Bridge OPC UA to MQTT in Node.js

The code

const { OPCUAClient, AttributeIds, TimestampsToReturn } = require("node-opcua");
const mqtt = require("mqtt");

const TOPIC = "plant/line1/machinetool1/state";

(async () => {
  const mq = mqtt.connect("mqtt://localhost:1883");
  await new Promise((r) => mq.on("connect", r));

  const client = OPCUAClient.create({ endpointMustExist: false });
  await client.connect("opc.tcp://opcua.umati.app:4840");
  const session = await client.createSession();

  const subscription = await session.createSubscription2({
    requestedPublishingInterval: 1000,
    requestedLifetimeCount: 100,
    requestedMaxKeepAliveCount: 10,
    publishingEnabled: true,
  });

  const item = await subscription.monitor(
    { nodeId: "i=2258", attributeId: AttributeIds.Value },
    { samplingInterval: 1000, queueSize: 10 },
    TimestampsToReturn.Both
  );

  item.on("changed", (dv) => {
    mq.publish(TOPIC, JSON.stringify({
      value: dv.value.value,
      status: dv.statusCode.name,
      sourceTimestamp: dv.sourceTimestamp,
    }));
  });

  // publish a connection-state topic so consumers can tell stale from unchanged
  subscription.on("terminated", () =>
    mq.publish("plant/line1/machinetool1/online", "false", { retain: true }));
  mq.publish("plant/line1/machinetool1/online", "true", { retain: true });
})();

What it prints

# in MQTT Explorer, plant/line1/machinetool1/state:
{"value":"2026-09-08T03:45:00.102Z","status":"Good","sourceTimestamp":"2026-09-08T03:45:00.102Z"}
# plant/line1/machinetool1/online: true  (retained)

Notes

• This is a client-server subscription republished over MQTT, not OPC UA PubSub. Say so precisely in design documents: the two secure differently.
• Publish a retained connection-state topic. Without it a consumer cannot distinguish a machine that is idle from a bridge that died.
• Derive topics from the browse path rather than a hand-written map. Hand-written maps break at machine two.
• Carry status and SourceTimestamp. A bare value cannot be audited later.
• For real OPC UA PubSub, use a Part 14 implementation such as the .NET reference stack or open62541.

What breaks first

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.