# ADR-011: Four-Layer Data Architecture

## Status
Accepted

## Context
We need a clear separation between raw data, extracted knowledge, and decision intelligence to avoid building expensive, tightly-coupled AI systems.

## Decision
Separate into four layers:

### Layer 1: Raw Archive (cheap, immutable)
- Original files never changed
- Hash guarantees integrity
- Storage policy: active (30 days) → archive (long-term, cheaper)

### Layer 2: Knowledge Extraction
- Objects, geometry, GPS, classifications, bounding boxes, embeddings
- Observations and Evidence created
- Structured knowledge used for most queries, not video files

### Layer 3: Knowledge Graph / Ontology
- Road A12 → has Observation → Crack → belongs to Area → Stockholm
- Answers: "Show all main roads where cracking increased last 12 months"
- Comes from knowledge graph, not by reading video files

### Layer 4: Decision Intelligence
- Verified Decision Cases
- Curated datasets for AI training
- Business impact tracking

## Key Principles

1. **Archive Artifact** = Original file, never modified
2. **Knowledge Artifact** = Extracted knowledge, can be re-generated
3. **Ontology before model** — taxonomy answers "what does it mean in our domain?"
4. **AI models trained on curated datasets**, not whole archive

## Data Lifecycle

```
Upload → Immutable Archive → Metadata Extraction → Knowledge Extraction
→ Ontology Mapping → Decision Pipeline → Learning → Archive Retention
```

Each step produces a new Artifact with version history.

## Long-term Goal

"Every real observation is converted once to structured knowledge and can then be reused infinitely for analysis, decision support, history, training, and future AI models."

## Consequences

### Positive
- Raw data = revisionable archive
- Real product = growing knowledge base + verified Decision Cases
- Can swap AI models without losing originals
- Cheaper storage (archive vs active)

### Negative
- More complex pipeline
- Need to manage extraction models
- Knowledge graph maintenance

## Related
- ADR-002: Artifact is common base contract
- ADR-003: Event Sourcing for traceability