understanding Docker compose mongodb and rails application - ruby-on-rails

Question

there is ruby on rails application which uses mongodb and postgresql databases. When I run it locally everything works fine, however when I try to open in a remote container, it throws error message
2021-03-14T20:22:27.985+0000 Failed: error connecting to db server: no reachable servers
the docker-compose.yml file defines following services:
redis mongodb db rails
I start remote containers with following command:
docker-compose build - build successful
docker-compose up -d - containers are up and running
when I connect to the rails container and try to do
bundle exec rake aws:restore_db
error mentioned above is thrown. I don't know what is wrong here. The mongodb container is up and running.
the docker-compose.yml is mentioned below:
version: '3.4'
services:
redis:
image: redis:5.0.5
mongodb:
image: mongo:3.6.13
volumes:
- mongo-data:/data/db
db:
image: postgres:11.3
volumes:
- db-data:/var/lib/postgresql/data
rails:
build: .
image: proj:latest
depends_on:
- db
- mongodb
- redis
volumes:
- .:/proj
ports:
- "3000:3000"
tty: true
stdin_open: true
env_file:
- .env/development.env
volumes:
db-data:
mongo-data:
this is how I start all four remote containers:
$ docker-compose up -d
Starting proj_db_1 ... done
Starting proj_redis_1 ... done
Starting proj_mongodb_1 ... done
Starting proj_rails_1 ... done
please help me to understand how remote containers should interact with each other.

Answer 1

Your configuration should point to the services by name and not to a port on localhost. For example, if you ware connecting to redis as localhost:6380 or 127.0.0.1:6380, now you need to use redis:6380
If this is still not helping, you can try to add links between containers in order the names given to them as services to be resolved. So the file will look something like this:
version: '3.4'
services:
redis:
image: redis:5.0.5
networks:
- front-end
links:
- "mongodb:mongodb"
- "db:db"
- "rails:rails"
mongodb:
image: mongo:3.6.13
volumes:
- mongo-data:/data/db
networks:
- front-end
links:
- "redis:redis"
- "db:db"
- "rails:rails"
db:
image: postgres:11.3
volumes:
- db-data:/var/lib/postgresql/data
networks:
- front-end
links:
- "redis:redis"
- "mongodb:mongodb"
- "rails:rails"
rails:
build: .
image: proj:latest
depends_on:
- db
- mongodb
- redis
volumes:
- .:/proj
ports:
- "3000:3000"
tty: true
stdin_open: true
env_file:
- .env/development.env
networks:
- front-end
links:
- "redis:redis"
- "mongodb:mongodb"
- "db:db"
volumes:
db-data:
mongo-data:
networks:
front-end:
The links will allow for a hostnames to be defined in the containers.
The link flag is legacy, and in new versions of docker-engine it's not required for user defined networks. Also, the links will be ignored in case of docker swarm deployment. However since there are sill old installations of Docker and docker-compose, this is one thing to try in troubleshooting.