OPC UA PubSub over MQTT

Before you begin

• Publish/subscribe and topicsHow a broker routes messages by topic, and how to name topics, use wildcards and design a topic tree.
• MQTT in PDCommWhere you use MQTT in PDComm and how the simulator connects to a broker.
• OPC UA overviewLearn how the OPC UA standard is organized, which two communication models it defines and what the parts of PDComm OPC UA do.

Goal

You build a setup in which a publisher on the equipment side and a subscriber on the server side share nothing but an MQTT broker:

publisher process              Mosquitto                 subscriber process
(publisher_id 100)  ──────▶  127.0.0.1:1883  ──────▶   (Received messages)
                    opcua/uadp/data/100/line1      │
                                                   └──▶ mosquitto_sub (observer)

• The publisher runs the headless simulator in scenario mode, sends two DataSets and exits.
• The subscriber runs the headless simulator interactively so that you can look at what it received.
• mosquitto_sub uses a wildcard filter to watch the data of every PublisherId.

OPC UA background explains DataSets, WriterGroups and DataSetWriters.

Before you begin

• The mqtt-lab directory and local.conf from Run a local Mosquitto broker
• The PDComm OPC UA simulator package
• Four terminal windows

Steps

1. Save the subscriber input file and the publisher scenario file in the mqtt-lab directory.

   subscriber.jsonl

   {"action":"configure","config":{"pubsub":{"transport":"mqtt","role":"subscriber","broker":"127.0.0.1:1883","topic":"opcua/uadp/data/100/line1"}}}
   {"action":"pubsub_start"}

   The subscriber input holds the JSON commands for the headless simulator, one per line.

   publisher-scenario.json

   [
     {
       "command": {
         "action": "configure",
         "config": {
           "pubsub": {
             "transport": "mqtt",
             "role": "publisher",
             "broker": "127.0.0.1:1883",
             "topic": "opcua/uadp/data/100/line1",
             "publisher_id": 100,
             "writer_group_id": 1,
             "dataset_writer_id": 1
           }
         }
       }
     },
     { "command": { "action": "pubsub_start" }, "expect": { "running": true, "transport": "mqtt" } },
     {
       "command": {
         "action": "pubsub_publish",
         "fields": [
           { "type": "Double", "value": 21.5 },
           { "type": "Boolean", "value": true },
           { "type": "String", "value": "RUN" }
         ]
       },
       "expect": { "sequence": 1 }
     },
     {
       "command": {
         "action": "pubsub_publish",
         "fields": [
           { "type": "Double", "value": 21.75 },
           { "type": "Boolean", "value": true },
           { "type": "String", "value": "RUN" }
         ]
       },
       "expect": { "sequence": 2 }
     },
     { "command": { "action": "pubsub_stop" }, "expect": { "running": false } }
   ]

   A scenario file is an array of steps. The simulator runs the command of each step in order and, when a step has expect, compares those values with the result. The publisher uses PublisherId 100, WriterGroupId 1 and DataSetWriterId 1, and publishes to opcua/uadp/data/100/line1, following the OPC UA PubSub topic convention.

2. In all four terminals, store the path of the simulator package in a variable and go to mqtt-lab:

   PDCOMM_APPS=<simulator package directory>
   cd mqtt-lab

3. In the first terminal, start the broker:

   mosquitto -c local.conf

4. In the second terminal, watch every OPC UA data topic:

   mosquitto_sub -V 5 -t 'opcua/uadp/data/+/#' -F '%t qos=%q retain=%r content-type=%C user-properties=%P %l bytes'

   + matches the PublisherId level, and # matches every level below it.

