`data` in EventTransform is always null

[sebastienblanc]

Describe the bug
I have : KafkaSource -> EventTransform -> Kafka Broker -> Trigger -> Sink (Knative Service)

My Event Transform is to extract a field from dataas Cloud Event attribute :

 apiVersion: eventing.knative.dev/v1alpha1
kind: EventTransform
metadata:
  name: simple-transform
spec:
  sink:
    ref:
      apiVersion: eventing.knative.dev/v1
      kind: Broker
      name: agent-broker
  jsonata:
    expression: |
      {
        "specversion": "1.0",
        "id": id,
        "time": time,
        "type": type,
        "source": source,
        "mycustomfield": "customvalue",
        "targetagent": data.target,
        "data": $
      }

KafaSource is reading this record from kafka :

{"target":"chatbot"}

But data is always null in the Event Transform, also "data": $ copies the entire cloud event (with an empty data) , here an record example from the internal kafka broker topic :

{"id":"partition:0/offset:55","time":"2025-12-19T07:19:05.531Z","type":"dev.knative.kafka.event","source":"/apis/v1/namespaces/scm-blanc-dev/kafkasources/kafka-source#agents","specversion":"1.0","subject":"partition:0#55","data":{},"knativekafkapartition":"0","knativekafkaoffset":"55"}

Expected behavior
datashould not be null and $ should only copy the payload.

[DCchoudhury15]

/assign

[DCchoudhury15]

Hi @sebastienblanc, thanks for the detailed report!

I've started looking into this. To help me reproduce the environment exactly, could you please share the YAML configuration for your KafkaSource?

I suspect this might be related to how the event data is being encoded. If the KafkaSource isn't explicitly configured with contentType: "application/json", the EventTransform might be receiving the payload as raw bytes/binary rather than a structured JSON object. If that happens, the JSONata expression data.target would likely fail (return null) because it cannot traverse the byte array.

Additionally, if you are able to inspect the CloudEvent headers coming out of the KafkaSource (before the transform), could you confirm what the datacontenttype attribute is currently set to?

[sebastienblanc]

Hi @DCchoudhury15 , thanks for looking into this !

Indeed I do not explicitly set the content type, here is my kafka source :

apiVersion: sources.knative.dev/v1
kind: KafkaSource
metadata:
  name: kafka-source
  labels:
    app: kafka-source
spec:
  bootstrapServers:
    - 'mycluster:9092'
  consumerGroup: agents
  consumers: 1
  initialOffset: latest
  net:
    tls:
      caCert:
        secretKeyRef:
          key: ca.pem
          name: kafka-tls
      cert:
        secretKeyRef:
          key: service.cert
          name: kafka-tls
      enable: true
      key:
        secretKeyRef:
          key: service.key
          name: kafka-tls
  ordering: ordered
  sink:
    ref:
      apiVersion: eventing.knative.dev/v1
      kind: Broker
      name: agent-broker
      namespace: scm-blanc-dev
  topics:
    - agents
    - chatbot
```

How can I set explicitly the content-type ? 

[DCchoudhury15]

Hey @sebastienblanc, thanks for posting the config.

I think I see what's happening here. Since contentType isn't explicitly set, the KafkaSource is likely defaulting to binary/raw bytes. That would explain why data is showing up as null in the transform—it's treating the payload as a byte array rather than parsing it as a JSON object.

Try adding contentType: "application/json" to your spec.

[sebastienblanc]

@DCchoudhury15 looks like spec.contentTypefor KafkaSource is not a known field, any idea how I can set it ?

[DCchoudhury15]

Apologies for the confusion, @sebastienblanc! i think i made a mistake —contentType isn't a top-level field in the KafkaSource spec.
To explicitly force the content type so the event is parsed as JSON, you should use ceOverrides to set the datacontenttype extension.
Could you try updating your KafkaSource with this configuration?

apiVersion: sources.knative.dev/v1
kind: KafkaSource
metadata:
  name: kafka-source
spec:
  # ... your existing bootstrapServers, consumerGroup, etc ...
  ceOverrides:
    extensions:
      datacontenttype: application/json

[sebastienblanc]

@DCchoudhury15 This works perfectly ! Thank you so much for looking into this. Maybe this could be added somewhere in the documentation ?

[DCchoudhury15]

Awesome ;

And you're totally right it is super easy to miss that KafkaSource defaults to binary. Adding the ceOverrides example to the docs would definitely save the next person some debugging time.

I'd be happy to take care of that. I can draft a PR for the documentation repo to add this example.

Do you want to keep this issue open until that PR is merged, or should we close this one out now since the bug itself is resolved?

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