[Bug]: Kafka Connect Image built using strimzi image 0.45.0 not compatible when running with operator 0.46.1

[rao2100]

Bug Description

I have a image build on top of strimzi like below to package my plugins, when i deploy it with strimzi operator 0.46.1 I get an error and it goes into crash loopback. It works fine if I build it using 0.46.1 image.

Is this by design or is it a bug? Meaning Kafka-Connect images are not compatible from version to version?

error:
2025-07-17 07:57:58,349 ERROR Stopping due to error (org.apache.kafka.connect.cli.AbstractConnectCli) [main] org.apache.kafka.common.config.ConfigException: Missing required configuration "key.converter" which has no default value. at org.apache.kafka.common.config.ConfigDef.parseValue(ConfigDef.java:534) at org.apache.kafka.common.config.ConfigDef.parse(ConfigDef.java:524) at org.apache.kafka.common.config.AbstractConfig.<init>(AbstractConfig.java:114) at org.apache.kafka.connect.runtime.WorkerConfig.<init>(WorkerConfig.java:441) at org.apache.kafka.connect.runtime.distributed.DistributedConfig.<init>(DistributedConfig.java:591) at org.apache.kafka.connect.runtime.distributed.DistributedConfig.<init>(DistributedConfig.java:585) at org.apache.kafka.connect.cli.ConnectDistributed.createConfig(ConnectDistributed.java:108) at org.apache.kafka.connect.cli.ConnectDistributed.createConfig(ConnectDistributed.java:55) at org.apache.kafka.connect.cli.AbstractConnectCli.startConnect(AbstractConnectCli.java:123) at org.apache.kafka.connect.cli.AbstractConnectCli.run(AbstractConnectCli.java:94) at org.apache.kafka.connect.cli.ConnectDistributed.main(ConnectDistributed.java:113)

checking the properties file:
`
$ k exec -it my-connect-cluster-connect-0 -- cat /tmp/strimzi-connect.properties

bootstrap.servers=

rest.advertised.host.name=my-connect-cluster-connect-0.my-connect-cluster-connect.cgf.svc
rest.advertised.port=8083

plugin.path=/opt/kafka/plugins

security.protocol=PLAINTEXT
producer.security.protocol=PLAINTEXT
consumer.security.protocol=PLAINTEXT
admin.security.protocol=PLAINTEXT

consumer.client.rack=
`

Image built using:
FROM dependencies.openet.com:5000/strimzi/kafka:0.45.0-kafka-3.8.1

Steps to reproduce

1. Deploy Strimzi, Kafka version 0.46.1
2. Deploy Kafka Connect using image built with 0.45.0
3. Observe crash loopback of kafka-connect

Expected behavior

Older kafka connect image should work with newer strimzi.

Strimzi version

0.46.1

Kubernetes version

1.32

Installation method

yaml

Infrastructure

Amazon AWS

Configuration files and logs

apiVersion: kafka.strimzi.io/v1beta2
kind: KafkaConnect
metadata:
name: my-connect-cluster
spec:
version: 3.9.0
replicas: 1
image: artifactory.XXX.com:5000/XXX/my-connect-package:2.20.3
bootstrapServers: odf-cluster-kafka-bootstrap:9092
config:
group.id: connect-cluster
offset.storage.topic: connect-cluster-offsets
config.storage.topic: connect-cluster-configs
status.storage.topic: connect-cluster-status
# -1 means it will use the default replication factor configured in the broker
config.storage.replication.factor: -1
offset.storage.replication.factor: -1
status.storage.replication.factor: -1
key.converter: org.apache.kafka.connect.json.JsonConverter
value.converter: org.apache.kafka.connect.json.JsonConverter

Additional context

No response

[rao2100]

If I set the Version as 3.8.0 I get the following error, as it is not compatible.

2025-07-17 08:17:09 WARN AbstractOperator:566 - Reconciliation #3192(watch) KafkaConnect(cgf/my-connect-cluster): Failed to reconcile io.strimzi.operator.cluster.model.KafkaVersion$UnsupportedKafkaVersionException: Unsupported Kafka.spec.kafka.version: 3.8.0. Supported versions are: [3.9.0, 4.0.0]

[im-konge]

It's completely normal that if you build Kafka Connect image with one set of images from version 0.45.0, then it's not compatible with different version.
There can be changes in the code that prevents it actually. Also, as you already mentioned, it has different set of supported Kafka versions.

[im-konge]

IIRC there is a difference in how the configuration is loaded for KafkaConnect between Strimzi versions 0.45.0 and 0.46.0.

I think that if you use the Connect Build feature, it should build the new image after the upgrade of operator (and using the supported Kafka version).

[rao2100]

Thanks for the reply. I do know there was some api change between 3.8.0 and 3.9.0 kafka connect api.

#11057

[im-konge]

It's not about just Kafka version, but also changes in the operator between the versions, as I mentioned in the previous replies.

Do you use the Connect Build feature or are you building it somewhere else?