Port mapping in Compose in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Understanding port mapping syntax in docker-compose

Given this snippet from a docker-compose.yml file, what port will be accessible on the host machine?

services:
  web:
    image: nginx
    ports:
      - "8080:80"

A Port 80 on the host will be accessible

B Port 8080 on the container will be accessible on port 80 of the host

C Port 80 on the container will be accessible on port 80 of the host

D Port 8080 on the host will be accessible

💡 Hint

Remember the format is hostPort:containerPort

The syntax "8080:80" means port 8080 on the host maps to port 80 inside the container. So you access the service on host port 8080.

🧠 Conceptual

intermediate

Effect of omitting host port in docker-compose ports

What happens if you specify only the container port in the ports section of docker-compose.yml like this?

services:
  app:
    image: myapp
    ports:
      - "5000"

A The container will not expose any ports to the host

B Port 5000 will be exposed on the host with the same port number

C Docker will randomly assign a free port on the host mapped to container port 5000

D This syntax is invalid and will cause a docker-compose error

💡 Hint

Think about how Docker handles ports when host port is not specified

When only the container port is specified, Docker assigns a random available port on the host and maps it to the container port.

❓ Troubleshoot

advanced

Troubleshooting port conflicts in docker-compose

You have two services in your docker-compose.yml:

services:
  service1:
    image: app1
    ports:
      - "8080:80"
  service2:
    image: app2
    ports:
      - "8080:80"

What will happen when you try to start both services together?

A Docker-compose will fail to start the second service due to port conflict

B The second service will start but override the first service's port mapping

C Both services will start successfully and share port 8080 on the host

D Docker will automatically assign a different host port to the second service

💡 Hint

Two services cannot bind the same host port at the same time

Port 8080 on the host cannot be used by two containers simultaneously. Docker-compose will error on the second service.

🔀 Workflow

advanced

Configuring multiple port mappings in docker-compose

You want to expose container ports 80 and 443 to host ports 8080 and 8443 respectively. Which docker-compose ports configuration achieves this?

A

ports:
  - "8080:80"
  - "8443:443"

B

ports:
  - "80:8080"
  - "443:8443"

C

ports:
  - "8080:443"
  - "8443:80"

D

ports:
  - "80"
  - "443"

💡 Hint

Remember the format is hostPort:containerPort

Option A correctly maps host port 8080 to container port 80 and host port 8443 to container port 443.

✅ Best Practice

expert

Best practice for exposing ports in production with docker-compose

Which approach is best to securely expose a web service running in a container on port 80 to the internet using docker-compose?

A Map container port 80 directly to host port 80 and open host firewall for port 80

B Use a reverse proxy container mapped to host ports 80 and 443, forwarding to the web service container on internal ports

C Expose container port 80 without mapping to host ports and rely on Docker internal networking

D Map container port 80 to a random host port and share that port publicly

💡 Hint

Think about security and flexibility in production environments

Using a reverse proxy container allows better control, SSL termination, and security while forwarding requests internally.