Sasha Shpota :

I setup a single node Kafka Docker container on my local machine like it is described in the Confluent documentation (steps 2-3).

In addition, I also exposed Zookeeper's port 2181 and Kafka's port 9092 so that I'll be able to connect to them from a client running on local machine:

$ docker run -d \
    -p 2181:2181 \
    --net=confluent \
    --name=zookeeper \
    -e ZOOKEEPER_CLIENT_PORT=2181 \
    confluentinc/cp-zookeeper:4.1.0

$ docker run -d \
    --net=confluent \
    --name=kafka \
    -p 9092:9092 \
    -e KAFKA_ZOOKEEPER_CONNECT=zookeeper:2181 \
    -e KAFKA_ADVERTISED_LISTENERS=PLAINTEXT://kafka:9092 \
    -e KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR=1 \
    confluentinc/cp-kafka:4.1.0

Problem: When I try to connect to Kafka from the host machine, the connection fails because it can't resolve address: kafka:9092.

Here is my Java code:

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("client.id", "KafkaExampleProducer");
props.put("key.serializer", LongSerializer.class.getName());
props.put("value.serializer", StringSerializer.class.getName());
KafkaProducer<Long, String> producer = new KafkaProducer<>(props);
ProducerRecord<Long, String> record = new ProducerRecord<>("foo", 1L, "Test 1");
producer.send(record).get();
producer.flush();

The exception:

java.io.IOException: Can't resolve address: kafka:9092
    at org.apache.kafka.common.network.Selector.doConnect(Selector.java:235) ~[kafka-clients-2.0.0.jar:na]
    at org.apache.kafka.common.network.Selector.connect(Selector.java:214) ~[kafka-clients-2.0.0.jar:na]
    at org.apache.kafka.clients.NetworkClient.initiateConnect(NetworkClient.java:864) [kafka-clients-2.0.0.jar:na]
    at org.apache.kafka.clients.NetworkClient.ready(NetworkClient.java:265) [kafka-clients-2.0.0.jar:na]
    at org.apache.kafka.clients.producer.internals.Sender.sendProducerData(Sender.java:266) [kafka-clients-2.0.0.jar:na]
    at org.apache.kafka.clients.producer.internals.Sender.run(Sender.java:238) [kafka-clients-2.0.0.jar:na]
    at org.apache.kafka.clients.producer.internals.Sender.run(Sender.java:176) [kafka-clients-2.0.0.jar:na]
    at java.lang.Thread.run(Thread.java:748) [na:1.8.0_144]
Caused by: java.nio.channels.UnresolvedAddressException: null
    at sun.nio.ch.Net.checkAddress(Net.java:101) ~[na:1.8.0_144]
    at sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:622) ~[na:1.8.0_144]
    at org.apache.kafka.common.network.Selector.doConnect(Selector.java:233) ~[kafka-clients-2.0.0.jar:na]
    ... 7 common frames omitted

Question: How to connect to Kafka running in Docker? My code is running from host machine, not Docker.

Note: I know that I could theoretically play around with DNS setup and /etc/hosts but it is a workaround - it shouldn't be like that.

There is also similar question here, however it is based on ches/kafka image. I use confluentinc based image which is not the same.

cricket_007 :

Disclaimer

tl;dr - At the end of the day, it's all the same Apache Kafka running in a container. You're just dependent on how it is configured. And which variables make it so.

_{The following uses confluentinc docker images, not wurstmeister/kafka, although there is a similar configuration, I have not tried it. If using that image, read their Connectivity wiki.}

_{Nothing against the wurstmeister image, but it's community maintained, not built in an automated CI/CD release... Bitnami ones are similarly minimalistic and run in multiple cloud providers}. _{For bitnami Kafka images, refer their README}

_{debezium/kafka docs on it are mentioned here.} _{Note: advertised host and port settings are deprecated. Advertised listeners covers both}

_{spotify/kafka is deprecated and outdated. fast-data-dev is great for an all in one solution, but it is bloated}

For supplemental reading, a fully-functional docker-compose, and network diagrams, see this blog by @rmoff

Answer

That Confluent quickstart document assumes all produce and consume requests will be within the Docker network.

You could fix the problem by running your Kafka client code within its own container, but otherwise you'll need to add some more environment variables for exposing the container externally, while still having it work within the Docker network.

First add a protocol mapping of PLAINTEXT_HOST:PLAINTEXT that will map the listener protocol to a Kafka protocol

-e KAFKA_LISTENER_SECURITY_PROTOCOL_MAP=PLAINTEXT:PLAINTEXT,PLAINTEXT_HOST:PLAINTEXT

Then setup two advertised listeners on different ports. (kafka:9092 here refers to the docker container name). Notice the protocols match the right side values of the mappings above

-e KAFKA_ADVERTISED_LISTENERS=PLAINTEXT://kafka:9092,PLAINTEXT_HOST://localhost:29092

When running the container, add -p 29092:29092 for the host port mapping

tl;dr (with the above settings)

When running any Kafka Client outside the Docker network (including CLI tools you might have installed locally), use localhost:29092 for bootstrap servers and localhost:2181 for Zookeeper

When running an app in the Docker network, use kafka:9092 for bootstrap servers and zookeeper:2181 for Zookeeper

See the example Compose file for the full Confluent stack