NATS

NATS is messaging stripped to the metal. A single Go binary that handles pub/sub, request/reply, and durable streaming (JetStream) with sub-millisecond latency and a tiny resource footprint. It runs everywhere from cloud clusters to Raspberry Pis.

JetStream adds at-least-once delivery and persistence without losing NATS's simplicity. Compared to Kafka (higher throughput, much more complex), NATS is easier to operate and faster for most workloads. Compared to RabbitMQ (more routing features, heavier), NATS is lighter and scales horizontally better. Compared to Redis Pub/Sub (simpler but no persistence), JetStream fills the gap.

Use this when you need fast, simple messaging for microservices without the operational weight of Kafka or RabbitMQ. Skip this if you need Kafka's exactly-once semantics or RabbitMQ's advanced routing patterns.

The catch: NATS core is at-most-once delivery by default — messages can be lost. JetStream adds durability but with different semantics to learn. The ecosystem is growing but still smaller than Kafka's or RabbitMQ's. Apache 2.0 license.