psycopg2.OperationalError: could not translate host name to address using psycopg2 and docker

Question

I've been trying for a few hours now but no solution from similar asked questions seem to work for me... I am using docker-compose to setup a postgresql database and run a python webserver from where I want to connect to my postgressql database (so it's running inside the container)

version: '3.8'

services:
  database:
    container_name: database
    hostname: database
    image: postgres
    restart: always
    environment:
      POSTGRES_DB: mydatabase
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: password
    volumes:
      - postgres:/pgdata
      - ./application/ressources/fixtures.sql:/docker-entrypoint-initdb.d/fixtures.sql 
    ports:
      - "5432:5432"

  application:
    container_name: application
    build: .
    ports:
      - "5001:5001"
    volumes: 
      - ./application:/application
    restart: always
    depends_on:
      - database

volumes:
    postgres:

I trying to connect as follows ( I have read that despite the depends on in my dockerfile the database needs some more time until it can accept connections so i added a retry logic):

        retries = 0
        while retries < 5:
            retries = retries + 1
            self.conn = psycopg2.connect(user='postgres', password='password',
                                         host='database', port="5432", database='mydatabase')
            if not self.conn:
                logging.info("retry to connect")
                sleep(5)

The weird thing is that when running it with docker-compose -f docker-compose.yml up everything works fine. But when I built the image (docker build -t myapp:0.1) and run it (docker run myapp:0.1) it gives me the following error:

File "/application/libraries/database.py", line 18, in establishConnection
    self.conn = psycopg2.connect(user=CONFIG.DATABASE_USER, password=CONFIG.DATABASE_PASSWORD,
  File "/usr/local/lib/python3.9/site-packages/psycopg2/__init__.py", line 127, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: could not translate host name "database" to address: Name or service not known

I've read that when using docker-compose a single network is created, so this can't be the error here i guess Docker Compose doku

Thanks in advance, Jacky

Answer 1

If you run docker run on an image, it does only what's on that command line and no more. In particular, the plain docker commands don't know about docker-compose.yml and any of the settings you might specify there.

The short answer is to always use docker-compose up to launch the containers.

In principle you could translate the docker-compose.yml file into explicit docker commands. At a minimum you'd need to manually create a Docker network and specify the container names:

docker network create app-net
docker run -d --net app-net --name database -p 5432:5432 postgres
docker run -d --net app-net -e PGHOST=database -p 5001:5001 myapp:0.1

This hasn't included the other options in the Compose setup, though, notably database persistence. There are enough settings that you'd want to write them down in a file, and at that point you've basically reconstructed the Compose environment.