Understanding Persistent Data in Containers

1. Introduction

In this tutorial, we will explore Docker volumes, a powerful feature that enables persistent data storage for your Docker containers. Docker volumes can be thought of as “folders” or “directories” that exist outside the lifecycle of the container, which allows data to persist even when the container is stopped or deleted.

Goals of this tutorial:

• Understand the concept of Docker volumes
• Learn how to create, manage, and use Docker volumes
• Understand best practices for using Docker volumes

Prerequisites:

• Basic understanding of Docker and Docker containers
• Docker installed on your local machine

2. Step-by-Step Guide

Docker volumes are created and managed by Docker. When a Docker volume is created, it is stored in a part of the host filesystem which is managed by Docker (/var/lib/docker/volumes/ on Linux).

Creating a Docker Volume

You can create a Docker volume using the docker volume create command:

docker volume create my-vol

This will create a Docker volume named "my-vol".

Listing Docker Volumes

You can list all volumes using the docker volume ls command:

docker volume ls

Using Docker Volumes

To use a volume, you need to specify it when you run your container with the -v option in the docker run command:

docker run -d -v my-vol:/app my-image

This command will start a new container and mount the volume "my-vol" into the container at the /app directory.

Deleting Docker Volumes

You can remove a volume using the docker volume rm command:

docker volume rm my-vol

Best Practice

To avoid data loss, it is advised to use Docker volumes for any data that needs to survive after a container is removed.

3. Code Examples

Example 1: Using Docker Volumes

# Create a volume
docker volume create my-vol

# Run a container with the volume
docker run -d -v my-vol:/app my-image

In this example, we first create a volume named "my-vol". Then, we run a new container and mount "my-vol" into the container at the /app directory.

Example 2: Deleting Docker Volumes

# Remove a volume
docker volume rm my-vol

In this example, we remove the volume named "my-vol". Be careful when removing volumes, as this operation is irreversible and any data stored in the volume will be lost.

4. Summary

In this tutorial, we have learned about Docker volumes and how to use them for persistent data storage in Docker containers. We've created, listed, used, and deleted Docker volumes.

Next steps include exploring Docker volumes in more depth, such as understanding volume drivers and options for sharing volumes between containers.

Additional resources:

• Docker's official documentation on volumes

5. Practice Exercises

Exercise 1: Create a Docker volume, run a container that uses this volume, and then list all volumes to confirm your volume is being used.

Solution:

# Create a volume
docker volume create exercise-vol

# Run a container with the volume
docker run -d -v exercise-vol:/app my-image

# List all volumes
docker volume ls

Exercise 2: Write a Dockerfile that creates a text file in a directory, builds an image from this Dockerfile, runs a container with a volume using this image, and then inspects the volume to confirm the text file exists.

Solution:

# Dockerfile
FROM alpine
WORKDIR /app
RUN echo "Hello, Docker volumes!" > message.txt

# Build an image from the Dockerfile
docker build -t exercise-image .

# Run a container with a volume using the image
docker run -d -v exercise-vol:/app exercise-image

# Inspect the volume
docker run -it --rm -v exercise-vol:/app alpine cat /app/message.txt

You should see the output "Hello, Docker volumes!".

Remember, practice makes perfect. Keep experimenting with Docker volumes and you'll soon get the hang of it. Happy Dockering!