Docker Ports

Understanding Ports in Docker

When running multiple containers on a single host:

• Host ports must be unique – Only one container can bind to a specific host port at a time.

• Container ports can overlap – Different containers can use the same internal port (e.g., two apps using port 3000 inside their containers).

Port Mapping Syntax

To expose a container’s port to the host, use:

docker run -p <HOST_PORT>:<CONTAINER_PORT> ...

• HOST_PORT: Port on your machine (must be free).

• CONTAINER_PORT: Port the app uses inside the container.

Example: Running PostgreSQL

docker run --name my_db \
  -p 5432:5432 \          # Maps host:5432 → container:5432
  -e POSTGRES_PASSWORD=password \
  -v pg_data:/var/lib/postgresql/data \  #named_volume
  -d postgres

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Key Concepts

1. Detached Mode (-d)

• Runs the container in the background (like a daemon).

• Without -d: The container ties up your terminal.

• With -d: Frees your terminal; use docker logs <container> to view output.

2. Testing Connections (Without IntelliJ Database Tab)

Option 1: Command Line (PSQL)

psql -h localhost -p 5432 -U postgres

Option 2: GUI Tools

• DBeaver (Free)

• TablePlus (Paid, free trial)

• PgAdmin (For PostgreSQL)

3. Stopping/Removing Containers

• Stop a running container:

    docker stop my_db
  

• Remove a stopped container:

    docker rm my_db
  

• Force remove (running container):

    docker rm -f my_db
  

  PS: You can’t remove a running container

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Common Pitfalls & Solutions

1. "Port Already in Use" Error

   • Cause: Another service (or container) is using the host port.

   • Fix:

     • Change the host port (e.g., -p 5433:5432).

     • Stop the conflicting service:

         sudo lsof -i :5432  # Find PID
         kill -9 <PID>       # Terminate
       

2. Firewall Blocking Connections

   • On Linux/macOS:

       sudo ufw allow 5432/tcp
     

   • On Windows: Adjust inbound rules in Windows Defender Firewall.

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Best Practices

• Use named volumes (as shown above) for persistent data.

• Avoid running as root inside containers for security:

    docker run --user 1000:1000 ...
  

Limit memory usage to prevent host exhaustion:

docker run -m 512m ...

Recap: Key Commands