User Data Function: 'NotFound' Error (WorkloadException) after Publishing, but Success in Develop

[Izyaan_sc]

I am setting up a monitoring workflow in Microsoft Fabric using Fabric Activator. I have created an alert based on a KQL query that monitors row ingestion. kql query - 

table1

| where ingestion_time() > ago(10m)

| summarize RowsIngested = count()
I am calling a User Data Function (UDF) as the action to push this data to an external Mimir endpoint via Remote Write.

@udf.function()
def testFunction(metricName: str, metricValue: float, labels: dict) -> str:
    logging.info(f"Starting Prometheus Remote Write push for: {metricName}")

    try:
        writer = RemoteWriter(
            url=MIMIR_URL,
            auth={
                "username": MIMIR_USERNAME,
                "password": MIMIR_PASSWORD
            }
        )

        if not isinstance(labels, dict):
            logging.error("Input 'labels' was not provided as a dictionary/object.")
            return "Push failed: labels must be a JSON object"

        timestamp_ms = int(datetime.datetime.now().timestamp() * 1000)

        metric_definition = labels.copy()
        metric_definition["__name__"] = metricName

        series = [
            {
                "metric": metric_definition,
                "values": [float(metricValue)],
                "timestamps": [timestamp_ms]
            }
        ]

        writer.send(series)

        logging.info(f"Successfully pushed {metricName} to Mimir.")
        return f"Metric pushed successfully at {datetime.datetime.now()}"

    except Exception as e:
        logging.error(f"Remote Write failed: {str(e)}")
        return f"Push failed: {str(e)}"

1. Develop Mode (Success)

When testing the function in Develop Mode using the Test Panel with the following parameters:

• metricName: "fabric_rows_ingested_test"

• metricValue: 111

• metricLabels: {"workspace": "dev", "table": "table1"}

Output: "Metric pushed successfully at 2026-03-23 06:53:51" Logs: Successfully pushed fabric_rows_ingested_test to Mimir.

above exposed metric can be accessed in grafana.

2. Run/Published Mode (Failure)

After clicking Publish and ensuring all libraries (prometheus-remote-writer, requests) are listed as "Published" in Library Management, I switch to Run only mode & invoke the function, 

{
  "functionName": "testFunction",
  "invocationId": "00000000-0000-0000-0000-000000000000",
  "status": "Failed",
  "errors": [
    {
      "errorCode": "WorkloadException",
      "subErrorCode": "NotFound",
      "message": "User data function: 'testFunction' invocation failed."
    }
  ]
}

Why does the Fabric backend fail to resolve the endpoint in Published mode (returning NotFound) when the same function is fully functional in the Develop sandbox?

[Izyaan_sc]

Hi @v-saisrao-msft, 
As mentioned in my post, I have already published the function multiple times. The function works as expected in Develop mode, but the invocation issue only persists in Run mode after publishing.

[v-saisrao-msft]

Hi @Izyaan_sc ,

Fabric User Data Functions need to be published before invocation, and only published functions are accessible in Run mode or from other services. In Develop mode, you can test functions without publishing, but Run mode uses only the deployed version. Each function is called through its public REST endpoint, requiring a published function and correct input parameters. If the function isn’t published or the request parameters are incorrect, the invocation will fail. Make sure the function is published and the parameters are correctly structured.

Quickstart - Create a Fabric User data functions item - Microsoft Fabric | Microsoft Learn

Tutorial - Invoke user data functions from a Python application - Microsoft Fabric | Microsoft Learn

Thank you.