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.

• 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, never subscribe to them.

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.

MQTT client with automatic reconnect

There’s a separate umqtt.robust module which builds on umqtt.simple and adds automatic reconnect support in case of network errors. Please see its documentation for further details.