Pub/sub messaging protocol originally developed for IoT devices. Clients connect to a broker and publish/subscribe on named topics; brokers support three quality-of-service levels — 0 (fire-and-forget), 1 (at-least-once delivery), 2 (exactly-once delivery). Higher performance than REST for many workloads and supports bidirectional communication, at the cost of introducing a broker that must be maintained and kept highly available. Demoed in Emiliano's wroclove.rb 2026 talk with the Ruby mqtt gem in a Rails initializer that subscribes to an 'orders' topic and creates an order whenever a message arrives; publishers fire-and-forget so error feedback requires a separate failure topic.