Make multiple Node js dockerized app communcate to mongodb docker - node.js

Question

I've built two NEST app which shares a common database. I've been trying to figure out a way to make these two apps to get data from the Mongo db container. I'm stuck with this process.
docker-compose.yml with app 1
version: '3'
services:
backend:
image: 'example-image_1:1.0.0'
working_dir: /app/example-app-1/backend/example-app-1-api
environment:
- DB_URL=mongodb://0.0.0.0:27017/example_app_1
- BACKEND_PORT=3333
- BACKEND_IP=0.0.0.0
restart: always
network_mode: "host"
ports:
- '3333:3333'
command: ['node', 'main.js']
depends_on:
- mongodb
expose:
- 3333
mongodb:
image: 'mongo:latest'
environment:
- 'MONGODB_DATABASE="example_app_1"'
ports:
- '27017:27017'
expose:
- 27017
docker-compose.yml with app-2
version: '3'
services:
backend:
image: 'example_app_2:1.0.0'
working_dir: /app/example_app_2/backend/example-app-2-api
environment:
- DB_URL=mongodb://0.0.0.0:27017/example_app_2
- BACKEND_PORT=8888
- BACKEND_IP=0.0.0.0
restart: always
ports:
- '8888:8888'
command: ['node', 'main.js']
expose:
- 8888
mongodb:
image: 'mongo:latest'
environment:
- 'MONGODB_DATABASE="example_app_2"'
ports:
- '27017:27017'
expose:
- 27017
I need help in making these app communicate with common container - mongodb
Click link for Architecture Setup

Answer 1

Since you're using 2 different docker-compose.yml you're actually running 2 backend and 2 mongodb on 2 docker-networks
One of the 2 mongo won't start cause the port is already occupied.
Option 1 (nicer):
services:
backend_1:
...
ports:
- '8888:8888'
backend_2:
...
ports:
- '8899:8899'
mongodb:
ports:
- '27017:27017'
This setup provides 3 container on the same network.
Now you can access at mongo from both backends at <mongo_ip>:27017
Option 2 (ugly):
services:
backend:
...
ports:
- '8888:8888'
mongodb:
ports:
- '27017:27017'
And in another docker-compose
services:
backend:
...
ports:
- '8888:8888'
This setup provides 3 container on 2 different network.
In this setup each docker-compose.yml has it's own docker network, so from the second backend service you have to connect to another docker network to access the container.