Networking & Data Persistence

Port Forwarding

Containers have their own network stack. To access services from outside, map host ports to container ports:

# Map host port 8080 to container port 80
docker container run -d --name web -p 8080:80 nginx
 
# Now accessible at http://localhost:8080
curl http://localhost:8080
 
# Map multiple ports
docker container run -d --name app \
  -p 3000:3000 \
  -p 9229:9229 \
  node:18

Format: -p HOST_PORT:CONTAINER_PORT

Docker Networks

Default Networks

docker network ls

Network	Purpose
`bridge`	Default — containers can communicate via IP
`host`	Container shares host's network (no isolation)
`none`	No networking

Creating Custom Networks

Custom networks enable DNS-based container communication (by name):

# Create a network
docker network create app-network
 
# Run containers on the same network
docker container run -d --name api \
  --network app-network \
  my-api-image
 
docker container run -d --name db \
  --network app-network \
  -e POSTGRES_PASSWORD=secret \
  postgres:15
 
# 'api' can reach 'db' by name:
# Connection string: postgresql://postgres:secret@db:5432

Container Communication

# Containers on the SAME network can communicate by name
docker container exec api ping db  # Works!
 
# Containers on DIFFERENT networks cannot communicate
docker container exec other-container ping db  # Fails!

Network Inspection

docker network inspect app-network

The Host Network

Removes network isolation — container uses host's ports directly:

docker container run -d --network host nginx
# Nginx is now on host's port 80 directly (no -p needed)

Data Persistence

By default, data inside containers is ephemeral — lost when the container is removed.

Docker Volumes (Recommended)

Volumes are managed by Docker and persist independently of containers:

# Create a volume
docker volume create my-data
 
# Mount it to a container
docker container run -d --name db \
  --mount type=volume,src=my-data,dst=/var/lib/postgresql/data \
  postgres:15
 
# Data persists even after container removal
docker container rm -f db
 
# Reattach to a new container
docker container run -d --name db2 \
  --mount type=volume,src=my-data,dst=/var/lib/postgresql/data \
  postgres:15
# All data is still there!

Volume Commands

docker volume ls              # List volumes
docker volume inspect my-data # View details
docker volume rm my-data      # Delete volume
docker volume prune           # Remove unused volumes

Bind Mounts (Development)

Map a host directory directly into the container — great for development:

# Mount current directory into container
docker container run -d --name dev \
  -v $(pwd):/app \
  -p 3000:3000 \
  node:18
 
# Changes on host immediately visible in container

Volumes vs Bind Mounts

Feature	Volumes	Bind Mounts
Managed by	Docker	You
Location	`/var/lib/docker/volumes/`	Anywhere on host
Portability	High	Low (path-dependent)
Security	Better (Docker-managed)	Less (direct access)
Use case	Production data	Development

# Create shared volume
docker volume create shared-data
 
# Container 1 writes
docker container run --rm \
  --mount type=volume,src=shared-data,dst=/data \
  ubuntu bash -c "echo 'Hello from container 1' > /data/message.txt"
 
# Container 2 reads
docker container run --rm \
  --mount type=volume,src=shared-data,dst=/data \
  ubuntu cat /data/message.txt
# Output: Hello from container 1

Practical Example: WordPress + MySQL

# Create network and volumes
docker network create wp-network
docker volume create wp-db
docker volume create wp-content
 
# Start MySQL
docker container run -d --name wp-db \
  --network wp-network \
  --mount type=volume,src=wp-db,dst=/var/lib/mysql \
  -e MYSQL_ROOT_PASSWORD=rootpass \
  -e MYSQL_DATABASE=wordpress \
  -e MYSQL_USER=wpuser \
  -e MYSQL_PASSWORD=wppass \
  mysql:8.0
 
# Start WordPress
docker container run -d --name wp-app \
  --network wp-network \
  --mount type=volume,src=wp-content,dst=/var/www/html \
  -p 8080:80 \
  -e WORDPRESS_DB_HOST=wp-db \
  -e WORDPRESS_DB_USER=wpuser \
  -e WORDPRESS_DB_PASSWORD=wppass \
  -e WORDPRESS_DB_NAME=wordpress \
  wordpress:latest
 
# Access at http://localhost:8080

Summary

-p HOST:CONTAINER maps ports for external access
Custom networks enable DNS-based container communication
Containers on the same network can reach each other by name
Volumes persist data beyond container lifecycle (production)
Bind mounts map host directories into containers (development)
Volumes can be shared between multiple containers

Next Steps

Next, we'll build custom Docker images with Dockerfiles and push them to registries.

Learning Objectives