Docker alias incorrectly resolved by application when container started - docker

Question

I am creating two docker-compose files (mainly because I don't want to have to keep restarting my infrastructure while developing my application.) that need to reside on the same docker network so they can use alias names to connect.
The files look similar to the following:
APP:
version: '3.5'
networks:
default:
name: kafka_network
driver: bridge
services:
client:
build:
context: .
dockerfile: ./Dockerfile
working_dir: /app/
command: ./client
environment:
BADDR: kafka:9092
CGROUP: test_group
TOPICS: my-topic
INFRASTRUCTURE:
version: '3.5'
networks:
default:
name: kafka_network
driver: bridge
services:
zookeeper:
image: confluentinc/cp-zookeeper:latest
ports:
- 2181:2181
environment:
ZOOKEEPER_CLIENT_PORT: 2181
ZOOKEEPER_TICK_TIME: 2000
kafka:
image: confluentinc/cp-kafka:latest
depends_on:
- zookeeper
ports:
- 9092:9092
environment:
KAFKA_BROKER_ID: 1
KAFKA_ZOOKEEPER_CONNECT: zookeeper:2181
KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://kafka:29092,PLAINTEXT_HOST://localhost:9092
KAFKA_LISTENER_SECURITY_PROTOCOL_MAP: PLAINTEXT:PLAINTEXT,PLAINTEXT_HOST:PLAINTEXT
KAFKA_INTER_BROKER_LISTENER_NAME: PLAINTEXT
KAFKA_OFFSETS_TOPIC_REPLICATION_FACTOR: 1
My issue is that the client doesn't resolve kafka:9092 correctly... it always resolves to 127.0.0.1:9092.
ERROR:
Broker: kafka:9092
Consumer_Group: my_group
Topics: [my-topic]
Created Consumer rdkafka#consumer-1
% Error: GroupCoordinator: Connect to ipv4#127.0.0.1:9092 failed: Connection refused (after 0ms in state CONNECT)
When run locally, it appears to run fine, so I am really confused as to what the issue might be. If anyone knows anything about this I would be very grateful!
LOCAL:
[procyclinsur#P-428 client]$ ./client
Broker: localhost:9092
Consumer_Group: my-group
Topics: [my-topic]
Created Consumer rdkafka#consumer-1
% AssignedPartitions: [my-topic[0]#unset]
% Message on my-topic[0]#0:
hello mate

Answer 1

That's problem related to your Kafka's config - not to docker at all.
Look on:
KAFKA_ADVERTISED_LISTENERS: PLAINTEXT://kafka:29092,PLAINTEXT_HOST://localhost:9092
It means that you setup 2 listeners for your Kafka which your clients will receive in Kafka's protocol when connecting.
So when you connect on port 9092 your client's will try to get Kafka at "localhost "and when you connect at port 29092 your clients will try to get Kafka at "kafka" DNS name.
It's working locally for you because your Kafka container is exposed on localhost:9092 via docker ports section.
Here is article which is well describing that topic: https://rmoff.net/2018/08/02/kafka-listeners-explained/