# "Choosing a Message Queue on Zerops"


## Keywords
queue, message queue, kafka, nats, event, stream, pub-sub, broker, choose queue, which queue, messaging

## TL;DR
**Use NATS** for most cases (simple, fast, JetStream persistence). Use **Kafka** only for enterprise event streaming with guaranteed ordering and unlimited retention.

## Decision Matrix

| Need | Choice | Why |
|------|--------|-----|
| **General messaging** | **NATS** (default) | Simple auth, JetStream built-in, fast |
| Enterprise event streaming | Kafka | SASL auth, 3-broker HA, unlimited retention |
| Lightweight pub/sub | NATS | Low overhead, 8MB default messages |
| Event sourcing / audit logs | Kafka | Indefinite topic retention, strong ordering |

## NATS (Default Choice)

- Ports: 4222 (client), 8222 (HTTP monitoring)
- Auth: user `zerops` + auto-generated password
- Connection: `nats://:@:4222`
- JetStream: Enabled by default (`JET_STREAM_ENABLED=1`)
- Storage: Up to 40GB memory + 250GB file store
- Max message: 8MB default, 64MB max (`MAX_PAYLOAD`)
- Health check: `GET /healthz` on port 8222
- **Config changes require restart** (no hot-reload)

## Kafka

- Port: 9092 (SASL PLAIN auth)
- Auth: `user` + `password` env vars (auto-generated)
- Bootstrap: `:9092`
- HA: 3 brokers, 6 partitions, replication factor 3
- Storage: Up to 40GB RAM + 250GB persistent
- Topic retention: **Indefinite** (no time or size limits)
- Schema Registry: Port 8081 (if enabled)

## Gotchas
1. **NATS config changes need restart**: No hot-reload — changing env vars requires service restart
2. **Kafka single-node has no replication**: 1 broker = 3 partitions but zero redundancy
3. **NATS JetStream HA sync interval**: 1-minute sync across nodes — brief data lag possible
4. **Kafka SASL only**: No anonymous connections — always use the generated credentials

## See Also
- zerops://themes/services — NATS, Kafka service cards and wiring
- zerops://decisions/choose-database