MQTT: A lightweight, publish-subscribe messaging protocol designed for low-bandwidth, high-latency, or unreliable networks. It's ideal for IoT applications, mobile devices, and scenarios where devices may have intermittent connectivity. MQTT is optimized for simplicity and efficiency in constrained environments.

Apache Kafka: A distributed event streaming platform that handles high-throughput data streams. Kafka is designed for real-time analytics, data integration, and stream processing at scale. It's suitable for applications requiring durable message storage, replayability, and complex event processing.