Lesson 1: The Vault (Persistent Data with Volumes)

Here's a terrifying fact: when a container is deleted, all its data is lost. If you're running a database in a container and you remove it — poof, all your data is gone. Volumes fix this.

The Data Problem

Container lifecycle:
  docker run → Container runs → Writes data → docker rm → 💀 DATA GONE

With a volume:
  docker run -v data:/db → Container runs → Writes to volume → docker rm → 📦 DATA SAFE

What is a Volume?

A volume is a persistent storage area managed by Docker that exists independently of any container. Even if you delete the container, the volume survives.

Types of Storage

| Type | Syntax | Use Case | |------|--------|----------| | Named Volume | -v mydata:/app/data | Production data (databases) | | Bind Mount | -v /host/path:/container/path | Development (live code editing) | | tmpfs | --tmpfs /tmp | Temporary data in memory |

Named Volumes (Recommended for Production)

# Create a volume
docker volume create app-data

# Use it with a container
docker run -d -v app-data:/var/lib/postgresql/data postgres

# The data persists even after the container is removed!
docker rm -f postgres_container
docker run -d -v app-data:/var/lib/postgresql/data postgres
# ✅ Your data is still there!

Bind Mounts (Great for Development)

Bind mounts link a host directory directly into the container. Changes on either side are instantly reflected:

docker run -d -v $(pwd)/src:/app/src -p 3000:3000 node-app

This is perfect for development — edit code on your laptop, and it's instantly updated inside the container.

Volume Management

docker volume ls                   # List all volumes
docker volume inspect app-data     # View volume details
docker volume rm app-data          # Remove a volume
docker volume prune                # Remove ALL unused volumes

Read-Only Mounts

For security, you can mount volumes as read-only:

docker run -v config:/app/config:ro nginx

The container can read the config but cannot modify it.

Mission Objective

Set up persistent storage:

1. Create the vault: Run docker volume create app-data.
2. Use it: Run docker run -d --name db -v app-data:/var/lib/postgresql/data postgres:alpine.
3. Inspect: Run docker volume inspect app-data to see where it's stored.

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