MQTT Basics Cheat Sheet

MQTT protocol for IoT: publish/subscribe model, Quality of Service levels, broker setup, retained messages, Last Will and Testament, and common IoT patterns.

Last Updated: May 1, 2025

MQTT Concepts

Item	Description
`Broker`	Central server that routes messages between publishers and subscribers
`Topic`	Hierarchical string (e.g., sensors/temperature/bedroom) — subscribers filter by topic
`Publish`	Send a message to a topic on the broker
`Subscribe`	Express interest in a topic (with optional wildcards)
`Client ID`	Unique identifier for each MQTT client connection
`Keep Alive`	Max idle time (seconds) — client sends PINGREQ to stay connected

Quality of Service (QoS)

Item	Description
`QoS 0: At most once`	Fire-and-forget — no acknowledgment, no retry. Fast, unreliable.
`QoS 1: At least once`	Message acknowledged (PUBACK). Retransmit until ACK received.
`QoS 2: Exactly once`	Four-way handshake — guaranteed exactly-once delivery. Slow but reliable.
`QoS is per-message`	Publisher and subscriber can have different QoS levels
`Downgrade rule`	Subscriber gets min(published QoS, subscribed QoS)

Advanced MQTT Features

Item	Description
`Retained Messages`	Broker stores last message on a topic — new subscribers get it immediately
`Last Will and Testament`	Pre-set message sent by broker if client disconnects unexpectedly
`Persistent Session`	Broker remembers subscriptions and queues messages for offline clients
`Clean Session = false`	Enable persistent session (messages queued between connections)
`Wildcard: +`	Single-level wildcard: sensors/+/temperature matches any sensor name
`Wildcard: #`	Multi-level wildcard: sensors/# matches everything under sensors/

Mosquitto Broker Quick Start

mosquitto -p 1883

Start broker on default MQTT port

mosquitto_sub -t 'sensors/#' -v

Subscribe to all sensor topics (verbose)

mosquitto_pub -t 'sensors/temp' -m '22.5'

Publish a temperature reading

mosquitto_pub -t 'alerts' -m 'OFFLINE' --will-topic 'alerts' --will-payload 'OFFLINE'

Publish with Last Will

mosquitto_pub -t 'config' -m 'v3' -r

Publish retained message (new subs get it)

mosquitto_sub -t '#' --qos 2

Subscribe with QoS 2 (exactly once)

Pro Tip: MQTT is the standard IoT messaging protocol — lightweight, binary, works over TCP or WebSocket. QoS 0 = fire-and-forget, QoS 1 = at-least-once, QoS 2 = exactly-once (slowest).