Subscribe to data changes in Node.js

The code

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

(async () => {
  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,   // what the consumer needs
    requestedLifetimeCount: 100,
    requestedMaxKeepAliveCount: 10,
    maxNotificationsPerPublish: 100,
    publishingEnabled: true,
    priority: 10,
  });

  subscription.on("keepalive", () => console.log("keepalive"));
  subscription.on("terminated", () => console.log("subscription terminated"));

  const item = await subscription.monitor(
    { nodeId: "i=2258", attributeId: AttributeIds.Value },
    {
      samplingInterval: 250,
      queueSize: 10,
      discardOldest: true,
      filter: new DataChangeFilter({
        trigger: DataChangeTrigger.StatusValue,
        deadbandType: DeadbandType.Absolute,
        deadbandValue: 0.5,
      }),
    },
    TimestampsToReturn.Both
  );

  item.on("changed", (dv) => {
    console.log(dv.value.value, dv.statusCode.toString(), dv.sourceTimestamp);
  });

  setTimeout(async () => {
    await subscription.terminate();
    await session.close();
    await client.disconnect();
  }, 5000);
})();

What it prints

2026-09-08T03:45:00.102Z Good (0x00000000) 2026-09-08T03:45:00.102Z
2026-09-08T03:45:01.104Z Good (0x00000000) 2026-09-08T03:45:01.104Z
keepalive

Notes

• requestedPublishingInterval is the delivery rate; samplingInterval is how often the server looks. Set the first from what the consumer needs, not from the fastest the server can manage.
• The server may revise your parameters. Read subscription.publishingInterval after creation to see what you actually got.
• Percent deadband requires EURange on the node. Absolute is the safe default.
• Listen for `terminated`. A subscription deleted server-side is a silent data stop otherwise.
• Keep the number of distinct publishing intervals small: each one is another subscription on the server.

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.