MQTT

MQTT is a lightweight messaging protocol built on publish and subscribe. Devices connect to a central broker and either publish messages to named topics, such as home/kitchen/temperature, or subscribe to topics to receive whatever is published there. Publishers and subscribers never talk directly. Andy Stanford-Clark of IBM and Arlen Nipper designed it in 1999 for monitoring oil pipelines over expensive, unreliable satellite links, and it became an open OASIS standard in 2014, with version 5 following in 2019.

It was built for constrained conditions, and that shows:

• Tiny overhead. A message header can be as small as two bytes, so it works over slow links and on small microcontrollers.
• Persistent connections. Clients hold a long-lived TCP connection open, so the broker can push messages instantly instead of devices polling, and clients behind Network Address Translation can still receive.
• Keep-alive. Periodic pings detect dead connections, a simple form of Failure Detection.

Topics are hierarchical, separated by slashes, and subscriptions can use wildcards: + matches one level and # matches everything below. A dashboard can subscribe to home/+/temperature to get every room.

MQTT offers three quality of service levels per message, mapping directly onto Delivery Guarantees:

• QoS 0, at most once: send and forget, fine for frequent sensor readings where a lost one doesn't matter.
• QoS 1, at least once: resend until acknowledged, so duplicates are possible and receivers should be idempotent.
• QoS 2, exactly once: a four-step handshake, rarely needed.

Two features suit unreliable devices. A retained message is stored by the broker and delivered immediately to any new subscriber, so a new dashboard learns a light's state without waiting for the next update. A last will and testament is a message the client registers when connecting, which the broker publishes on its behalf if the client disconnects uncleanly, announcing "this sensor went offline".

Brokers include the open-source Mosquitto, EMQX, and HiveMQ. MQTT is widespread in home automation, where hubs and custom devices share state through a local broker; in industrial telemetry; and in Facebook Messenger, which adopted it in 2011 to save phone battery.

Compared with a Message Queue built for work distribution, MQTT is about routing current state and events to whoever is interested, and it keeps little history. Compared with a Webhook, which pushes HTTP requests to a server, it suits devices that can't accept inbound connections. Clients reconnecting after an outage should use Exponential Backoff, or thousands of devices returning at once become a thundering herd.

Security is often neglected. Port 1883 carries plain text, while 8883 wraps it in Transport Layer Security. Scans regularly find many thousands of brokers exposed to the internet without authentication, so brokers belong behind a Firewall with credentials and access rules per topic.