PubSub publisher

The opcua-pubsub service emits OPC UA Part 14 UADP (OPC UA Binary over UDP) NetworkMessages every 500 ms. Built on the UA-.NETStandard PubSub stack. Subscribers on the host machine listen on 127.0.0.1:14850.

How to reach it

A single UDP socket — no OPC UA session, no handshake. The subscriber opens a UDP listener and decodes incoming datagrams.

Why two services

PubSub natively uses UDP multicast. Multicast doesn't reliably traverse the Docker Desktop VM boundary across Linux/macOS/Windows. To make the publisher work the same everywhere, the suite splits it:

opcua-pubsub  ── pubsub-net ──►  opcua-pubsub-relay  ── host.docker.internal:14850 ──►  subscriber
   (publisher)                       (socat sidecar)                                       (your test)

The publisher sends unicast to the relay (a stateless socat forwarder) on a shared compose bridge. The relay re-emits each packet to host.docker.internal:14850, which Docker Desktop and Docker Engine both bridge to the physical host's 127.0.0.1:14850.

This works on:

• Linux + Docker Engine (bare metal, GitHub Actions runners)
• Docker Desktop on Linux, macOS, Windows

Subscribers don't know the relay exists — they listen on 127.0.0.1:14850 directly.

DataSet shape

Every NetworkMessage carries one DataSet with three fields:

NetworkMessage headers

Headers are written into each UADP frame's chunk encoding — your subscriber's demux uses them to route to the right DataSetReader.

Configuration via env vars

Test patterns

Receive first message

A subscriber bound to 127.0.0.1:14850:

socket = bind UDP on 0.0.0.0:14850
data, addr = socket.recv()
# Decode UADP NetworkMessage

Expected: a NetworkMessage with one DataSet, three fields, PublisherId=100, WriterGroupId=1, DataSetWriterId=1.

Counter increments

Receive 10 messages over ~5 seconds. The counter field is strictly increasing, but not necessarily by 1 per message. The simulator increments _counter every OPCUA_TICK_INTERVAL_MS (default 250 ms) but the publisher emits a NetworkMessage only every OPCUA_PUBLISH_INTERVAL_MS (default 500 ms). With the defaults you should see counter advance by 2 between consecutive frames. To get a strict +1 step, set both intervals to the same value (e.g. OPCUA_TICK_INTERVAL_MS=500).

If you miss a message (UDP isn't reliable), counter jumps by more than the expected step. That's by design — UDP losses are part of the test surface.

Value sequence

The value field follows sin(counter × π / 20). So:

(counter is never 0 on the wire — see the note in the DataSet shape table above.)

Verify the formula holds for any received message.

Filter by PublisherId

A subscriber that exercises filtering:

config.PublisherIdToAccept = 100
→ all messages pass (only one PublisherId here)

config.PublisherIdToAccept = 999
→ no messages pass

The publisher only emits with PublisherId=100 unless reconfigured.

Multiple writers

The default config has one DataSetWriter. To test multi-writer demux, fork and add a second OPCUA_DATASET_WRITER_ID in src/TestPublisher/Program.cs.

Decoding the wire

UADP frame structure (high-level):

NetworkMessage Header
  PublisherId, WriterGroupId, ...
  DataSet Message Header (per DataSet)
    DataSetWriterId, Sequence number, Timestamp
    Field Encoding
      Field 0 (counter)  — UInt32, little-endian
      Field 1 (timestamp) — DateTime
      Field 2 (value)     — Double, IEEE-754

The exact byte-level wire format is defined in OPC UA Part 14 §7.2.

UA-.NETStandard's OPCFoundation.NetStandard.Opc.Ua.PubSub NuGet package implements this fully on the .NET side — what the suite emits matches what that library expects to decode.

Sequence numbers

The wire includes a per-message sequence number that increments independently of the counter field:

SequenceNumber: 0, 1, 2, 3, ...

Subscribers verify the sequence is contiguous (no skips) to detect UDP drops. UDP packet loss rates depend heavily on the host and network: on a quiet loopback it is normally near-zero, but a busy CI host or a Docker Desktop VM can drop several percent. Treat any observed rate as environmental rather than a guaranteed bound.

Notes

• The publisher is unsecured (mode None). For secured PubSub testing, pair with the Security Key Service — full end-to-end secured PubSub is on the roadmap.
• The publisher doesn't expose a UA server interface — there's no DataSet discovery beyond what's in the wire-level metadata.
• The relay sidecar (opcua-pubsub-relay) is stateless — it forwards every UDP packet it receives. No buffering, no retransmission.

Where to read next

• GitHub Action — invoke the publisher (or a subset) from CI.
• Security Key Service — the complementary feature.