Resolving No Events Found When Sampling Data with Azure Stream Analytics

Data is making it to the Event Hub…at least you’re 99% sure it is, because successful requests are being made by your app/device and received by the Hub.

The trouble is, when you attempt to sample data with Azure Stream Analytics, you come up empty with a message like

No events found for HubName.

or

Diagnostics: While sampling data, no data was received from # partitions.

In my particular case, everything was fine with both the Event Hub and the Stream Analytics Job configurations. My issue was how my client app was encoding things as JSON.

I was using Node to simulate a device sending data to the Event Hub.

I had a standard import of the required library objects, and the creation of an EventHubClient as follows:

const { EventHubClient, EventPosition } = require('@azure/event-hubs');
const client = EventHubClient.createFromConnectionString(process.env["EVENTHUB_CONNECTION_STRING"], process.env["EVENTHUB_NAME"]);

But my main function contained a flaw. It was sending valid JSON, but not in the “shape” that Azure Stream Analytics expected it. It resulted in a JSON parsing error.

What was the flaw? Compare the following snippets inline:

function main() {
    const incorrectDataFormat = { DeviceID: "SomeDeviceID", LocationID: "SomeLocationID", Measurement: "SomeMeasurement" };
    const CORRECTDataFormat = { body: { DeviceID: "SomeDeviceID", LocationID: "SomeLocationID", Measurement: "SomeMeasurement" } };
    
    client.send(incorrectDataFormat); // won't get picked up by Stream Analytics
    client.send(CORRECTDataFormat); // WILL get picked up by Stream Analytics
}

What’s the difference?

For receiving events from Azure Stream Analytics, you’ve got to send Events to an EventHub where the body is a JSON object/array…which means you’ve got to have a body property with a value…an object, or an array… that contains the data you want to send.

const CORRECTDataFormat = { body: { DeviceID: "SomeDeviceID", LocationID: "SomeLocationID", Measurement: "SomeMeasurement" } };

If you’re running into issues with sampling Event Hub data using Stream Analytics, it might be worth the effort to double-check that the structure of your JSON object is in line with the expectation described above. 🙌🏻