postgres container throws "forward host lookup failed: Unknown host" - postgresql

Question

I'm working with docker image that uses Python3.6 as its base. All of the sudden it started crashing (exiting right after start up). So I bashed into the container and found out that
it crashes because connection to containerized postgres database fails all of the sudden.
The only error output I managed to get is forward host lookup failed: Unknown host which isn't telling me much.
entrypoint.sh:
echo "Waiting for postgres..."
while ! nc -z users-db 5432; do
sleep 0.1
done
echo "PostgreSQL started"
python manage.py run -h 0.0.0.0
error output:
Waiting for postgres...
users-db: forward host lookup failed: Unknown host
users-db: forward host lookup failed: Unknown host
users-db: forward host lookup failed: Unknown host
...
...
Dockerfile:
FROM python:3.6.9-slim
LABEL maintainer="abc"
RUN apt-get update && \
apt-get install -y netcat && \
apt-get clean
WORKDIR /usr/src/app
COPY ./requirements.txt /usr/src/app/requirements.txt
RUN pip install -r requirements.txt
COPY ./entrypoint.sh /usr/src/app/entrypoint.sh
RUN chmod +x /usr/src/app/entrypoint.sh
COPY . /usr/src/app
CMD ["/usr/src/app/entrypoint.sh"]
What strikes me about this is that it worked wonderfully until now and without me making any changes to the database container the connection failed.
What can I do to troubleshoot this ? If you need to review any files just ask and I'll share it here.
docker ps:
72d344cc61bf tdd_nginx "nginx -g 'daemon of…" 25 minutes ago Restarting (1) 55 seconds ago tdd_nginx_1
8ee2f8082e69 tdd_client "npm start" 26 minutes ago Up 25 minutes 0.0.0.0:3007->3000/tcp tdd_client_1
1ccfc3ca5600 tdd_users-db "docker-entrypoint.s…" 26 minutes ago Up 26 minutes 0.0.0.0:5435->5432/tcp tdd_users-db_1
--> 62af29277b78 tdd_users "/bin/bash -s" 22 minutes ago Exited (130) 2 minutes ago # <-- keeps crashing
docker-compose file:
version: '3.7'
services:
users:
build:
context: ./services/users
dockerfile: Dockerfile
volumes:
- './services/users:/usr/src/app'
ports:
- 5001:5000
environment:
- FLASK_ENV=development
- APP_SETTINGS=project.config.DevelopmentConfig
- DATABASE_URL=postgres://postgres:postgres#users-db:5432/users_dev
- DATABASE_TEST_URL=postgres://postgres:postgres#users-db:5432/users_test
- SECRET_KEY=bart_simpson
depends_on:
- users-db
client:
build:
context: ./services/client
dockerfile: Dockerfile
volumes:
- './services/client:/usr/src/app'
- '/usr/src/app/node_modules'
ports:
- 3007:3000
environment:
- NODE_ENV=development
- REACT_APP_USERS_SERVICE_URL=${REACT_APP_USERS_SERVICE_URL}
depends_on:
- users
nginx:
build:
context: ./services/nginx
dockerfile: Dockerfile
restart: always
ports:
- 80:80
depends_on:
- users
- client
users-db:
build:
context: './services/users/project/db'
dockerfile: Dockerfile
ports:
- 5435:5432
environment:
- POSTGRES_USER=postgres
- POSTGRES_PASSWORD=postgres

Answer 1

Problem was that I've got pyjwt library installed for generating web tokens and I used pyjwt.encode() instead of jwt.encode() in my code. That made a major difference to the functionality of the connection between containers. Still don't know why though. Containers are now running again. If somebody will vote to close this topic I'll understand as nobody would've guessed this.