How to set 'spring.datasource.url' inside a Docker Container

Question

I created a Spring Boot application which uses a MySQL database. I use a docker-compose to launch the database.

services:
  adminer:
    image: adminer
    restart: always
    ports:
      - 8888:8080
  db:
    image: mysql:latest
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: 'example' # TODO: Change this      
    volumes:
      - "./config/my.conf:/etc/mysql/conf.d/config-file.cnf"
      - "./data:/var/lib/mysql:rw"

The Spring Boot Application (Backend) currently does not use Docker, I run it inside Eclipse. Before launching the Backend I have to grep the Docker Container for IPAddress:

docker inspect mysql_ex_db_1 | grep 'IPAddress'

which results something like this (this exact address changes time-to time)

                "IPAddress": "",
                    "IPAddress": "172.21.0.2", 

Then I take this value and I set spring.datasource.url inside Eclipse in the file Application.properties with it.

spring.datasource.url=jdbc:mysql://172.21.0.2:3306/employee_management_system?allowPublicKeyRetrieval=true&useSSL=false&createDatabaseIfNotExist=true

After this I can launch the Backend in Eclipse the Connection to database is there, everything works.

Now I want to move the launching of Backend from Eclipse to the same docker-compose file I use to launch the database. Therefore I built an image, and appended the docker-compose file:

version: '3.1'

services:
  adminer:
    image: adminer
    restart: always
    ports:
      - 8888:8080
  db:
    image: mysql:latest
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: 'example' # TODO: Change this      
    volumes:
      - "./config/my.conf:/etc/mysql/conf.d/config-file.cnf"
      - "./data:/var/lib/mysql:rw"
  backend:
    image: backend:latest
    restart: always
    ports:
      - 8090:8080

In this case how can I configure the IPAddress in spring.datasource.url? The exact IPAddress changes whenever I re-launch the mysql containers.

spring.datasource.url=jdbc:mysql://172.21.0.2:3306/employee_management_system?allowPublicKeyRetrieval=true&useSSL=false&createDatabaseIfNotExist=true

So what should I write instead of '172.21.0.2' ?

I tried localhost here but it does not seem to work.

Answer 1

First of all, you can set environment variables like spring.datasource.url outside of your docker image. This allows you to dynamically set these variables according to your deployment needs (like connecting to a dev or prod database).

All docker containers running from your docker-compose file run in the same virtual network and their service names correspond to their hostnames within this network. When you want to access your database from your dockerized spring backend the hostname and port will be db:3306. You can overwrite spring.datasource.url in your docker-compose file by introducing an environment variable like:

version: '3.1'

services:
  adminer:
    image: adminer
    restart: always
    ports:
      - 8888:8080
  db:
    image: mysql:latest
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: 'example' # TODO: Change this      
    volumes:
      - "./config/my.conf:/etc/mysql/conf.d/config-file.cnf"
      - "./data:/var/lib/mysql:rw"
  backend:
    image: backend:latest
    restart: always
    ports:
      - 8090:8080
    environment:
      spring.datasource.url: "jdbc:mysql://db:3306/employee_management_system?allowPublicKeyRetrieval=true&useSSL=false&createDatabaseIfNotExist=true"