# ADR-004: SurrealDB como Database Único

**Status**: Accepted | Implemented
**Date**: 2024-11-01
**Deciders**: Backend Architecture Team
**Technical Story**: Selecting unified multi-model database for relational, graph, and document workloads

---

## Decision

Usar **SurrealDB 2.3** como base de datos única (no PostgreSQL + Neo4j, no MongoDB puro).

---

## Rationale

1. **Multi-Model en una sola DB**: Relational (SQL), graph (queries), document (JSON) sin múltiples conexiones
2. **Multi-Tenancy Nativa**: SurrealDB scopes permiten aislamiento a nivel de database sin lógica en aplicación
3. **WebSocket Connection**: Soporte nativo de conexiones bidireccionales (vs REST)
4. **SurrealQL**: Sintaxis SQL-like + graph traversal en una sola query language
5. **VAPORA Requirements**: Almacena projects (relational), agent relationships (graph), execution history (document)

---

## Alternatives Considered

### ❌ PostgreSQL + Neo4j (Two Database Approach)
- **Pros**: Maduro, comunidad grande, especializados
- **Cons**: Sincronización entre dos DBs, dos conexiones, transacciones distribuidas complejas

### ❌ MongoDB Puro (Document Only)
- **Pros**: Flexible, escalable
- **Cons**: Sin soporte graph nativo, requiere aplicación para traversal, sin SQL

### ✅ SurrealDB (CHOSEN)
- Unifica relational + graph + document
- Multi-tenancy built-in
- WebSocket para real-time

---

## Trade-offs

**Pros**:
- ✅ Una sola DB para todos los modelos de datos
- ✅ Scopes para isolamiento de tenants (no en aplicación)
- ✅ Transactions ACID
- ✅ SurrealQL es SQL + graph en una query
- ✅ WebSocket bidireccional

**Cons**:
- ⚠️ Ecosistema más pequeño que PostgreSQL
- ⚠️ Drivers/herramientas menos maduras
- ⚠️ Soporte de clusters más limitado (vs Postgres)

---

## Implementation

**Database Connection**:
```rust
// crates/vapora-backend/src/main.rs:48-59
let db = surrealdb::Surreal::new::<surrealdb::engine::remote::ws::Ws>(
    &config.database.url
).await?;

db.signin(surrealdb::opt::auth::Root {
    username: "root",
    password: "root",
}).await?;

db.use_ns("vapora").use_db("main").await?;
```

**Scope-Based Multi-Tenancy**:
```rust
// All queries use scope for tenant isolation
db.query("SELECT * FROM projects WHERE tenant_id = $tenant_id")
    .bind(("tenant_id", tenant_id))
    .await?
```

**Key Files**:
- `/crates/vapora-backend/src/main.rs:45-59` (connection setup)
- `/crates/vapora-backend/src/services/` (query implementations)
- `/crates/vapora-shared/src/models.rs` (Project, Task, Agent models with tenant_id)

---

## Verification

```bash
# Connect to SurrealDB
surreal sql --conn ws://localhost:8000 --user root --pass root

# Verify namespace and database exist
USE ns vapora db main;
INFO FOR DATABASE;

# Test multi-tenant query
SELECT * FROM projects WHERE tenant_id = 'workspace:123';

# Test graph traversal
SELECT
    *,
    ->assigned_to->agents AS assigned_agents
FROM tasks
WHERE project_id = 'project:123';

# Run backend tests with SurrealDB
cargo test -p vapora-backend -- --nocapture
```

**Expected Output**:
- SurrealDB connects via WebSocket
- Projects table exists and is queryable
- Graph relationships (->assigned_to) resolve
- Multi-tenant queries filter correctly
- 79+ backend tests pass

---

## Consequences

### Data Model Changes
- All tables must include `tenant_id` field for scoping
- Relations use SurrealDB's `->` edge syntax for graph queries
- No foreign key constraints (SurrealDB uses references instead)

### Query Patterns
- Services layer queries must include tenant_id filter (defense-in-depth)
- SurrealQL instead of raw SQL learning curve for team
- Graph traversal enables efficient knowledge graph queries

### Scaling Considerations
- Horizontal scaling requires clustering (vs Postgres replication)
- Backup/recovery different from traditional databases (see ADR-020)

---

## References

- [SurrealDB Documentation](https://surrealdb.com/docs/surrealql/queries)
- `/crates/vapora-backend/src/services/` (query patterns)
- `/crates/vapora-shared/src/models.rs` (model definitions with tenant_id)
- ADR-025 (Multi-Tenancy with Scopes)

---

**Related ADRs**: ADR-001 (Workspace), ADR-025 (Multi-Tenancy)