# Single Aggregate

> A rule or precondition that a shared measurement keeps one aggregate, overwritten by each run that can honestly replace it, while a run that cannot streams its verdict instead of writing a second report.

Record: `architecture:single-aggregate`
Kind: constraint
Layer: [Execution Core](https://banes-lab.com/records/layer/execution-core.md)
Severity: mandatory
Scope: measurement, report
Canonical: https://banes-lab.com/ontology#architecture-single-aggregate

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Joinable Mandated Field](https://banes-lab.com/records/architecture/joinable-mandated-field.md) and before [Contradicted Invariant](https://banes-lab.com/records/architecture/contradicted-invariant.md).

## Repair

- Refactored by: Overwrite the Aggregate From Full Runs, Stream Narrowed Verdicts
- Detected by: a second report describing the aggregate's subject
- Violated by: writing a report named for the scope or caller of a run beside the aggregate, or overwriting the aggregate from a narrowed run
- Measured by: reports beside the aggregate
- Enforced by: coordination review

## Requires

- [Read Population](https://banes-lab.com/records/lexicon/read-population.md)

## Reinforces

- [Single Source of Truth](https://banes-lab.com/records/architecture/single-source-of-truth.md)

## Enables

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

## Conflicts with

- [Invocation-Keyed Report](https://banes-lab.com/records/architecture/invocation-keyed-report.md)
- [Narrowed Aggregate](https://banes-lab.com/records/architecture/narrowed-aggregate.md)

## Severity

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

## Category

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

## Linked from

- [Schema / Canonical Data / Semantics](https://banes-lab.com/ontology/principles/architecture-category-schema-canonical-data-semantics.md)
- [Coordination Surfaces](https://banes-lab.com/ontology/principles/architecture-category-coordination-surfaces.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/substrate.model.md](https://banes-lab.com/source/coordination/models/substrate.model.md.md)