracts Core](https://banes-lab.com/records/layer/contracts-core.md)
Severity: mandatory
Scope: entity, aggregate, module, system
Canonical: https://banes-lab.com/ontology#architecture-invariant

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Postconditions](https://banes-lab.com/records/architecture/postconditions.md) and before [Backward Compatibility](https://banes-lab.com/records/architecture/backward-compatibility.md).

## Repair

- Refactored by: Encapsulate State, Add Factory, Add Validation
- Detected by: mutable public state, missing invariant checks
- Violated by: invalid domain states, broken aggregate rules
- Measured by: invariant test coverage
- Enforced by: domain tests, constructors, type system

## Requires

- [Encapsulation](https://banes-lab.com/records/architecture/encapsulation.md)
- [Validation](https://banes-lab.com/records/architecture/validation.md)

## Reinforces

- [Correctness](https://banes-lab.com/records/architecture/correctness.md)
- [Consistency](https://banes-lab.com/records/architecture/consistency.md)

## Enables

- [Safe Refactoring](https://banes-lab.com/records/lexicon/safe-refactoring.md)

## Conflicts with

- [External State Mutation](https://banes-lab.com/records/lexicon/external-state-mutation.md)

## In tension with

- [Flexibility](https://banes-lab.com/records/lexicon/flexibility.md)

## Tensions

- [Invariant / Flexibility](https://banes-lab.com/records/tension/flexibility-invariant.md)

## Severity

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

## Category

- [Contracts / Interfaces / Compatibility](https://banes-lab.com/records/architecture-category/contracts-interfaces-compatibility.md)

## Concept

- [Invariant](https://banes-lab.com/records/reasoning/substrate-node-invariant.md)

## Enforced by

- [rules/eslint/factory-owned-value.eslint.rule.ts](https://banes-lab.com/source/governance/rules/eslint/factory-owned-value.eslint.rule.ts.md)
- [rules/eslint/validated-construction.eslint.rule.ts](https://banes-lab.com/source/governance/rules/eslint/validated-construction.eslint.rule.ts.md)

## Required by

- [Domain Model](https://banes-lab.com/records/architecture/domain-model.md)
- [Design by Contract](https://banes-lab.com/records/architecture/design-by-contract.md)
- [Postconditions](https://banes-lab.com/records/architecture/postconditions.md)
- [Liskov Substitution Principle](https://banes-lab.com/records/architecture/liskov-substitution.md)

## Reinforced by

- [Aggregate](https://banes-lab.com/records/architecture/aggregate.md)

## Linked from

- [Domain Architecture](https://banes-lab.com/ontology/principles/architecture-category-domain-architecture.md)
- [Contracts / Interfaces / Compatibility](https://banes-lab.com/ontology/principles/architecture-category-contracts-interfaces-compatibility.md)
- [Correctness / Determinism / Verification](https://banes-lab.com/ontology/principles/architecture-category-correctness-determinism-verification.md)
- [Core Modular Design](https://banes-lab.com/ontology/principles/architecture-category-core-modular-design.md)
- [SOLID / Object-Oriented Design](https://banes-lab.com/ontology/principles/architecture-category-solid-object-oriented-design.md)
- [Transactions / State / Concurrency](https://banes-lab.com/ontology/principles/architecture-category-transactions-state-concurrency.md)
- [Contracts / Interfaces / Compatibility](https://banes-lab.com/ontology/lexicon/lexicon-category-contracts-interfaces-compatibility.md)
- [Core Vocabulary](https://banes-lab.com/ontology/lexicon/lexicon-category-core-vocabulary.md)
- [SOLID / Object-Oriented Design](https://banes-lab.com/ontology/lexicon/lexicon-category-solid-object-oriented-design.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)
- [The resolutions](https://banes-lab.com/ontology/schema/the-resolutions.md)