MQTT Events debugging

[TePe]

Hi,

I have a MQTT Broker (HiveMQ) which is receiving data from Home Assistant (Mosquitto).

I see data flowing into the MQTT source, at least in the "Data Insights" the chart shows events arriving. Runtime Logs are empty so I assume that there are no errors.

In the Message Stream I see in the "Data Insights" tab again a chart, so there should be data arriving. However, the Data Preview tab is empty, I can't see any events.

I added an Eventhouse destination. The Data Preview is again empty. But there I get Data Deserialization Errors: 

{
    "ErrorCode": {
        "value": "Diagnostic: Diagnostic Error",
        "type": "string"
    },
    "TimeGenerated": {
        "value": "2026-01-02 16:18:40Z",
        "type": "datetime"
    },
    "Level": {
        "value": "Warning",
        "type": "string"
    },
    "Category": {
        "value": "Warning",
        "type": "string"
    },
    "ResourceId": {
        "value": "",
        "type": "string"
    },
    "SubscriptionId": {
        "value": "",
        "type": "string"
    },
    "ResourceGroup": {
        "value": "",
        "type": "string"
    },
    "JobName": {
        "value": "",
        "type": "string"
    },
    "OperationName": {
        "value": "",
        "type": "string"
    },
    "Status": {
        "value": "Warning",
        "type": "string"
    },
    "Source": {
        "value": "",
        "type": "string"
    },
    "TypeProperty": {
        "value": "Warning",
        "type": "string"
    },
    "DataErrorType": {
        "value": "",
        "type": "string"
    },
    "ErrorCategory": {
        "value": "Warning",
        "type": "string"
    },
    "BriefMessage": {
        "value": "",
        "type": "string"
    },
    "Message": {
        "value": "First Occurred: 1/2/2026 4:18:38 PM UTC | Resource Name: es-embeddedeventhub | Message: Source 'EventHubInputAdapter' had 14 occurrences of kind 'InputDeserializerError.InvalidData' between processing times '2026-01-02T16:18:36.6674295Z' and '2026-01-02T16:18:38.3290880Z'. Json input stream should either be an array of objects or line separated objects. Found token type: Float\r\n",
        "type": "string"
    },
    "FromTimestamp": {
        "value": null,
        "type": "datetime"
    },
    "ToTimestamp": {
        "value": null,
        "type": "datetime"
    },
    "EventCount": {
        "value": 0,
        "type": "long"
    },
    "GroupSize": {
        "value": 1,
        "type": "long"
    }
}

Here's my question: How should I debug these errors when I have no chance to see the data flowing in? From what I can see in "external" tools everything is looking great....

THANKS


Thomas

[TePe]

So it turned out to be what @lbendlin mentioned, the data was not JSON as I expected. I couldn't believe but it was true. And Kudos to @svelde which pointed me in the right direction to analyze that. 
Perhaps I'll go and add an idea, still the "debugging" of the Eventstreams is a pain. If there would be a way how to really look at the source events as they flow in (before parsing) the issue would have been solved in minutes. Now I had to connect with 3rd party solutions looking into the stream to fix it. Doable, but a lot of work. I found out some other glitches which make Eventstreams really hard to handle, will add ideas around that as well. But we can close this for now.

[v-aatheeque]

Hi @TePe 

Have you had a chance to look through the responses shared earlier? If anything is still unclear, we’ll be happy to provide additional support.

[TePeBox]

Fully agree... But how to see that JSON befor any transformation? I think this is a basic requirement to debug stuff... 

[svelde · Accepted solution]

Hello @TePeBox ,

Welcome to this Microsoft Fabric community forum.

Your messages are passed on by the HiveMQ MQTT broker to Eventstream (as a subscriber).

Can you get a client like MQTT Explorer or MQTTX connected to the same topics on the HiveMQ MQTT Broker?

If so, you should be able to peek at the messages being generated and it should look like JSON.

If this is also the case, copy one or more of these messages and test them online (it the messages do not contain sensitive data) like https://jsonlint.com/ 

This will check the JSON format of your messages. 

Next to that, do you know Azure is offering a great MQTT broker, part of Azure Event Grid Namespace? Notice that security is based on certificates. Check out this blog post for details.

--

If this answer helps you, a thumbs-up or marking it as accepted answer is appreaciated.

[lbendlin]

InputDeserializerError.InvalidData

Looks like your data is not coming in in the expected JSON format?