Multiple variables in one subscription - node-opcua

Question

I've read that it is better practice to put multiple variables into one subscription compared to creating more subscriptions.
I'm using this example (https://github.com/node-opcua/node-opcua-htmlpanel) and want to read more variables in subscription.
My code responsible for reading one variable - one node.
const nodeIdToMonitor = 'ns=3;s="OP_UA_FB_DB"."OPC_Data"."Data_integer"';
const itemToMonitor = {
nodeId: nodeIdToMonitor,
attributeId: AttributeIds.Value,
};
const parameters = {
samplingInterval: 100,
discardOldest: true,
queueSize: 100,
};
const monitoredItem = await subscription.monitor(
itemToMonitor,
parameters,
TimestampsToReturn.Both
);
monitoredItem.on("changed", (dataValue) => {
io.sockets.emit("message", {
value: dataValue.value.value,
timestamp: dataValue.serverTimestamp,
nodeId: nodeIdToMonitor,
browseName: "ISP",
});
});

Answer 1

To monitor multiple variables in one subscription you can make use of the monitorItems method call. This method call takes an array of nodes to be monitored as input and create monitored items for all the nodes present in the array.
Here is a sample code snippet that you can make use of to monitor multiple nodes,
const parameters1: MonitoringParametersOptions = {
discardOldest: true,
queueSize: 100,
samplingInterval: 100,
filter: new DataChangeFilter({
deadbandType: DeadbandType.Absolute,
deadbandValue: 0.1,
trigger: DataChangeTrigger.StatusValueTimestamp,
}),
};
const itemsToMonitor2: ReadValueIdOptions[] = [
{
attributeId: AttributeIds.Value,
nodeId: nodeIdToMonitor
},
{
attributeId: AttributeIds.Value,
nodeId: nodeIdToMonitor1
}];
const item2 = (await subscription.monitorItems(
itemsToMonitor2,
parameters1,
TimestampsToReturn.Both))
// If a data change is detected print the value
item2.on("changed", (monitoredItem: ClientMonitoredItemBase, dataValue: DataValue, index: number) => {
console.log("Index", index);
console.log(" The value has changed : ", dataValue.toString());
});
I see that you are using open source OPC UA implementations. Here are a list open source implementations that you might be interested in trying out:
open62541 – https://open62541.org/certified-sdk.html – C stack | Mozilla License | Embedded ready | TSN ready
NodeOPCUA – https://node-opcua.github.io/ – NodeJS | MIT License | Cloud ready
FreeOpcUa – https://github.com/FreeOpcUa/freeopcua – C++ | LGPL-3.0 License | Python bindings
UA .NET stack – https://github.com/OPCFoundation/UA-.NETStandard – RCL License for corporate members of OPC Foundation & GPL 2.0 for others | Standard profile | Web oriented implementation
If you are looking for more hands-on information, you can also check out these resources:
Free documentation: You can look at the open source documentation page: https://open62541.org/doc/current/
Paid online course: Practical introduction to OPC UA – code walk-through and examples in this course use the open62541 stack: https://opcfoundation.org/products/view/practical-introduction-to-opc-ua-part-i