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umqtt.simple2

umqtt.simple2 is a MQTT client for MicroPython. (Note that it uses some MicroPython shortcuts and doesn't work with CPython).

Support MQTT Version 3.1.1 only.

It certainly works with micropython ports: esp8266 and esp32. It should also work with other ports, but the library was not tested under other ports.

MQTT client with more features

There's a separate umqtt.robust2 module which builds on umqtt.simple2 adds the ability to reconnect. It is able to send unsent messages itself. And many more...

Differences between umqtt.simple and umqtt.simple2

  • When sending messages from QoS=1, there is no problem with "suspending" the script while waiting for confirmation of message receipt by the server.
  • When subscribing to a channel, there is no problem with "suspending" the script while waiting for confirmation of the subscription by the server.
  • Information about receiving or failing to receive a message from QoS=1 or subscription can only be received by registering a callback using the set_callback_status() method.
  • Currently, the module informs about errors in more detailed way. See the umqtt/errno.py file.
  • The application should also not hang up when using check_msg()
  • The code compiled for MPY files, is about 30% larger than the original one. So this library has gained more functionality (maybe reliability), but this was done at the expense of the amount of code.

Problems and solutions

  • ImportError: no module named 'umqtt.simple2'

    Versions of micropython from http://micropython.org/download/ since version 1.12 include the umqtt library, which conflicts with the current library. To avoid conflicts, you need to change the order of importing libraries. You need to import the '/lib' libraries first and then the system libraries. Just add the following lines of code to the boot.py file:

import sys
sys.path.reverse()

How and where to install this code?

You can install using the upip:

import upip
upip.install("micropython-umqtt.simple2")

or

micropython -m upip install -p modules micropython-umqtt.simple2

You can also clone this repository, and install it manually:

git clone https://github.com/fizista/micropython-umqtt.simple2.git

Manual installation gives you more possibilities:

  • You can compile this library into MPY files using the compile.sh script.
  • You can remove comments from the code with the command: python setup.py minify
  • You can of course copy the code as it is, if you don't mind.

Please note that the PyPi repositories contain optimized code (no comments).

Design requirements

  • Memory efficiency.
  • Avoid infamous design anti-patterns like "callback hell".
  • Support for both publishing and subscription via a single client object (another alternative would be to have separate client classes for publishing and subscription).

API design

Based on the requirements above, there are following API traits:

  • All data related to MQTT messages is encoded as bytes. This includes both message content AND topic names (even though MQTT spec states that topic name is UTF-8 encoded). The reason for this is simple: what is received over network socket is binary data (bytes) and it would require extra step to convert that to a string, spending memory on that. Note that this applies only to topic names (because they can be both sent and received). Other parameters specified by MQTT as UTF-8 encoded (e.g. ClientID) are accepted as strings.
  • Subscribed messages are delivered via a callback. This is to avoid using a queue for subscribed messages, as otherwise they may be received at any time (including when client expects other type of server response, so there're 2 choices: either deliver them immediately via a callback or queue up until an "expected" response arrives). Note that lack of need for a queue is delusive: the runtime call stack forms an implicit queue in this case. And unlike explicit queue, it's much harder to control. This design was chosen because in a common case of processing subscribed messages it's the most efficient. However, if in subscription callback, new messages of QoS>0 are published, this may lead to deep, or infinite recursion (the latter means an application will terminate with RuntimeException).

API reference

Taking into account API traits described above, umqtt pretty closely follows MQTT control operations, and maps them to class methods:

  • connect(...) - Connect to a server. Returns True if this connection uses persisten session stored on a server (this will be always False if clean_session=True argument is used (default)).
  • disconnect() - Disconnect from a server, release resources.
  • ping() - Ping server (response is processed automatically by wait_msg()).
  • publish() - Publish a message.
  • subscribe() - Subscribe to a topic.
  • set_callback() - Set callback for received subscription messages. call(topic, msg, retained)
  • set_callback_status() - Set callback for received subscription messages. call(pid, status)
  • set_last_will() - Set MQTT "last will" message. Should be called before connect().
  • wait_msg() - Wait for a server message. A subscription message will be delivered to a callback set with set_callback(), any other messages will be processed internally.
  • check_msg() - Check if there's pending message from server. If yes, process the same way as wait_msg(), if not, return immediately.

wait_msg() and check_msg() are "main loop iteration" methods, blocking and non-blocking version. They should be called periodically in a loop, wait_msg() if you don't have any other foreground tasks to perform (i.e. your app just reacts to subscribed MQTT messages), check_msg() if you process other foreground tasks too.

Note that you don't need to call wait_msg()/check_msg() if you only publish messages with QoS==0, never subscribe to them.

If you are using a subscription and/or sending QoS>0 messages, you must run one of these commands ( wait_msg() or check_msg() ).

For more detailed information about API please see the source code (which is quite short and easy to review) and provided examples.

Supported MQTT features

QoS 0 and 1 are supported for both publish and subscribe. QoS2 isn't supported to keep code size small. Besides ClientID, only "clean session" parameter is supported for connect as of now.

Simple library testing

The current tests are not only to test the code, but also to check it in a real environment. Therefore, a good idea, before we use this library in our own project, is to test its operation with the MQTT broker.

To test if the library works well with your device and MQTT broker, use the TestMQTT class from the tests.py module.

If you don't have your own MQTT broker yet, you can use the free MQTT test broker (test.mosquitto.org).

There is also a sample file main.py`(`example_test_main.py), In this file we add only network configuration. Upload this file to your device with umqtt.simple2 library and tests.py module. Then reset the device and watch the results in the console.

Different problems

  • Wrong topic format during subscription - you'll get OSError: [Errno 104] ECONNRESET in subscribe() or MQTTException: 1 in the wait_msg()/check_msg()

Additional resources