Broker
The server in a publish-subscribe system that receives messages from publishers, matches them to subscriptions and delivers them, decoupling devices from applications.
A broker is the hub of MQTT and similar protocols. Every client connects to it, and it alone knows who has subscribed to which topic. It authenticates clients, enforces access rules, queues messages for offline subscribers with persistent sessions, stores retained messages and honors QoS handshakes. Popular implementations include Eclipse Mosquitto, EMQX, HiveMQ, and the managed endpoints built into cloud IoT platforms.
The design simplifies devices enormously, since a sensor needs one outbound connection and no knowledge of consumers. The cost is that the broker becomes the component everything depends on. Sizing it for connection count and message rate, clustering it for availability, securing it with TLS and per-client credentials, and placing it where latency and data residency rules allow are central architecture decisions. Brokerless alternatives such as DDS avoid the dependency at the cost of complexity. See MQTT explained.
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