# Storage

**Last Updated:** 2025-01-31
**Confidence:** High

## Summary

Episteme uses a Log-Structured, Content-Addressed storage model. Writes append to WAL, then index asynchronously. Reads query indexes and apply Lenses.

**Key Facts:**
- Append-only (never mutate)
- WAL for durability (fsync on write)
- KV store for indexes (sled MVP, trait-abstracted)
- Content-addressed by BLAKE3 hash

**File Pointer:** `crates/stemedb-core/src/store.rs` (planned)

## KV Layout

| Key Pattern | Value | Purpose |
|-------------|-------|---------|
| `H:{Hash}` | `Assertion` (serialized) | Main content store |
| `S:{Subject}` | `Vec<Hash>` | Subject index |
| `SP:{Subject}:{Predicate}` | `Vec<Hash>` | Triple index |
| `A:{AgentId}` | `ReputationScore` | TrustRank storage |

## Write Path

```
1. Agent submits signed Assertion
2. Validate signature
3. Append to WAL (fsync)
4. Return 202 Accepted with Hash
5. Background: tail WAL -> update indexes
```

## Read Path

```
1. Query: GET(Subject, Predicate, Lens)
2. Lookup: SP:{Subject}:{Predicate} -> [Hash...]
3. Hydrate: Load assertions from H:{Hash}
4. Resolve: Apply Lens
5. Return: Deterministic answer
```

## Related Topics

- [Assertion](./assertion.md)
- [Architecture](../../../architecture.md)