MQTT

MQTT is a pubsub protocol using a single broker and arbitrary number of clients and topics. Clients subscribe to topics, informing the broker of their subscriptions (clients do not receive messages on topics to which they are unsubscribed, and publishers do not need to know about clients). MQTT mandates a reliable transport. Each topic can retain a message, which will immedaitely be provided to new clients. There is otherwise no queue, despite MQTT's origins in IBM's MQ. Clients and publishers can maintain persistent connections to the broker.

Eclipse Mosquitto is a popular open source implementation of an MQTT publisher, client, and broker.