flask app service can't connect to postgres service within docker-compose

Question

I am trying to run flask, postgres and nginx services with following docker-compose:

version: '3.6'


services:

  postgres:
    image: postgres:10.5
    container_name: postgres
    hostname: postgres
    user: postgres  
    ports:
      - "5432:5432"
    networks:
      - db-tier
    environment:
      CUSTOM_CONFIG: /etc/postgres/postgresql.conf
    volumes:
      - ./postgres/sql/create_tables.sql:/docker-entrypoint-initdb.d/create_tables.sql
      - ./postgres/postgresql.conf:/etc/postgres/postgresql.conf
    command: postgres -c config_file=/etc/postgres/postgresql.conf
    restart: always

  app:
    image: python/app:0.0.1
    container_name: app
    hostname: app
    build:
      context: .
      dockerfile: Dockerfile
    depends_on:
     - postgres
    networks:
      - app-tier
      - db-tier
    stdin_open: true

  nginx:
    image: nginx:1.22
    container_name: nginx-reverse-proxy-flask
    ports:
      - "8080:80"
    depends_on:
      - app
    volumes:
      - ./nginx/nginx.conf:/etc/nginx/nginx.conf:ro
    networks:
      - app-tier


networks:
  app-tier:
    driver: bridge
  db-tier:
    driver: bridge

This is what app.config["SQLALCHEMY_DATABASE_URI"] equals to postgresql://denis:1234Five@postgres:5432/app

The error after docker-compose up is:

app         | psycopg2.OperationalError: could not connect to server: Connection refused
app         |   Is the server running on host "postgres" (192.168.32.2) and accepting
app         |   TCP/IP connections on port 5432?

What could cause this type of error? I double checked the container name of postgres service, and it running with this name postgres why flask app doesn’t "see" it?

Asked By: PatrickHellman

||

Source

Answer 1

It could be issue with no propper startup probe in your postgres container and docker-compose.yml
Look at for reference how to setup them at Docker – check if postgres is ready

So after postgres starts as container, docker starts your app, but postgres inside of container is not ready yet and you get your error

Answered By: Ryabchenko Alexander

Answer 2

This issue was resolved with postgres pg_isready adding following lines to postgres service:

healthcheck:
  test: ["CMD-SHELL", "sh -c 'pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB}'"]
  interval: 10s
  timeout: 3s
  retries: 3

Took as solution from here Safe ways to specify postgres parameters for healthchecks in docker compose

Answered By: PatrickHellman

flask app service can't connect to postgres service within docker-compose

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