Error with linking containers in docker-compose - docker

Question

I am trying to build an image and a container of a application with docker compose. However when i try to link the containers it keeps giving the error "Service api has a link to service 'containername' which is undefined". Does someone know what i am doing wrong?
This is my code:
version: "3.9"
services:
db:
image: mysql:5.6
ports:
- 23306:3306
volumes:
- .:/var/lib/mysql
environment:
MYSQL_ROOT_PASSWORD: quint
MYSQL_DATABASE: cddb_quin
MYSQL_USER: cddb_quint
MYSQL_PASSWORD: quint
container_name: cddb_mysql
api:
build: ./backend
ports:
- 28080:8080
image: javaimage:latest
links:
- cddb_mysql:mysql
container_name: cddb_backend
web:
build: ./frontend
ports:
- 20080:80
image: angularimage:latest
links:
- cddb_backend
container_name: cddb_frontend

Answer 1

Compose links: are an obsolete feature. In even vaguely modern Compose – any Compose file with a top-level services: block – Compose will automatically create a Docker network for you and attach all of the services to it. You can almost always safely delete links: without losing any functionality.
The one thing that it's possible to do with links: but not Docker networking is to make another container visible under a different name. The links: [cddb_mysql:mysql] syntax you have does this. There's not particularly any benefit to doing this, though.
So the easiest way to fix the error you're getting is to just delete all of the links: blocks. You don't show how you're configuring your database connection, but you need to configure it to point to the Compose service name of the database db. Networking in Compose in the Docker documentation describes this setup further.
(You can also delete all of the container_name: settings, and on the images you build:, you can delete their manual image: names if you're not planning to push them to a registry.)
version: '3.8'
services:
api:
build: ./backend
ports:
- 28080:8080
environment:
- MYSQL_HOST=db
db: { ... }
backend: { ... }