Not able to connect to Postgres container from another container

Question

This question has been asked many times here, here and here but these solutions are not working for me.

I have created a Postgres and a AppServer container with this docker-compose.yml file

version: "3.7"
services: 
  db:
    image: postgres:alpine
    container_name: db
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      POSTGRES_DB: mydb
      POSTGRES_INITDB_ARGS: '-A md5'
    volumes:
      - ./pgdata:/var/lib/postgressql/data
    ports:
      - "5432:5432"
  api:
    build: api
    container_name: api
    volumes:
      - ./database/migrations:/migrations
    ports:
      - "8080:8080"
    links:
      - db
    depends_on:
      - db

After running this, I can successfully do

docker exec -it db psql -U user mydb

and I connect to Postgres successfully. I can also successfully login into terminals of both containers with

docker exec -it api bash
docker exec -it db bash

from inside of bash of api I can ping db without any problem

However from my api container, I cannot establish a JDBC connection to the Postgres database.

api    | Flyway Community Edition 7.3.2 by Redgate
api    | ERROR: 
api    | Unable to obtain connection from database (jdbc:postgresql://db:5432/mydb) for user 'user': Connection to db:5432 refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.
api    | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
api    | SQL State  : 08001
api    | Error Code : 0
api    | Message    : Connection to db:5432 refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.
api    | 
api    | Caused by: org.postgresql.util.PSQLException: Connection to db:5432 refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.
api    | Caused by: java.net.ConnectException: Connection refused (Connection refused)

Why am I getting connection refused when I can connect via psql? This is my flyway conf

flyway.url=jdbc:postgresql://db:5432/mydb
flyway.user=user
flyway.password=password
flyway.locations=filesystem:/migrations

Edit:: So if I wait and then execute flyway migrate after some time from docker exec -it api bash everything works fine. I think what is happening above is that my flyway migrate command is running even before the database is ready.

Why is this happening? because I have specified dependency so my API container should start only when the database has fully started. but it seems that is not the case.

Tal · Accepted Answer · 2021-01-02 04:58:13Z

1

Specifying the database container as a dependency doesn't guarantee that it will be ready before your other services/containers. It only guarantees that it will start before your other services.

One way to get around this is to implement a retry attempt(s) in your API application when failing to connect to your database during startup.

Here is a link to an article that uses a shell script to wait for a service to be ready.

IMO your application should be smart enough to retry a few times when it cannot establish a database connection. It will make it more robust anyways.

edited Jan 2, 2021 at 4:58

answered Jan 2, 2021 at 4:52

Tal

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1 Comment

David Maze Over a year ago

This doesn't actually describe how to solve the problem. What changes should the asker make in their code or container environment?

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