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An MQTT sensor on a dashboard

This guide takes a sensor that publishes over MQTT from a Mosquitto broker to a tag and a screen. It is for the engineer who has a broker and wants the first topic in the gateway. The MQTT driver has run against Mosquitto; nothing else has been tried.

Prerequisites​

  • A gateway with the MQTT driver module (io.scadex.driver.mqtt) running; see Modules.

  • A Mosquitto broker the gateway can reach, for example on mqtt://broker:1883. A local one for trying things out:

    docker run -d --name mosquitto -p 1883:1883 eclipse-mosquitto:2 \
      sh -c "printf 'listener 1883\nallow_anonymous true\n' > /mosquitto/config/mosquitto.conf && /usr/sbin/mosquitto -c /mosquitto/config/mosquitto.conf"

  • The mosquitto_pub and mosquitto_sub command line tools (package mosquitto-clients on Debian and Ubuntu).

  • The engineer role for the connection and tag, the operator role to write.

Steps​

1. Add the broker​

  1. Open Connections, then MQTT brokers.
  2. Press Add a broker. A panel opens beside the list.
  3. Fill in Name (Plant_broker; letters, digits, dots, hyphens or underscores) and Address (mqtt://broker:1883; the address starts with its protocol). Leave Enabled on.
  4. Under Sign-in, leave User name and Password empty if the broker lets anyone connect. Client ID is optional; left empty, the gateway picks one.
  5. Under Delivery, set Default QoS: At most once (0), At least once (1) or Exactly once (2). The gateway publishes with this QoS.
  6. Press Add broker.

The same over the API:

curl -b cookies.txt -H "Origin: https://app.scadex.io" -H "Content-Type: application/json" \
  https://app.scadex.io/api/v1/modules/io.scadex.driver.mqtt/connections \
  -d '{"name":"Plant_broker","enabled":true,"configuration":{"url":"mqtt://broker:1883","qos":"1"}}'

The configuration keys are url, username, password, clientId and qos. Test it with Test connection on the broker's detail page, or POST .../connections/<id>/test.

2. Publish something to subscribe to​

From any machine that reaches the broker:

mosquitto_pub -h broker -t plant/line1/temperature -m 21.5 -r

-r asks the broker to retain the message, so a late subscriber gets the last value at once.

3. Find the topic and make it a tag​

  1. Select the broker in the list and open its Browse tab (or press Browse topics on the empty tags tab).
  2. The browser lists topics the gateway has seen. Type a filter in Listen on, e.g. plant/# and confirm it to make the gateway subscribe to that filter while you browse; plant/# covers everything under plant/. A pin marks a topic the broker retains; the row shows its last payload.
  3. On plant/line1/temperature press the Create a tag from ... action. A New tag panel opens, bound to the topic.
  4. Give the tag its path (Line1.Temperature), choose how the payload is read, and save.

The binding of an MQTT tag has two keys:

KeyValues
topicThe topic, or a filter with + or #. A tag on a filter receives every matching message and can't be written.
payloadFormatnumber, boolean, json, or left empty for text

How a payload is decoded:

payloadFormatPayloadTag value
number21.521.5. Anything that is not a finite number gives a bad quality with the reason '...' is not a number.
booleantrue, 1, on, false, 0, offtrue or false
json{"t":21.5,"h":40}The parsed document as one JSON value. Read a field from it with a derived tag whose property is the field name (t), see Tags.
emptyanythingThe text as it is

Over the API:

curl -b cookies.txt -H "Origin: https://app.scadex.io" -H "Content-Type: application/json" \
  https://app.scadex.io/api/v1/tags \
  -d '{"path":"Line1.Temperature","kind":"external","producerModuleId":"io.scadex.driver.mqtt","producerInstanceId":"<connection id>","binding":{"topic":"plant/line1/temperature","payloadFormat":"number"},"engUnit":"°C"}'

4. See values arrive​

Open Tags and types and select Line1.Temperature. Then publish a few values:

for v in 21.7 22.0 22.4 21.9; do mosquitto_pub -h broker -t plant/line1/temperature -m $v; sleep 2; done

Each one shows up with good quality and the time the gateway received it. Publish abc and the quality turns bad with 'abc' is not a number.; the next good value clears it.

The broker's detail page shows Tags subscribed, Topics seen and Last message in its Driver section, and the status tags are under @gateway.connection.mqtt.Plant_broker (state, connected, tags, error, since). Values are published to the tag runtime in batches, and a Dropped samples figure appears when values arrived faster than they could be stored.

5. Show it on a screen​

  1. Open Screens, create a screen and open it in the designer (Designer).
  2. Add a widget that shows a value and bind it to Line1.Temperature; the tag's unit and format come along.
  3. Add a trend widget on the same tag for the last hour. A trend reads from the historian when the module runs, otherwise from what the event backbone retains (History and replay).
  4. Save and run the screen. Publish another value; the number and the trend move.

Widgets lists the widgets and what each one binds to.

6. Write back to the broker​

A tag on an exact topic (no + or #) can be written. Writing publishes the value to that topic with the broker's Default QoS, not retained, as text: numbers with full precision, booleans as true/false, bytes as Base64. Watch it arrive:

mosquitto_sub -h broker -t 'plant/line1/#' -v

Then write the tag from the tag browser, from a screen widget that writes (a button or an input, see Widgets), over /sync with tag.write {path, value}, which is what the SDK's client.write sends (operator role; see Live data over /sync), or with POST /api/v1/runtime/tags/{id}/write (see Tags). A write is refused when the broker is not connected ("The MQTT broker for this tag is not connected."), when the tag is on a wildcard topic, or when the broker answers with a reason code ("The broker refused the write: ...").

Because the tag is also subscribed to the topic, the written value comes back from the broker and becomes the tag's value.

Where to go next​