Unexpected Offset Increment of 2 Per Message in Single-Partition Kafka Topic

[vcanuel]

Description:

Hi,

I’ve encountered an issue where, after writing messages to a Kafka topic with a single partition, the offsets increment by 2 instead of the expected increment of 1. For example, the sequence of offsets is 0, 2, 4, etc.
Is this normal ?

Basic information

• Axon Framework version: 4.10.2

• JDK version: 21

• Kafka Extension version: 4.10.0

• My application properties :

axon.kafka.publisher.confirmation-mode=transactional
axon.kafka.bootstrap-servers=localhost:9092
axon.kafka.message-converter-mode=default
axon.kafka.client-id=anonymous
axon.kafka.default-topic=axon-events
axon.kafka.publisher.confirmation-mode=wait_for_ack
axon.kafka.producer.transaction-id-prefix=axontx
axon.kafka.producer.retries=1
axon.kafka.producer.event-processor-mode=subscribing
axon.kafka.producer.acks=all

Steps to Reproduce:

1. Produce messages to a Kafka topic configured with a single partition.
2. Observe the offsets assigned to each message.

Expected Behavior:

Offsets should increment by 1 for each new message (e.g., 0, 1, 2, 3, …).

Actual Behavior:

Offsets increment by 2 for each new message (e.g., 0, 2, 4, …).

[smcvb]

Thanks for filing this concern with us, @vcanuel.
Although this might be asking a lot, I am going to regardless: would you mind trying to reproduce this offset-issue with the example present in this repository?
The example resides in the "kafka-axon-example" folder, by the way.
Sharing a reproducible will help us quite a bit to ensure nothing gets lost in translation.

[vcanuel]

Hi @smcvb,

I have run the Axon example without any issues.

I have pinpointed the problem: it occurs when the confirmation mode is set to transactional:

  kafka:
    publisher:
      confirmation-mode: transactional
    clientid: kafka-axon-example
    producer:
      retries: 1
      bootstrap-servers: localhost:29092
      transaction-id-prefix: axontx

Is this normal ?