The message model

The primary use case for this project is a domestic environment with multiple devices of any type: lights, audio video components, security devices, heating, air conditioning, controllers, keypads, etc… For many (good) reasons, MQTT has been selected as the communication protocol. But only a few, if any, devices are MQTT enabled. For those that are not, there is a need to develop ad-hoc gateways to bridge whatever interface they use natively (serial for example) to one that is MQTT based. Even for those devices that communicate natively through MQTT, there is a need to agree on a syntax that makes the exchange of messages coherent.


In the example below, a smart home has some lighting connected in four different rooms through a proprietary network, four audio-video devices connected through another proprietary network, and some other devices that are already MQTT-enabled, but which still need to speak a common language.

Diagram of a smart home with some connected devices

One of the objectives of this project is not only to define a common MQTT syntax, but also to make it as intuitive as possible. Ideally, a human should be able to write an MQTT message off-hand and operate successfully any device in the network.

Message Addressing

The first step of any message is to define its destination. A flexible addressing model should allow for a heuristic approach based on a combination of characteristics of the recipient, on top of the standard deterministic approach (e.g. a unique device id). Four characteristics are usually considered:

  • the function of the device: lighting, security, audio-video, etc;
  • its location;
  • its gateway: which application is managing that device, if any;
  • the name of the device.

In our example, the MQTT point of view shows how those four characteristics, or just a subset, can define all the devices in the network.

Diagram of a smart home from the MQTT network point of view

Some considerations about those four characteristics:

  • not all four characteristics need to be provided to address succesfully a device;
  • the device name can be generic (e.g. spotlight) or specific and unique within the network (e.g. lightid1224); in the generic case, obviously other characteristics are needed to address the device.
  • any device can have more than one value for each characteristics, particularly the function and device ones (it is probable that the gateway and the location characteristics are unique for a given device);
  • the location is important and probably the most intuitive characteristic of all; preferably it should represent the place where the device operates and not where it is physically located (e.g. an audio amplifier might be in the basement but it powers speakers in the living room; the location should be the living room); but the location might even not be defined (e.g. to address the security system of the whole house, or an audio network player that can broadcast to different channels or zones).
  • the gateway is the most deterministic characteristic (alongside a unique device id); this should be the chosen route for fast and unambiguous messaging.
  • the function is another important intuitive characteristic; not only it helps in addressing devices (combined with a location for example), but it also clarifies ambiguous commands (e.g. POWER_ON with lighting or with audiovideo means different things). However things can get more complicated if a device has more than one function; this should be allowed, it is up to the gateway to make sure any ambiguity is resolved from the other characteristics.

Those four characteristics should ensure that the messaging model is flexible enough to be heuristic or deterministic. A gateway will decide how flexible it wants to be. If it has enough processing bandwidth, it can decide to subscribe to all lighting messages for example, and then parse all messages received to check if they are actually addressed to it. Or it can subscribe only to messages addressed specifically to itself (through the gateway name for example), restricting access only to the senders that know the name of that gateway.

Message Content

The content of a message in the context of domestic IoT can be modelled in many different ways. This project splits it into 3 characteristics:

  • a type with 2 possible values: command for messages that are requiring an action to be performed, or status for messages that only broadcast a state;
  • an action that indicates what to do or what the status is referring to;
  • a set of arguments that might complete the action characteristic.

The key characteristic here is the action, a string representing the what to do, with the optional arguments helping to define by how much for example. It can be POWER_ON and POWER_OFF on their own for example (no argument), or SET_POWER with the argument power:ON or power:OFF, or both. The interface decides what actions it recognises, the more the better.

Message Source

The sender, which can be a device or another gateway for example, is an optional characteristic in our message model. It can be very useful in answering status requests in a targeted way, for example.

Bridging MQTT and the interface

There are therefore a total of 8 characteristics in our message model:

  • function,
  • gateway,
  • location,
  • device,
  • type,
  • action,
  • argument of action,
  • sender.

