Incorporating topic names

Jan-Piet Mens edited this page Feb 15, 2016 · 3 revisions


A MQTT topic name contains information you may want to use in transformation data. After having the information decoded into transformation data, it can be reused in format or topic targets directives.

Let's have a look into the details by means of two examples.

Incorporate topic names into outgoing messages

As a rather extreme example, consider the OwnTracks program (the artist formerly known as MQTTitude).

When an OwnTracks device detects a change of a configured waypoint or geo-fence (a region monitoring a user can set up on the device), it emits a JSON payload which looks like this, on a topic name consisting of owntracks/_username_/_deviceid_:

owntracks/jane/phone -m '{"_type": "location", "lat": "52.4770352" ..  "desc": "Home", "event": "leave"}'

In order to be able to obtain the username (jane) and her device name (phone) for use in transformations (see previous section), we would ideally want to parse the MQTT topic name and add that to the item data our plugins obtain. Yes, we can.

An optional datamap in our configuration file, defines the name of a function we provide, also in the configuration file, which accomplishes that.

datamap = OwnTracksTopicDataMap()

This specifies that when a message for the defined topic owntracks/jane/phone is processed, our function OwnTracksTopicDataMap() should be invoked to parse that. (As usual, topic names may contain MQTT wildcards.)

The function we define to do that is:

def OwnTracksTopicDataMap(topic):
    if type(topic) == str:
            # owntracks/username/device
            parts = topic.split('/')
            username = parts[1]
            deviceid = parts[2]
            deviceid = 'unknown'
            username = 'unknown'
        return dict(username=username, device=deviceid)
    return None

The returned dict is merged into the transformation data, i.e. it is made available to plugins and to transformation rules (format). If we then create the following rule

format = "{username}: {event} => {desc}"

the above PUBlish will be transformed into

jane: leave => Home

Incorporate topic names into topic targets


In another example we use information from the MQTT topic path to dynamically resolve topic targets in order to dispatch messages to specified receivers.

Decode topic names into transformation data

The Hiveeyes bee monitoring system uses MQTT topics like {realm}/{network}/{gateway}/{node}/{field} for transmitting sensor measurement values. An example message looks like:

hiveeyes/0ef-917-40b-a4-5b5/8sf83id9/1/temp1 22.5

Similar to the OwnTracks scenario, we first need to split the topic path into segments and put them into a dictionary. We take a slightly different approach and decode the MQTT topic using regular expressions by defining a datamap function hiveeyes_topic_to_topology:

import re

def hiveeyes_topic_to_topology(topic):
    if type(topic) == str:
            pattern = r'^(?P<realm>.+?)/(?P<network>.+?)/(?P<gateway>.+?)/(?P<node>.+?)/(?P<field>.+?)$'
            p = re.compile(pattern)
            m = p.match(topic)
            topology = m.groupdict()
            topology = {}
        return topology
    return None

Let's hook this function into the mqttwarn.ini configuration file:

datamap = hiveeyes_topic_to_topology()

When receiving appropriate messages, we have the topic segments accessible inside the transformation data dictionary:

    "realm":   "hiveeyes",
    "network": "0ef-917-40b-a4-5b5",
    "gateway": "8sf83id9",
    "node":    "1",
    "field":   "temp1"

Format an appropriate outgoing message

The format configuration directive

format = "{field} of {gateway}_{node}@{network} is {payload}°C"

provides a nicely formatted, human readable message:

temp1 of 8sf83id9_1@0ef-917-40b-a4-5b5 is 22.5°C

Dispatch message to appropriate receivers

To recap, the goal is to inform members of our Beekeepersclub of measurement values by dispatching messages to registered addresses.

Let's use xmpp by defining the following addressbook:

sender   = ''
password = 'yourcatsname'
targets  = {
                '0ef-917-40b-a4-5b5' : [''],
                '8sf83id9'           : [''],
                '8sf83id9-1'         : ['']

In this example, the researchers of the Beekeepersclub would get informed about every message on the network, while the Berlin section would only get messages from a specific gateway. Peng, who is responsible for a single beehive, would only get measurement data from a specific node.

By using a configuration like:

datamap = hiveeyes_topic_to_topology()
targets = log:info, xmpp:{network}, xmpp:{gateway}, xmpp:{gateway}-{node}

this allows us to make mqttwarn compute the right hand side of the designated topic target dynamically based on information from the transformation data, while only having to define a single service section in the configuration file.

Otherwise, we would have to define a service section for each and every MQTT topic we want to monitor and dispatch messages from.

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