# ADR-005: Storage Strategy

## Status
Accepted

## Context
The system needs to store: structured data (users, documents, comments), document content, file attachments, and search indexes. Requirements include: zero external dependencies, backup simplicity, and resistance to data corruption.

## Decision
**libsql (SQLite) for structured data + content-addressed filesystem for files**

### Implementation

1. **Database (libsql/SQLite)**:
   - Single file: `data/db.sqlite`
   - WAL mode for concurrent reads during writes
   - Embedded — no separate process needed
   - Backup: `cp db.sqlite db.sqlite.backup` (atomic with WAL checkpoint)

2. **File Storage (Content-Addressed)**:
   - Directory: `data/files/`
   - Layout: `data/files/ab/cd/abcdef1234567890abcdef1234567890abcdef12`
   - SHA-256 hash as filename (hex encoded)
   - First two characters as subdirectory (for filesystem performance)
   - Immutable: write-once, never modify

3. **Document Mapping**:
   - Database stores: `document_path → current_hash`
   - History table stores all previous hashes
   - Attachment metadata stores original filename + content hash

## Consequences

### Positive
- SQLite is the most tested database in the world (billions of deployments)
- libsql adds replication option without changing the API
- Content-addressed files are naturally deduplicated
- Filesystem storage is trivial to backup (rsync, tar)
- No network dependencies, works offline
- Single directory contains all state — easy to migrate

### Negative
- SQLite has write concurrency limits (mitigated by WAL + short transactions)
- Filesystem storage doesn't scale horizontally without shared storage
- No built-in file expiration/garbage collection (must implement orphan cleanup)
- Large attachments can bloat filesystem (mitigated by quotas)

## Alternatives Considered

### PostgreSQL
- **Pros**: Excellent concurrency, rich feature set, horizontal scaling
- **Cons**: Requires separate process, more complex backup, network dependency
- **Rejected**: Overkill for anticipated scale; SQLite is simpler and faster for read-heavy workloads

### Object Storage (S3/MinIO)
- **Pros**: Unlimited scale, CDN-friendly
- **Cons**: Network dependency, vendor lock-in, complex local development
- **Rejected**: Violates zero-dependency and simple backup requirements

### SQLite for Everything (BLOBs)
- **Pros**: Single file for everything
- **Cons**: Large BLOBs slow down database, harder to serve directly, backup size bloat
- **Rejected**: Separation of concerns — database for metadata, filesystem for content

## References
- SQLite WAL mode: https://sqlite.org/wal.html
- Content-addressable storage: https://git-scm.com/book/en/v2/Git-Internals-Git-Objects
- libsql: https://github.com/tursodatabase/libsql
