Cannot connect to kafka docker container from logstash docker container

Question

I am trying to connect to a kafka docker container from a logstash docker container but I always get the following message:

 Connection to node 0 (localhost/127.0.0.1:9092) could not be established. Broker may not be available.

My docker-compose.yml file is

version: '3.2'

services:
  elasticsearch:
    build:
      context: elasticsearch/
      args:
        ELK_VERSION: $ELK_VERSION
    volumes:
      - type: bind
        source: ./elasticsearch/config/elasticsearch.yml
        target: /usr/share/elasticsearch/config/elasticsearch.yml
        read_only: true
      - type: volume
        source: elasticsearch
        target: /usr/share/elasticsearch/data
    ports:
      - "9200:9200"
      - "9300:9300"
    environment:
      ES_JAVA_OPTS: "-Xmx256m -Xms256m"
      ELASTIC_PASSWORD: changeme
    networks:
      - elk
    depends_on:
      - kafka

  logstash:
    build:
      context: logstash/
      args:
        ELK_VERSION: $ELK_VERSION
    volumes:
      - type: bind
        source: ./logstash/config/logstash.yml
        target: /usr/share/logstash/config/logstash.yml
        read_only: true
      - type: bind
        source: ./logstash/pipeline
        target: /usr/share/logstash/pipeline
        read_only: true
    ports:
      - "5000:5000"
      - "9600:9600"
    links:
      - kafka
    environment:
      LS_JAVA_OPTS: "-Xmx256m -Xms256m"
    networks:
      - elk
    depends_on:
      - elasticsearch

  kibana:
    build:
      context: kibana/
      args:
        ELK_VERSION: $ELK_VERSION
    volumes:
      - type: bind
        source: ./kibana/config/kibana.yml
        target: /usr/share/kibana/config/kibana.yml
        read_only: true
    ports:
      - "5601:5601"
    networks:
      - elk
    depends_on:
      - elasticsearch

  zookeeper:
    image: strimzi/kafka:0.11.3-kafka-2.1.0
    container_name: zookeeper
    command: [
      "sh", "-c",
      "bin/zookeeper-server-start.sh config/zookeeper.properties"
    ]
    ports:
      - "2181:2181"
    networks:
      - elk
    environment:
      LOG_DIR: /tmp/logs

  kafka:
    image: strimzi/kafka:0.11.3-kafka-2.1.0
    command: [
      "sh", "-c",
      "bin/kafka-server-start.sh config/server.properties --override listeners=$${KAFKA_LISTENERS} --override advertised.listeners=$${KAFKA_ADVERTISED_LISTENERS} --override zookeeper.connect=$${KAFKA_ZOOKEEPER_CONNECT}"
    ]
    depends_on:
      - zookeeper
    ports:
      - "9092:9092"
    networks:
      - elk
    environment:
      LOG_DIR: "/tmp/logs"
      KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://localhost:9092
      KAFKA_LISTENERS: PLAINTEXT://0.0.0.0:9092
      KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181

networks:
  elk:
    driver: bridge

volumes:
  elasticsearch:

and my logstash.conf file is

input {
    kafka{
        bootstrap_servers => "kafka:9092"
        topics => ["logs"]
    }
}

## Add your filters / logstash plugins configuration here

output {
    elasticsearch {
        hosts => "elasticsearch:9200"
        user => "elastic"
        password => "changeme"
    }
}

All my containers are running normally and I can send messages to Kafka topics outside of the containers.

Answer 1

You need to define your listener based on the hostname at which it can be resolved from the client. If the listener is localhost then the client (logstash) will try to resolve it as localhost from its own container, hence the error.

I've written about this in detail here but in essence you need this:

KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://localhost:9092, PLAINTEXT://kafka:29092
KAFKA_LISTENERS: PLAINTEXT://0.0.0.0:9092, PLAINTEXT://kafka:29092

Then any container on the Docker network uses kafka:29092 to reach it, so logstash config becomes

bootstrap_servers => "kafka:29092"

Any client on the host machine itself continues to use localhost:9092.

You can see this in action with Docker Compose here: https://github.com/confluentinc/demo-scene/blob/master/build-a-streaming-pipeline/docker-compose.yml#L40