server and postgresql cannot connect when both running on Docker `getaddrinfo ENOTFOUND`

Question

after creating docker containers with docker compose file (below), I call

$ docker run myApp

However, I get

Error: getaddrinfo ENOTFOUND main_db

this only happens when both server and postgresql are in docker containers (I am able to connect to postgresql on localhost)

I'm running a NestJS app using TypeOrm to connect to a postgresql server

inside the app.module.ts where it boots up the connection my config should match my docker postgresql config. the host points to the container I created on docker main_db and I declared this as a dependency of my server, the main service. Everything should be on the same network webnet.:

TypeOrmModule.forRoot({
      type: 'postgres', 
      host: 'main_db',
      port: +process.env.POSTGRES_PORT,
      username: process.env.POSTGRES_USER,
      password: process.env.POSTGRES_PASSWORD,
      database: process.env.POSTGRES_DB,
      autoLoadEntities: true,
      synchronize: true,
      logging: dbLogging,
    }),

docker-compose.yml

version: '3.7'

services:
  main:
    container_name: main
    build:
      context: .
      target: development
    volumes:
      - .:/usr/src/app
      - /usr/src/app/node_modules
    ports:
      - ${SERVER_PORT}:${SERVER_PORT}
      - 9229:9229
    command: npm run start:dev
    env_file:
      - .env
    networks:
      - webnet
    depends_on:
      - main_db
  main_db:
    container_name: main_db
    image: postgres:12
    restart: always
    networks:
      - webnet
    environment:
      POSTGRES_DB: ${POSTGRES_DB}
      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
      POSTGRES_USER: ${POSTGRES_USER}
      PG_DATA: /var/lib/postgresql/data
    ports:
      - '${POSTGRES_PORT}:${POSTGRES_PORT}'
    volumes:
      - pgdata:/var/lib/postgresql/data
networks:
  webnet:
volumes:
  pgdata:

.env file

POSTGRES_PORT=5432
POSTGRES_USER=test
POSTGRES_PASSWORD=test
POSTGRES_DB=test
SERVER_PORT=3001

Answer 1

When creating a Dockerfile and a docker-compose.yml file and you call docker run myApp for the app defined inside of the Dockerfile instead of calling docker-compose up, you will see the app running; however, it will not start the containers defined in the docker-compose file. In the case of the NestJS server, it was running the server, but could not find the container with the database, since this container was not being spun up. Although the distinction between the app setup in the Dockerfile and the definition of the containers in the docker-compose.yml was clear, I didn't realize that the docker command didn't reference the docker-compose.yml. Thus, posting here in case anyone else has a similar confusion.