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I am running a testing kafka "cluster" consisting of a single Kafka broker and a single Zookeeper node from docker-compose, using the following (official) docker-compose.yml:

--- version: '2' services: zookeeper: image: confluentinc/cp-zookeeper:6.0.0 hostname: zookeeper container_name: zookeeper ports: - "2181:2181" environment: ZOOKEEPER_CLIENT_PORT: 2181 ZOOKEEPER_TICK_TIME: 2000 broker: image: confluentinc/cp-kafka:6.0.0 hostname: broker container_name: broker depends_on: - zookeeper ports: - "29092:29092" environment: KAFKA_BROKER_ID: 101 KAFKA_ZOOKEEPER_CONNECT: 'zookeeper:2181' KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: PLAINTEXT:PLAINTEXT,PLAINTEXT_HOST:PLAINTEXT KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://broker:9092,PLAINTEXT_HOST://localhost:29092 KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1 KAFKA_TRANSACTION_STATE_LOG_MIN_ISR: 1 KAFKA_TRANSACTION_STATE_LOG_REPLICATION_FACTOR: 1 KAFKA_GROUP_INITIAL_REBALANCE_DELAY_MS: 0

After starting this "cluster" with docker-compose up I successfully create a topic using console command:

docker exec broker kafka-topics --create --topic myTopic --bootstrap-server localhost:29092 --replication-factor 1 --partitions 1

and programmatically create messages with a Golang producer, while listening to them with a console client:

docker exec broker kafka-console-consumer --from-beginning --bootstrap-server localhost:29092 --topic myTopic --timeout-ms 15000

Every once in a while both the console consumer and the Golang programmatic producer hang and stop connecting to kafka. When I look into the logs of Kafka broker to figure out, what's wrong, I see a wall of repeating messages of the following structure about topics being not in preferred replica:

broker | [2021-05-09 06:08:50,144] INFO [Controller id=101] Processing automatic preferred replica leader election (kafka.controller.KafkaController) broker | [2021-05-09 06:08:50,149] TRACE [Controller id=101] Checking need to trigger auto leader balancing (kafka.controller.KafkaController) broker | [2021-05-09 06:08:50,165] DEBUG [Controller id=101] Topics not in preferred replica for broker 101 HashMap() (kafka.controller.KafkaController) broker | [2021-05-09 06:08:50,166] TRACE [Controller id=101] Leader imbalance ratio for broker 101 is 0.0 (kafka.controller.KafkaController)

Could you explain, what's going wrong? How could a wrong replica be assigned, if there's just 1 partition and 1 broker?

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  • Since you're exec-ing into the broker, you'd use kafka:9092 as the connection string, but why do you have TRACE logs enabled? Commented May 9, 2021 at 12:58
  • @OneCricketeer Hi, thanks for comments and response. Didn't modify the log level in server.properties. Apparently, that's the default. 9092 and 29092 are both allowed, I believe. Commented May 9, 2021 at 13:48
  • You should only use localhost:29092 outside of the Docker network, specially only with code connecting from the Docker host. That's why it's named PLAINTEXT_HOST... More specifically, if your Go code was in its own container, using that address would try to connect to itself, and not the broker Commented May 9, 2021 at 13:54
  • @OneCricketeer Ok, thanks for pointing this out. Commented May 9, 2021 at 14:27
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    Hi @Neil. My final solution is here: github.com/BurkovBA/go-kafka-adapter/tree/master. I can't exactly recall TBH, if I fixed it or circumvented it (it's been a while ago). See instructions in README.md. Commented Aug 5, 2024 at 11:00

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I don't think this small section of logs is your issue

Those logs say there's no topics not in preferred replica set... It's printing an empty Hashmap

Similarly, the imbalance ratio is zero, so the cluster is perfectly balanced

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