# ADR-005: NATS JetStream para Agent Coordination

**Status**: Accepted | Implemented
**Date**: 2024-11-01
**Deciders**: Agent Architecture Team
**Technical Story**: Selecting persistent message broker for reliable agent task queuing

---

## Decision

Usar **async-nats 0.45 con JetStream** para coordinación de agentes (no Redis Pub/Sub, no RabbitMQ).

---

## Rationale

1. **At-Least-Once Delivery**: JetStream garantiza persistencia + retries (vs Redis Pub/Sub que pierde mensajes)
2. **Lightweight**: Ninguna dependencia pesada (vs RabbitMQ/Kafka setup)
3. **Async Native**: Diseñado para Tokio (mismo runtime que VAPORA)
4. **VAPORA Use Case**: Coordinar tareas entre múltiples agentes con garantías de entrega

---

## Alternatives Considered

### ❌ Redis Pub/Sub
- **Pros**: Simple, fast
- **Cons**: Sin persistencia, mensajes perdidos si broker cae

### ❌ RabbitMQ
- **Pros**: Maduro, confiable
- **Cons**: Pesado, require seperate server, más complejidad operacional

### ✅ NATS JetStream (CHOSEN)
- At-least-once delivery
- Lightweight
- Tokio-native async

---

## Trade-offs

**Pros**:
- ✅ Persistencia garantizada (JetStream)
- ✅ Retries automáticos
- ✅ Bajo overhead operacional
- ✅ Integración natural con Tokio

**Cons**:
- ⚠️ Cluster setup requiere configuración adicional
- ⚠️ Menos tooling que RabbitMQ
- ⚠️ Fallback a in-memory si NATS cae (degrada a at-most-once)

---

## Implementation

**Task Publishing**:
```rust
// crates/vapora-agents/src/coordinator.rs
let client = async_nats::connect(&nats_url).await?;
let jetstream = async_nats::jetstream::new(client);

// Publish task assignment
jetstream.publish("tasks.assigned", serde_json::to_vec(&task_msg)?).await?;
```

**Agent Subscription**:
```rust
// Subscribe to task queue
let subscriber = jetstream
    .subscribe_durable("tasks.assigned", "agent-consumer")
    .await?;

// Process incoming tasks
while let Some(message) = subscriber.next().await {
    let task: TaskMessage = serde_json::from_slice(&message.payload)?;
    process_task(task).await?;
    message.ack().await?; // Acknowledge after successful processing
}
```

**Key Files**:
- `/crates/vapora-agents/src/coordinator.rs:53-72` (message dispatch)
- `/crates/vapora-agents/src/messages.rs` (message types)
- `/crates/vapora-backend/src/api/` (task creation publishes to JetStream)

---

## Verification

```bash
# Start NATS with JetStream support
docker run -d -p 4222:4222 nats:latest -js

# Create stream and consumer
nats stream add TASKS --subjects 'tasks.assigned' --storage file

# Monitor message throughput
nats sub 'tasks.assigned' --raw

# Test agent coordination
cargo test -p vapora-agents -- --nocapture

# Check message processing
nats stats
```

**Expected Output**:
- JetStream stream created with persistence
- Messages published to `tasks.assigned` persisted
- Agent subscribers receive and acknowledge messages
- Retries work if agent processing fails
- All agent tests pass

---

## Consequences

### Message Queue Management
- Streams must be pre-created (infra responsibility)
- Retention policies configured per stream (age, size limits)
- Consumer groups enable load-balanced processing

### Failure Modes
- If NATS unavailable: Agents fallback to in-memory queue (graceful degradation)
- Lost messages only if dual failure (server down + no backup)
- See disaster recovery plan for NATS clustering

### Scaling
- Multiple agents subscribe to same consumer group (load balancing)
- One message processed by one agent (exclusive delivery)
- Ordering preserved within subject

---

## References

- [NATS JetStream Documentation](https://docs.nats.io/nats-concepts/jetstream)
- `/crates/vapora-agents/src/coordinator.rs` (coordinator implementation)
- `/crates/vapora-agents/src/messages.rs` (message types)

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**Related ADRs**: ADR-001 (Workspace), ADR-018 (Swarm Load Balancing)