Docker Kafka w/ Python consumer

Question

I am using dockerized Kafka and written one Kafka consumer program. It works perfectly when I run Kafka in docker and application at my local machine. But when I configured the local application in docker I am facing issues. The issue may be due to a topic not created until time application started.

docker-compose.yml

version: '3' services:   zookeeper:     image: wurstmeister/zookeeper     ports:       - "2181:2181"   kafka:     image: wurstmeister/kafka     ports:       - "9092:9092"     environment:       KAFKA_ADVERTISED_HOST_NAME: localhost       KAFKA_CREATE_TOPICS: "test:1:1"       KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181     volumes:       - /var/run/docker.sock:/var/run/docker.sock   parse-engine:     build: .     depends_on:       - "kafka"     command: python parse-engine.py     ports:      - "5000:5000" 

parse-engine.py

from kafka import KafkaConsumer import json  try:     print('Welcome to parse engine')     consumer = KafkaConsumer('test', bootstrap_servers='localhost:9092')     for message in consumer:         print(message) except Exception as e:     print(e)     # Logs the error appropriately.      pass 

Error log

kafka_1         | [2018-09-21 06:27:17,400] INFO [SocketServer brokerId=1001] Started processors for 1 acceptors (kafka.network.SocketServer) kafka_1         | [2018-09-21 06:27:17,404] INFO Kafka version : 2.0.0 (org.apache.kafka.common.utils.AppInfoParser) kafka_1         | [2018-09-21 06:27:17,404] INFO Kafka commitId : 3402a8361b734732 (org.apache.kafka.common.utils.AppInfoParser) kafka_1         | [2018-09-21 06:27:17,431] INFO [KafkaServer id=1001] started (kafka.server.KafkaServer) **parse-engine_1  | Welcome to parse engine parse-engine_1  | NoBrokersAvailable  parseengine_parse-engine_1 exited with code 0** kafka_1         | creating topics: test:1:1 

As I already added depends_on property in docker-compose but before starting topic application connecting so error occurred.

I read that I can possible to add the script in the docker-compose file but I am looking for some easy way.

Thanks for help

Answer 1

Your problem is the networking. In your Kafka config you're setting

KAFKA_ADVERTISED_HOST_NAME: localhost 

but this means that any client (including your python app) will connect to the broker, and then be told by the broker to use localhost for any connections. Since localhost from your client machine (e.g. your python container) is not where the broker is, requests will fail.

You can read more about fixing problems with your Kafka listeners in detail here

• Kafka Listeners - Explained
• My Python/Java/Spring/Go/Whatever Client Won’t Connect to My Apache Kafka Cluster in Docker/AWS/My Brother’s Laptop. Please Help!

So to fix your issue, you can do one of two things:

1. Simply change your compose to use the internal hostname for Kafka (KAFKA_ADVERTISED_HOST_NAME: kafka). This means any clients within the docker network will be able to access it fine, but no external clients will be able to (e.g. from your host machine):

    version: '3'  services:  zookeeper:      image: wurstmeister/zookeeper      ports:      - "2181:2181"  kafka:      image: wurstmeister/kafka      ports:      - "9092:9092"      environment:      KAFKA_ADVERTISED_HOST_NAME: kafka      KAFKA_CREATE_TOPICS: "test:1:1"      KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181      volumes:      - /var/run/docker.sock:/var/run/docker.sock  parse-engine:      build: .      depends_on:      - "kafka"      command: python parse-engine.py      ports:      - "5000:5000" 

   Your clients would then access the broker at kafka:9092, so your python app would change to

    consumer = KafkaConsumer('test', bootstrap_servers='kafka:9092') 

2. Add a new listener to Kafka. This enables it to be accessed both internally and externally to the docker network. Port 29092 would be for access external to the docker network (e.g. from your host), and 9092 for internal access.

   You would still need to change your python program to access Kafka at the correct address. In this case since it's internal to the Docker network, you'd use:

    consumer = KafkaConsumer('test', bootstrap_servers='kafka:9092') 

   Since I'm not familiar with the wurstmeister images, this docker-compose is based on the Confluent images which I do know:

   (editor has mangled my yaml, you can find it here)

    ---  version: '2'  services:    zookeeper:      image: confluentinc/cp-zookeeper:latest      environment:        ZOOKEEPER_CLIENT_PORT: 2181        ZOOKEEPER_TICK_TIME: 2000     kafka:      # "`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-      # An important note about accessing Kafka from clients on other machines:       # -----------------------------------------------------------------------      #      # The config used here exposes port 29092 for _external_ connections to the broker      # i.e. those from _outside_ the docker network. This could be from the host machine      # running docker, or maybe further afield if you've got a more complicated setup.       # If the latter is true, you will need to change the value 'localhost' in       # KAFKA_ADVERTISED_LISTENERS to one that is resolvable to the docker host from those       # remote clients      #      # For connections _internal_ to the docker network, such as from other services      # and components, use kafka:9092.      #      # See https://rmoff.net/2018/08/02/kafka-listeners-explained/ for details      # "`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-'"`-._,-      #      image: confluentinc/cp-kafka:latest      depends_on:        - zookeeper      ports:        - 29092:29092      environment:        KAFKA_BROKER_ID: 1        KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181        KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://kafka:9092,PLAINTEXT_HOST://localhost:29092        KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: PLAINTEXT:PLAINTEXT,PLAINTEXT_HOST:PLAINTEXT        KAFKA_INTER_BROKER_LISTENER_NAME: PLAINTEXT        KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1 

---

Disclaimer: I work for Confluent