Module 4 Lesson 4: Volumes and Persistent Storage

As we learned in Module 2, the "Writable Layer" of a container is temporary. If you run a database inside a container and then delete that container, your data is Deleted forever.

To save data permanently, we use Volumes.

1. Two Ways to Save Data

A. Docker Volumes (Recommended for Databases)

Managed by Docker. You don't know (or care) where they are on your hard drive.

• Pros: Best performance, managed by Docker CLI, easy to backup.
• Syntax: -v <volume_name>:<container_path>

B. Bind Mounts (Recommended for Development)

Maps a specific folder on your laptop to a folder in the container.

• Pros: You can edit code on your laptop and see it change instantly inside the running container.
• Syntax: -v /absolute/path/on/host:/container_path

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2. Using Volumes in the CLI

# 1. Create a volume
docker volume create my-db-data

# 2. Start a container and "Mount" the volume
docker run -d \
  --name my-db \
  -v my-db-data:/var/lib/postgresql/data \
  postgres

Now, even if you run docker rm -f my-db, the volume my-db-data stays safe. When you start a new container and mount that same volume, all your data is right there.

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3. Bind Mounts for Dev Speed

Imagine you are writing a website in Python.

docker run -v $(pwd):/app my-python-app

• $(pwd) is a shortcut for "Present Working Directory."
• Now, whenever you save your file in your code editor, the file inside the container changes too. No need to docker build every time!

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4. Summary Table

Feature	Docker Volume	Bind Mount
Location	/var/lib/docker/volumes/	Anywhere on your host
Managed by	Docker	You (The User)
Best For...	Databases / Production state	Source Code / Config files
Performance	High	Host-dependent

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Exercise: The Survival Test

1. Create a volume called note-taker.
2. Run an alpine container and mount that volume: docker run --rm -v note-taker:/data alpine sh -c "echo 'hello from volume' > /data/test.txt"
   • Note: the --rm flag means the container is deleted the moment it finishes.
3. Now, run a second container and mount the same volume: docker run --rm -v note-taker:/data alpine cat /data/test.txt
4. Did you see the message? Why did it survive if the first container was deleted?
5. Cleanup: Delete the volume using docker volume rm note-taker.

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Summary

Persistence is what separates "Stateless" apps from "Stateful" apps. By using Volumes for your databases and Bind Mounts for your code, you ensure that your data is safe and your development workflow is fast.

Next Lesson: Configuring on the fly: Environment variables and configuration.