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The short version

Kafka stores messages in named lists called topics. A producer writes messages to a topic and a consumer reads them. Kafka ships with a console producer and a console consumer, so you need no code yet.

Details

A broker is a running Kafka server. Start one in Docker:

docker run -d --name kafka -p 9092:9092 apache/kafka:4.1.0

Kafka keeps every message after it is read. Reading does not remove anything, so many consumers can read the same topic.

Try it yourself

Create a topic called greetings:

docker exec kafka /opt/kafka/bin/kafka-topics.sh \
  --bootstrap-server localhost:9092 --create --topic greetings

Open a producer, type two lines and press Enter after each. Press Ctrl+C when you are done.

docker exec -it kafka /opt/kafka/bin/kafka-console-producer.sh \
  --bootstrap-server localhost:9092 --topic greetings
>hello
>hello again

Read the topic from the start:

docker exec -it kafka /opt/kafka/bin/kafka-console-consumer.sh \
  --bootstrap-server localhost:9092 --topic greetings --from-beginning

You should see:

hello
hello again

Explain it to the duck

Why does reading a message from Kafka not delete it?

Tested with: Kafka 4.1.0 in Docker.