PubSub over UDP

Goal

Run publishing and subscribing in one process and watch UADP messages go through.
Split publisher and subscriber into two processes that talk over UDP.
Choose the fields to publish and read the identifiers of received messages.

What you need

The apps package and the OPCUA_APPS variable from the lab setup.
Two terminals.

PubSub needs no OPC UA server or session, so this tutorial doesn’t use server_start or connect.

1. Publish and receive in one process

Start the headless simulator:

"$OPCUA_APPS/bin/pdcomm-opcua-headless"

The default configuration uses UDP with the both role, publisher and subscriber at once: the subscriber listens on a free port of 127.0.0.1 and the publisher sends to that address.

{"action":"pubsub_start"}

{"ok":true,"result":{"acknowledged":0,"destination":"127.0.0.1:64578","dropped":0,"error":null,"listen":"127.0.0.1:64578","queued":0,"received":0,"rejected":0,"role":"both","running":true,"sent":0,"transport":"udp"}}

Publish one DataSet with the default fields:

{"action":"pubsub_publish"}

{"ok":true,"result":{"bytes":60,"fields":4,"sequence":1}}

One 60-byte UADP NetworkMessage went out. Take the received messages:

{"action":"pubsub_received"}

{"ok":true,"result":{"messages":[{"bytes":60,"datasets":[{"fields":[{"type":"Boolean","value":true},{"type":"Int32","value":42},{"type":"Double","value":3.5},{"type":"String","value":"pdcomm"}],"major":1,"minor":1,"sequence":1}],"publisher_id":77,"source":"127.0.0.1:54425","writer_group":1,"writer_ids":[17]}]}}

Field	Meaning
`publisher_id`	The PublisherId in the NetworkMessage header (UInt16 77)
`writer_group`	The WriterGroupId (1)
`writer_ids`	The DataSetWriterIds in the message ([17])
`datasets[].sequence`	The DataSetMessage sequence number
`datasets[].major`, `minor`	The configuration version
`datasets[].fields`	The field values of the key frame
`source`	The UDP address the message came from

2. Choose the fields

Put 1 to 64 typed values in fields:

{"action":"pubsub_publish","fields":[{"type":"Double","value":21.5},{"type":"String","value":"line-1"}]}

{"ok":true,"result":{"bytes":53,"fields":2,"sequence":2}}

Check the status:

{"action":"pubsub_status"}

{"ok":true,"result":{"acknowledged":0,"destination":"127.0.0.1:64578","dropped":0,"error":null,"listen":"127.0.0.1:64578","queued":1,"received":2,"rejected":0,"role":"both","running":true,"sent":2,"transport":"udp"}}

sent and received are running totals, queued counts messages you haven’t taken yet and rejected counts messages that couldn’t be decoded.

{"action":"pubsub_received"}

{"ok":true,"result":{"messages":[{"bytes":53,"datasets":[{"fields":[{"type":"Double","value":21.5},{"type":"String","value":"line-1"}],"major":1,"minor":1,"sequence":2}],"publisher_id":77,"source":"127.0.0.1:54425","writer_group":1,"writer_ids":[17]}]}}

Stop PubSub and end the program:

{"action":"pubsub_stop"}

{"action":"shutdown"}

3. Start the subscriber process

Now split publisher and subscriber. In the first terminal, start the simulator again:

"$OPCUA_APPS/bin/pdcomm-opcua-headless"

Set the role to subscriber and the listen address to 127.0.0.1:4840, the default opc.udp port defined by Part 14. When you pass only part of the configuration, the rest keeps its defaults.

{"action":"configure","config":{"pubsub":{"role":"subscriber","listen":"127.0.0.1:4840"}}}

{"action":"pubsub_start"}

{"ok":true,"result":{"acknowledged":0,"destination":null,"dropped":0,"error":null,"listen":"127.0.0.1:4840","queued":0,"received":0,"rejected":0,"role":"subscriber","running":true,"sent":0,"transport":"udp"}}

4. Start the publisher process

In the second terminal, start another simulator:

"$OPCUA_APPS/bin/pdcomm-opcua-headless"

Set the role to publisher and the destination to the subscriber’s address:

{"action":"configure","config":{"pubsub":{"role":"publisher","destination":"127.0.0.1:4840"}}}

{"action":"pubsub_start"}

{"ok":true,"result":{"acknowledged":0,"destination":"127.0.0.1:4840","dropped":0,"error":null,"listen":null,"queued":0,"received":0,"rejected":0,"role":"publisher","running":true,"sent":0,"transport":"udp"}}

{"action":"pubsub_publish","fields":[{"type":"Int32","value":5},{"type":"Boolean","value":false}]}

{"ok":true,"result":{"bytes":40,"fields":2,"sequence":1}}

5. Check the subscriber

In the first terminal, take the received messages:

{"action":"pubsub_received"}

{"ok":true,"result":{"messages":[{"bytes":40,"datasets":[{"fields":[{"type":"Int32","value":5},{"type":"Boolean","value":false}],"major":1,"minor":1,"sequence":1}],"publisher_id":77,"source":"127.0.0.1:53076","writer_group":1,"writer_ids":[17]}]}}

6. Clean up

Type this in both terminals:

{"action":"shutdown"}

{"ok":true,"result":{"client_closed":null,"pubsub_stopped":true,"server_stopped":false}}

Common errors

`error` in the reply	Cause and fix
`PubSub is already running`	PubSub is already started. To change its configuration, send `pubsub_stop` first, then `configure`.
`PubSub is not running`	You sent `pubsub_publish` or `pubsub_received` before `pubsub_start`.
`UDP transport I/O: address in use (os error 48)`	Another program uses the listen address. Use another port or stop that program; the error number differs by operating system.
`This PubSub connection has no publisher`	You sent `pubsub_publish` with the `subscriber` role.
`fields must be an array of 1 to 64 typed values`	`fields` is empty or has more than 64 entries.

The simulator doesn’t sign or encrypt UADP messages, so use it only on networks you trust. In the GUI, the PubSub tab does the same.

Next steps

PubSub over MQTT