---
tags:
  - micro-controllers
---

# MQTT

_Message Queuing Telemetry Transport_

Messaging protocol that runs on top of TCP/IP like HTTP. Particularly well
suited for IoT devices.

Has publisher/subscriber architecture. Devices publish data to a broker which
manages topics. Other devices subscribe to those topics. Broker would typically
run on a server as the central messaging hub.

MQTT handles connection drops gracefully - if an ESP32 temporarily loses Wi-Fi,
it can queue messages and send them when reconnected. The protocol uses minimal
bandwidth (just a few bytes of overhead vs HTTP's much larger headers), which
means lower power consumption on your ESP32s. MQTT also maintains persistent
connections rather than creating new ones for each data point, reducing network
overhead and latency.

The main Linux client for setting up an MQTT is `mosquitto`.

There are different **Quality of Service** levels you can use to manage how
messages are sent and what to do in event of failure:

- _fire and forget_
- _acknowledged delivery_
- _acknowledged only once delivery_
- _retained message_ (broker remembers last message)
- _last will and testament_ (automatica notification if device disconnects
  unexpectedly)

## Example use case

As an example, my usecase would be to have several ESP32 micro-controllers in
different rooms publish humidity data to a small computer that acts as broker.
This computer will also subcribe to the topics belonging to each room e.g
`bathroom_sensor`, `bedroom_sensor` and store this data in a database.