Why MQTT Isn’t a Message Queue—and What It Really Is

Despite its name, MQTT (Message Queuing Telemetry Transport) does not provide message queuing in the traditional sense (like in systems such as RabbitMQ, IBM MQ, or ActiveMQ).

Here’s the clarification:

• The “MQ” in MQTT is a historical artifact from its origins at IBM in the late 1990s. It was inspired by IBM’s MQSeries (now IBM MQ), but MQTT itself is not a message queue system.
• MQTT provides publish-subscribe messaging, not queue-based messaging. There is:
  • No built-in message persistence across subscribers (unless retained messages or persistent sessions are used).
  • No message routing or load balancing like you’d find in real queuing systems.
  • No message backlog per subscriber (unless configured with features like persistent sessions).

However, MQTT can simulate some queue-like behavior:

• Persistent sessions: Allow a subscriber to receive messages published while it was offline.
• QoS levels: Ensure messages are delivered reliably (to some extent).
• Retained messages: Let the broker store the last message on a topic and send it to new subscribers.
• Some MQTT brokers (e.g., EMQX, HiveMQ) offer extensions/plugins to integrate with real message queues or provide queue-like semantics.

Summary:

Feature	MQTT	Real Message Queue (e.g., RabbitMQ)
Pub/Sub model	✅ Yes	❌ Usually point-to-point
Traditional queuing	❌ No	✅ Yes
Message persistence	🟡 Optional	✅ Yes
Load balancing	❌ Not built-in	✅ Yes

So: MQTT does not inherently offer traditional message queuing, despite the name. It’s optimized for pub/sub communication, especially in IoT and constrained environments.

Reference: https://chatgpt.com/share/684d561a-f4b8-8002-a9b6-69a21beed17a

Specifications: https://mqtt.org/mqtt-specification/