They are all strings except type which can only have 2 predefined values. They are all the fields that can appear in a MQTT message, either in the topic or in the payload. They are all attributes of the internal message class that is used to exchange messages between the library and the interface being developed. They are all the characteristics available to the developer to code its interface.

The internal message class

The internal message class internalMsg defines the message objects stored in the lists that are shared by the library and the interface. There is a list for incoming messages and a list for outgoing messages. At its essence, the library simply parses MQTT messages into internal ones, and back. The library therefore defines the MQTT syntax by the way it converts the messages.

The conversion process

The conversion process happens inside the class msgMap with the methods MQTT2Internal() and Internal2MQTT(). These methods achieve 2 things:

  • define the syntax of the MQTT messages in the way the various characteristics are positioned within the MQTT topic and payload;
  • if mapping is enabled, map the keywords for every characteristic between the MQTT vocabulary and the internal one; this is done via dictionaries initialised by a mapping file.

The MQTT syntax

The library currently defines the MQTT syntax as follows. The topic is structured like this:


where root can be anything the developer wants (home for example) and type can be only C or S.

The payload is simply the action alone if there are no arguments:


or the action with the arguments all in a JSON string like this:


where the first action key is written as is and the other argument keys can be chosen by the developer and will be simply copied in the argument dictionary.

This syntax is defined within the 2 methods doing the conversions. The rest of the library is agnostic to the MQTT syntax. Therefore one only needs to change these 2 methods to change the syntax. However in that case, all the devices and other gateways obviously have to adopt the same new syntax.

The mapping data

By default, when the mapping option is disabled, the keywords used in the MQTT messages are simply copied in the internal class. So, for example, if the function in the MQTT message is lighting, then the attribute function in the internalMsg will also be lighting. If for any reason a keyword has to change on either side, it has to be reflected on the other one, which is unfortunate. For example, let’s assume a location name in the MQTT vocabulary is basement and that is what is used in the internal code of the interface to start with. For some reason the name in the MQTT vocabulary needs to be changed to lowergroundfloor. In order for the interface to recognise this new keyword, a mapping can be introduced that links the keyword lowergroundfloor in the MQTT messages to basement in the internal representation of messages. This mapping is defined in a separate JSON file, and the code does not need to be modified.

The mapping option can be enabled (it is off by default) in the configuration file, in which case the location of the JSON file is required. All the keyword characteristics (except type) can (but do not have to) be mapped in that file: function, gateway, location, device, sender, action, argument keys and argument values. To give more flexibility, there are 3 mapping options available for each characteristic that need to be specified:

  • none: the keywords are left unchanged, so there is no need to provide the mapping data for that characteristic;
  • strict: the conversion of the keywords go through the provided map, and any missing keyword raises an exception; the message with that keyword is probably ignored;
  • loose: the conversion of the keywords go through the provided map, but missing keywords do not raise any error but are passed unchanged.

The mapping between internal keywords and MQTT ones is a one-to-many relationship for each characteristic. For each internal keyword there can be more than one MQTT keyword, even if there will have to be one which has priority in order to define without ambiguity the conversion from internal to MQTT keyword. In practice, this MQTT keyword will be the first one in the list provided in the mapping (see below) and the other keywords of that list can be considered aliases. Going back to the example above, for the unique internal location keyword basement, we could define a list of MQTT keywords as ["lowergroundfloor", "basement"], so that basement in internal code gets converted to lowergroundfloor in MQTT (as it is the new official keyword) but basement in MQTT is still accepted as a keyword that gets converted to basement in internal messages.

In practice, the mapping data is provided by a JSON formatted file. The JSON schema mqtt_map_schema.json is available in the gateway package. New JSON mapping files can be tested against this schema (I use the online validation tool The mapping file also contains the topics to subscribe to and the root token for all the topics. These values override the ones found in the configuration file if the mapping feature is enabled.