Could not translate host name "db" to address using Postgres, Docker Compose and Psycopg2

Question

In one folder I have 3 files: base.py, Dockerfile and docker-compose.yml.

base.py:

import psycopg2

conn = psycopg2.connect("dbname='base123' user='postgres' host='db' password='pw1234'")

Dockerfile:

FROM ubuntu:16.04

RUN apt-get update
RUN apt-get -y install python-pip
RUN apt-get update
RUN pip install --upgrade pip
RUN pip install psycopg2-binary

COPY base.py base.py

RUN python base.py

docker-compose.yml:

version: '3'
services:
  db:
    image: 'postgres:latest'
    expose:
      - "5432"
    environment:
      POSTGRES_PASSWORD: pw1234
      POSTGRES_DB: base123
  aprrka:
    build: .    
    depends_on:
      - db

After I ran docker-compose up, I got the following error:

Traceback (most recent call last):
  File "base.py", line 5, in <module>
conn = psycopg2.connect("dbname='base123' user='postgres' host='db' password='pw1234'")
   File "/usr/local/lib/python2.7/dist-packages/psycopg2/__init__.py", line 130, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: could not translate host name "db" to address: Name or service not known

ERROR: Service 'aprrka' failed to build: The command '/bin/sh -c python base.py' returned a non-zero code: 1

I don't know why I have this error. I exposed port 5432. By default Compose sets up a single network for app. Each service joins the default network, I think that my app with postgres should work together. Did I write incorrect docker-compose.yml?

Answer 1

The problem is you should not be running python base.py as part of the RUN directive.

The RUN directive is executed only when you are building the image. The postgres container is not running at this point, nor has the network been created. Instead you want to use the CMD directive.

Change the Dockerfile to this:

FROM ubuntu:16.04

RUN apt-get update
RUN apt-get -y install python-pip
RUN apt-get update
RUN pip install --upgrade pip
RUN pip install psycopg2-binary

COPY base.py base.py

CMD ["python", "base.py"]

The above should result in the hostname db to be resolved. However if your python code doesn't have any reconnection logic for connecting to the database the container will likely still error out. This because the postgres container will be running but the database won't be ready to accept connections.

This can be temporarily fixed by adding restart: always to your docker-compose.yml.

version: '3'
services:
  db:
    image: 'postgres:latest'
    expose:
      - "5432"
    environment:
      POSTGRES_PASSWORD: pw1234
      POSTGRES_DB: base123
  aprrka:
    restart: always
    build: .    
    depends_on:
      - db

Hopefully this will get you up and running.