Django 'failed connection' between two containers - django

Question

I'm currently trying to make a connection between two of my docker containers (The requesting container running Gunicorn/Django and the api container running kroki).
I've had a look at other answers but seem to be coming up blank with a solution, so was hoping for a little poke in the right direction.
Docker-compose:
version: '3.8'
services:
app:
build:
context: ./my_app
dockerfile: Dockerfile.prod
command: gunicorn my_app.wsgi:application --bind 0.0.0.0:8000 --access-logfile -
volumes:
- static_volume:/home/app/web/staticfiles
expose:
- 8000
environment:
- DJANGO_ALLOWED_HOSTS=localhost 127.0.0.1 kroki
env_file:
- ./.env.prod
depends_on:
- db
db:
image: postgres:13.0-alpine
volumes:
- postgres_data:/var/lib/postgresql/data/
env_file:
- ./.env.prod.db
nginx:
build: ./nginx
volumes:
- static_volume:/home/app/web/staticfiles
ports:
- 1337:80
depends_on:
- app
kroki:
image: yuzutech/kroki
ports:
- 7331:8000
volumes:
postgres_data:
static_volume:
settings.py
ALLOWED_HOSTS = os.environ.get("DJANGO_ALLOWED_HOSTS").split(" ")
Requesting code in django
url = 'http://kroki:7331/bytefield/svg/' + base64_var
try:
response = requests.get(url)
return response.text
except ConnectionError as e:
print("Connection to bytefield module, unavailable")
return None
I'm able to access both containers via my browser successfully, however initiating the code for an internal call between the two throws out
requests.exceptions.ConnectionError: HTTPConnectionPool(host='kroki', port=7331): Max retries exceeded with url: /bytefield/svg/<API_URL_HERE> (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f286f5ecaf0>: Failed to establish a new connection: [Errno 111] Connection refused'))
I've had a go accessing the url via localhost:7331 & 127.0.0.1:7331 however neither seem to help at all

Answer 1

When you access to other containers within the same network you don't have to access to them using the exposed port in the host but using instead the actual port where the application within the container is listening.
I made a really simple example so you can understand what's your problem:
version: '3.8'
services:
app:
image: busybox
entrypoint: tail -f /dev/null
kroki:
image: yuzutech/kroki
ports:
- 7331:8000
From Host
❯ curl -s -o /dev/null -w "%{http_code}" localhost:7331
200
From App
/ # wget kroki:7331
Connecting to kroki:7331 (172.18.0.3:7331)
wget: can't connect to remote host (172.18.0.3): Connection refused
/ # wget kroki:8000
Connecting to kroki:8000 (172.18.0.3:8000)
saving to 'index.html'
index.html 100% |************************************************************************| 51087 0:00:00 ETA
'index.html' saved