AMQP
Enterprise messaging protocol — started by JPMorgan in 2003 for banking messages. Standardised in 0.9.1 (RabbitMQ) and 1.0 (more divergent).
AMQP 0.9.1 (de facto via RabbitMQ): producers post to "exchanges" that route to "queues" based on routing keys + binding rules. Exchange types: direct, topic, fanout, headers. AMQP 1.0 is a more minimal protocol, focused on message structure rather than broker topology — Azure Service Bus, Apache Qpid use it. Different from Kafka (log-based, replay) and MQTT (lightweight IoT). More reliable than Kafka for request/response flows.