What is the use of enviroument variable if the .env already existed

Docker Engine Compose
1

I am watching the example from [digitalocean] to introduce docker compose.(Containerizing a Node.js Application for Development With Docker Compose | DigitalOcean)
.env:

MONGO_USERNAME=sammy
MONGO_PASSWORD=your_password
MONGO_PORT=27017
MONGO_DB=sharkinfo

The script use the .env variable

...
const {
  MONGO_USERNAME,
  MONGO_PASSWORD,
  MONGO_HOSTNAME,
  MONGO_PORT,
  MONGO_DB
} = process.env;

const url = `mongodb://${MONGO_USERNAME}:${MONGO_PASSWORD}@${MONGO_HOSTNAME}:${MONGO_PORT}/${MONGO_DB}?authSource=admin`;

mongoose.connect(url, {useNewUrlParser: true});

If we can already use the env or use proccess.env.VARIABLE_NAME(or any other programming language own getting .env variable method) in the script, why do we still need to add environment variable inside docker-compose file, like this:

version: '3'

services:
  nodejs:
    build:
      context: .
      dockerfile: Dockerfile
    image: nodejs
    container_name: nodejs
    restart: unless-stopped
    env_file: .env
    environment:
      - MONGO_USERNAME=$MONGO_USERNAME
      - MONGO_PASSWORD=$MONGO_PASSWORD
      - MONGO_HOSTNAME=db
      - MONGO_PORT=$MONGO_PORT
      - MONGO_DB=$MONGO_DB 
    ports:
      - "80:8080"
    volumes:
      - .:/home/node/app
      - node_modules:/home/node/app/node_modules
    networks:
      - app-network
    command: ./wait-for.sh db:27017 -- /home/node/app/node_modules/.bin/nodemon app.js

What is the purpose of adding these environment variable?

2

Hi,

Its not needed, things defined in the yaml, environment block, will override the variable in .env

3

So if Iā€™m happy with using environment variable directly reading the ā€˜.envā€™, I no need to add another environment section inside docker compose file? Is there any benefit of people using the environment section rather than reading the ā€˜.envā€™? I knew there are levels of environment variable, environment variables inside docket compose will replace the ā€˜.envā€™.

4

Personally i only use the environment block in docker-compose, in that way i have all configuration in 1 file :slight_smile:

5

The .env files are useful when you want to place your docker-compose.yml in git, but donā€™t want your secrets in there.

Other static env settings you can place inside docker-compose.yml, keeping it all in one place.

6

Nothing is really required, just an option for specific usecases. If your application can read a .env file, thatā€™s fine, but that means, only your application will know about the variables and you need to read the file every time when you execute a command in the container using docker exec for example.

Also worth noting that variables in a .env file can be used to choose a docker network dynamically or set a port number which has nothing to do with the application in the container. It is also possible that the application expects a URL as an endpoint, but you want to create separate variables like EP_PROTOCOL, EP_HOSTNAME, EP_PATH and so on and you just concatenate it and optionally generate a new .env file for the application.

You could also have an appliation which was written without the support of reading a file for the variables but you still want to run it in a container.

So Docker and Docker Compose just makes things possible, but eventually it is up to you how you use the features.