MQTT is an easily used networking protocol designed for IOT (internet of things) applications. It is well suited for controlling hardware devices and for reading sensors across a local network or the internet.

It is a means of communicating between multiple clients. A single server, also known as a broker, manages the network. Clients may include ESP8266, ESP32 and Pyboard D modules and other networked computers. Typical server hardware is a Raspberry Pi or other small Linux machine which may be left running 24/7. Public brokers also exist.

An effective PC client and server is mosquitto.

This contains two separate projects:

1. A "resilient" asynchronous non-blocking MQTT driver.
2. A means of using a cheap ESP8266 module to bring MQTT to MicroPython platforms which lack a WiFi interface.

This is an alternative to the official driver. It has been tested on the following platforms.

1. ESP8266
2. ESP32
3. Pyboard D

The principal features of this driver are:

1. Non-blocking operation for applications using uasyncio.
2. Automatic recovery from WiFi and broker outages.
3. True qos == 1 operation with retransmission.
4. Improved WiFi range because of its tolerance of poor connectivity.

It has the drawback of increased code size which is an issue on the ESP8266. Run as frozen bytecode it uses about 50% of the RAM on the ESP8266. On ESP32 and Pyboard D it may be run as a standard Python module.

This comprises an ESP8266 firmware image and a MicroPython driver. The target hardware is linked to an ESP8266 running the firmware image using a 5-wire interface. The driver runs on the target which can then access MQTT. The driver is non-blocking and is designed for applications using uasyncio.

The current version of this library is in the bridge directory and is documented

It uses the new version of uasyncio.

An old version is archived to the pb_link directory, although I plan to delete this.

The MQTT Bridge replaces this library. It was written a long time ago when the issues around portability were less clear. The client code is substantially revised with API changes. Objectives:

1. Compatibility with uasyncio V3.
2. True portability between platforms. In particular:
3. Tested compatibility with the Raspberry Pi Pico.
4. A more consistent API with significant simplifications.
5. Replace the non-portable RTC code with a means of retrieving NTP time.
6. Enable a choice of time server.
7. Bugs fixed!

There seems little hope of a portable machine.RTC class, so setting the RTC is now the responsibility of the application (if required).

The ESP8266 code has only minor changes.