How to debug user-defined functions in Azure Digital Twins


A new version of the Azure Digital Twins service has been released. In light of the new service's expanded capabilities, the original Azure Digital Twins service (described in this documentation set) has been retired.

To view the documentation for the new service, visit the active Azure Digital Twins Documentation.

This article summarizes how to diagnose and debug user-defined functions in Azure Digital Twins. Then, it identifies some of the most common scenarios found when debugging them.


Read How to configure monitoring and logging to learn more about setting up debugging tools in Azure Digital Twins using Activity Logs, Diagnostic Logs, and Azure Monitor.

Debug issues

Knowing how to diagnose issues within Azure Digital Twins allows you to effectively analyze issues, identify the causes of problems, and provide appropriate solutions for them.

A variety of logging, analytics, and diagnostic tools are provided to that end.

Enable logging for your instance

Azure Digital Twins supports robust logging, monitoring, and analytics. Solutions developers can use Azure Monitor logs, diagnostic logs, activity logs, and other services to support the complex monitoring needs of an IoT app. Logging options can be combined to query or display records across several services and to provide granular logging coverage for many services.

Once configured, you'll be able to select all log categories, metrics, and use powerful Azure Monitor log analytics workspaces to support your debugging efforts.

Trace sensor telemetry

To trace sensor telemetry, verify that diagnostic settings are enabled for your Azure Digital Twins instance. Then, ensure that all desired log categories are selected. Lastly, confirm that the desired logs are being sent to Azure Monitor logs.

To match a sensor telemetry message to its respective logs, you can specify a Correlation ID on the event data being sent. To do so, set the x-ms-client-request-id property to a GUID.

After sending telemetry, open Azure Monitor log analytics to query for logs using the set Correlation ID:

| where CorrelationId == 'YOUR_CORRELATION_IDENTIFIER'
Query value Replace with
YOUR_CORRELATION_IDENTIFIER The Correlation ID that was specified on the event data

To read all recent telemetry logs query:

| order by CorrelationId desc

If you enable logging for your user-defined function, those logs appear in your log analytics instance with the category UserDefinedFunction. To retrieve them, enter the following query condition in log analytics:

| where Category == 'UserDefinedFunction'

For more information about powerful query operations, read Getting started with queries.

Identify common issues

Both diagnosing and identifying common issues are important when troubleshooting your solution. Several issues that are commonly encountered when developing user-defined functions are summarized in the following subsections.

In the examples below, YOUR_MANAGEMENT_API_URL refers to the URI of the Digital Twins APIs:
Name Replace with
YOUR_INSTANCE_NAME The name of your Azure Digital Twins instance
YOUR_LOCATION The region your instance is hosted on

Check if a role assignment was created

Without a role assignment created within the Management API, the user-defined function doesn't have access to perform any actions such as sending notifications, retrieving metadata, and setting computed values within the topology.

Check if a role assignment exists for your user-defined function through your Management API:

GET YOUR_MANAGEMENT_API_URL/roleassignments?path=/&traverse=Down&objectId=YOUR_USER_DEFINED_FUNCTION_ID
Parameter value Replace with
YOUR_USER_DEFINED_FUNCTION_ID The ID of the user-defined function to retrieve role assignments for

Learn How to create a role assignment for your user-defined function, if no role assignments exist.

Check if the matcher works for a sensor's telemetry

With the following call against your Azure Digital Twins instances' Management API, you're able to determine if a given matcher applies for the given sensor.

Parameter Replace with
YOUR_MATCHER_IDENTIFIER The ID of the matcher you wish to evaluate
YOUR_SENSOR_IDENTIFIER The ID of the sensor you wish to evaluate


    "success": true,
    "logs": [
        "$.dataType: \"Motion\" Equals \"Motion\" => True"

Check what a sensor triggers

With the following call against the Azure Digital Twins Management APIs, you're able to determine the identifiers of your user-defined functions triggered by the given sensor's incoming telemetry:

GET YOUR_MANAGEMENT_API_URL/sensors/YOUR_SENSOR_IDENTIFIER/matchers?includes=UserDefinedFunctions
Parameter Replace with
YOUR_SENSOR_IDENTIFIER The ID of the sensor to send telemetry


        "id": "48a64768-797e-4832-86dd-de625f5f3fd9",
        "name": "MotionMatcher",
        "spaceId": "2117b3e1-b6ce-42c1-9b97-0158bef59eb7",
        "conditions": [
                "id": "024a067a-414f-415b-8424-7df61392541e",
                "target": "Sensor",
                "path": "$.dataType",
                "value": "\"Motion\"",
                "comparison": "Equals"
        "userDefinedFunctions": [
                "id": "373d03c5-d567-4e24-a7dc-f993460120fc",
                "spaceId": "2117b3e1-b6ce-42c1-9b97-0158bef59eb7",
                "name": "Motion User-Defined Function",
                "disabled": false

Issue with receiving notifications

When you're not receiving notifications from the triggered user-defined function, confirm that your topology object type parameter matches the type of identifier that's being used.

Incorrect Example:

var customNotification = {
    Message: 'Custom notification that will not work'

sendNotification(telemetry.SensorId, "Space", JSON.stringify(customNotification));

This scenario arises because the used identifier refers to a sensor while the topology object type specified is Space.

Correct Example:

var customNotification = {
    Message: 'Custom notification that will work'

sendNotification(telemetry.SensorId, "Sensor", JSON.stringify(customNotification));

The easiest way to not run into this issue is to use the Notify method on the metadata object.


function process(telemetry, executionContext) {
    var sensorMetadata = getSensorMetadata(telemetry.SensorId);

    var customNotification = {
        Message: 'Custom notification'

    // Short-hand for above methods where object type is known from metadata.

Common diagnostic exceptions

If you enable diagnostic settings, you might encounter these common exceptions:

  1. Throttling: if your user-defined function exceeds the execution rate limits outlined in the Service Limits article, it will be throttled. No further operations are successfully executed until the throttling limits expire.

  2. Data Not Found: if your user-defined function attempts to access metadata that does not exist, the operation fails.

  3. Not Authorized: if your user-defined function doesn't have a role assignment set or lacks enough permission to access certain metadata from the topology, the operation fails.

Next steps