How ports notation in docker compose service works? - docker

Question

in docker-compose.yml,
What is the difference between in following ports notations?
ports:
- "5000:5000"
resp:
ports:
- "8080"
or no ports at all.
For example in following docker-compose.yml, the mongodb service must be exposing a port to communicate with node service, but no port is specified
services:
node:
build:
context: .
dockerfile: node.dockerfile
ports:
- "3000:3000"
networks:
- nodeapp-network
depends_on:
- mongodb
mongodb:
image: mongo
networks:
- nodeapp-network
networks:
nodeapp-network:
driver: bridge
source: https://github.com/DanWahlin/NodeExpressMongoDBDockerApp
However in these docker-compose.yml, there are ports awlays specified with either 27017:27017 or 8080 notation.
services:
nginx:
container_name: nginx
image: ${DOCKER_ACCT}/nginx
build:
context: .
dockerfile: .docker/nginx.${APP_ENV}.dockerfile
links:
- node1:node1
- node2:node2
- node3:node3
ports:
- "80:80"
- "443:443"
networks:
- codewithdan-network
node1:
container_name: node-codewithdan-1
image: ${DOCKER_ACCT}/node-codewithdan
build:
context: .
dockerfile: .docker/node-codewithdan.${APP_ENV}.dockerfile
ports:
- "8080"
volumes:
- .:/var/www/codewithdan
working_dir: /var/www/codewithdan
env_file:
- ./.docker/env/app.${APP_ENV}.env
depends_on:
- mongo
- redis
networks:
- codewithdan-network
node2:
container_name: node-codewithdan-2
image: ${DOCKER_ACCT}/node-codewithdan
build:
context: .
dockerfile: .docker/node-codewithdan.${APP_ENV}.dockerfile
ports:
- "8080"
volumes:
- .:/var/www/codewithdan
working_dir: /var/www/codewithdan
env_file:
- ./.docker/env/app.${APP_ENV}.env
depends_on:
- mongo
- redis
networks:
- codewithdan-network
node3:
container_name: node-codewithdan-3
image: ${DOCKER_ACCT}/node-codewithdan
build:
context: .
dockerfile: .docker/node-codewithdan.${APP_ENV}.dockerfile
ports:
- "8080"
volumes:
- .:/var/www/codewithdan
working_dir: /var/www/codewithdan
env_file:
- ./.docker/env/app.${APP_ENV}.env
depends_on:
- mongo
- redis
networks:
- codewithdan-network
mongo:
container_name: mongo
image: ${DOCKER_ACCT}/mongo
build:
context: .
dockerfile: .docker/mongo.dockerfile
ports:
- "27017:27017"
env_file:
- ./.docker/env/mongo.${APP_ENV}.env
networks:
- codewithdan-network
redis:
container_name: redis
image: ${DOCKER_ACCT}/redis
build:
context: .
dockerfile: .docker/redis.${APP_ENV}.dockerfile
ports:
- "6379"
networks:
- codewithdan-network
networks:
codewithdan-network:
driver: bridge
source: https://github.com/DanWahlin/CodeWithDanDockerServices
Can you explain the difference?

Answer 1

Typical Docker containers run a long-running server listening on some TCP port. Other containers on the same Docker network can reach that container using the container’s name (docker run --name, container_name: directive) as a DNS name and the port the server is running on. In Docker Compose, Compose creates a Docker network per Compose YAML file, and also makes services available under their key in the YAML file. This works even if no ports: are specified.
So, for instance, if your docker-compose.yml file says
services:
mongo:
image: mongo
others:
env:
MONGODB_HOST: mongo
MONGODB_PORT: 27017
then the MongoDB container will be reachable on that host name and (default) port, even though it doesn’t explicitly have a ports:.
If you do declare a ports: then the container will be reachable from outside Docker space. If you only have one port it’s the port number of the server, and Docker picks the host port; this isn’t useful in most cases (but it’s guaranteed to not hit a port conflict). If you have two ports they’re the host port and internal service port. You can also specify a host IP address to bind(2) to.
Presence or absence of ports: doesn’t affect inter-dontainer communication. Always use the container’s name (or Docker-compose.yml service name) and the “internal” port number the server is listening on.

Answer 2

Either specify both ports (HOST:CONTAINER), or just the container port (an ephemeral host port is chosen). So in your case 8080 is container port
ports:
- "3000"
- "3000-3005"
- "8000:8000"
- "9090-9091:8080-8081"
- "49100:22"
- "127.0.0.1:8001:8001"
- "127.0.0.1:5000-5010:5000-5010"
- "6060:6060/udp"
From here
The ephemeral port range is configured by /proc/sys/net/ipv4/ip_local_port_range kernel parameter, typically ranging from 32768 to 61000.
Either way, you should be able to peek at what Docker has accomplished in your network stack by examining your NAT tables. from here
In docker compose by default no ports will be created in case they collide with already opened ports