5. In the third terminal, start the subscriber:

   cat subscriber.jsonl - | "$PDCOMM_APPS/bin/pdcomm-opcua-headless"

   cat sends the two lines of the file first and then passes your keyboard input on to the simulator (-). When the second output line contains "role":"subscriber","running":true, the subscription is ready:

   {"ok":true,"result":{"acknowledged":0,"broker":"127.0.0.1:1883","destination":null,"dropped":0,"error":null,"listen":null,"queued":0,"received":0,"rejected":0,"role":"subscriber","running":true,"sent":0,"topic":"opcua/uadp/data/100/line1","transport":"mqtt"}}

   In Windows PowerShell, run bin\pdcomm-opcua-headless.exe and paste the two lines of subscriber.jsonl.

6. In the fourth terminal, run the publisher scenario:

   "$PDCOMM_APPS/bin/pdcomm-opcua-headless" --scenario publisher-scenario.json

   The simulator prints one result line per step and exits with code 0. The first line, the configuration result, is long and left out here:

   {"result":{"acknowledged":0,"broker":"127.0.0.1:1883","destination":null,"dropped":0,"error":null,"listen":null,"queued":0,"received":0,"rejected":0,"role":"publisher","running":true,"sent":0,"topic":"opcua/uadp/data/100/line1","transport":"mqtt"},"step":2}
   {"result":{"bytes":52,"fields":3,"sequence":1},"step":3}
   {"result":{"bytes":52,"fields":3,"sequence":2},"step":4}
   {"result":{"running":false},"step":5}

   The observer in the second terminal prints two lines:

   opcua/uadp/data/100/line1 qos=0 retain=0 content-type=application/opcua+uadp user-properties=UAMessageType:ua-data 52 bytes
   opcua/uadp/data/100/line1 qos=0 retain=0 content-type=application/opcua+uadp user-properties=UAMessageType:ua-data 52 bytes

7. In the third terminal (the subscriber), type:

   {"action":"pubsub_received"}

   {"ok":true,"result":{"messages":[{"bytes":52,"datasets":[{"fields":[{"type":"Double","value":21.5},{"type":"Boolean","value":true},{"type":"String","value":"RUN"}],"major":1,"minor":1,"sequence":1}],"publisher_id":100,"source":"mqtt","writer_group":1,"writer_ids":[1]},{"bytes":52,"datasets":[{"fields":[{"type":"Double","value":21.75},{"type":"Boolean","value":true},{"type":"String","value":"RUN"}],"major":1,"minor":1,"sequence":2}],"publisher_id":100,"source":"mqtt","writer_group":1,"writer_ids":[1]}]}}

   publisher_id 100, writer_ids [1] and sequence numbers 1 and 2 match the publisher’s settings. The subscriber receives by topic, whatever its own publisher_id (77 by default) is.

Subscribe from the GUI

The result is the same when the subscriber is the GUI:

1. Open the OPC UA simulator GUI and go to the PubSub tab.
2. Set Transport to UADP over MQTT and Role to Subscriber.
3. Enter 127.0.0.1:1883 in MQTT broker and opcua/uadp/data/100/line1 in MQTT topic, then click Start PubSub.
4. Run the publisher scenario from step 6 again and click Show received. The table shows two rows.

The simulator PubSub tab describes the screen.

Clean up

1. Type {"action":"shutdown"} in the third terminal to stop the subscriber.
2. Stop mosquitto_sub in the second terminal and the broker in the first terminal with Ctrl+C.

Common problems

Symptom	Cause	Fix
messages from pubsub_received is empty	You started the subscriber after the publisher. The simulator doesn’t retain data and starts a new session on every connection.	Start the subscriber first, then run the publisher scenario again.
The observer sees the messages but the subscriber doesn’t	The subscriber’s topic differs from the publisher’s topic.	Compare topic in the two files.
The scenario ends with Scenario step 2 failed: MQTT broker connection failed: …	The publisher couldn’t connect to the broker.	Check the broker from step 3 and the broker address. Look up the rest of the message in Connection problems.

Tip

You can use the publisher scenario as is in automated tests. When a step fails, the simulator exits with code 1 and prints the number of the failed step to standard error.