Understanding Container Data Storage
Containers Are Temporary and Don't Save Data by Default
What Does "Ephemeral" Mean?
Ephemeral means something that lasts for a very short time. Think of it like a sandcastle on the beach - it's there when you build it, but when the tide comes in, it disappears.
In Docker, containers are ephemeral - they're designed to be temporary. When you delete a container, everything inside it is gone forever.
Two Types of Data in Docker:
Temporary Data
This is data that doesn't need to be saved long-term.
- Stored directly inside the container
- Easy to use - just save files like normal
- Disappears when the container is deleted
- Good for: Cache files, temporary logs, session data
Permanent Data
This is important data that needs to be saved.
- Stored outside the container using Volumes
- Volumes are like special folders that survive container deletion
- Data stays safe even if containers are destroyed
- Good for: Databases, user files, application data
Real-World Example:
Think of a container like a hotel room:
- Temporary data = The towels and toiletries in the room (they get replaced when you leave)
- Permanent data = Your luggage that you store at the front desk (it stays safe even after you check out)
Data Management Examples
Temporary Data (Inside Container)
# This data disappears when container stops
docker run -it ubuntu bash
# Inside the container:
echo "This will be lost" > /tmp/temp-file.txt
cat /tmp/temp-file.txt
# Output: This will be lost
# But if you stop and restart the container,
# the file is gone forever!
docker run -it ubuntu bash
# Inside the container:
echo "This will be lost" > /tmp/temp-file.txt
cat /tmp/temp-file.txt
# Output: This will be lost
# But if you stop and restart the container,
# the file is gone forever!
Permanent Data (Using Volumes)
# Create a volume for permanent storage
docker volume create my-data
# Use the volume when running a container
docker run -v my-data:/app/data ubuntu bash
# Inside the container:
echo "This will be saved" > /app/data/important-file.txt
# Even if you delete the container,
# the data in 'my-data' volume remains safe
docker volume create my-data
# Use the volume when running a container
docker run -v my-data:/app/data ubuntu bash
# Inside the container:
echo "This will be saved" > /app/data/important-file.txt
# Even if you delete the container,
# the data in 'my-data' volume remains safe
Best Practices for Docker Data
Do This:
- Use volumes for database files
- Store configuration files in volumes
- Keep user uploads in persistent storage
- Use environment variables for settings
Avoid This:
- Storing important data directly in containers
- Keeping sensitive information in container images
- Assuming data will survive container restarts
- Using containers as permanent storage solutions
Remember:
Containers are like disposable cameras - great for temporary use, but not for keeping your precious photos safe long-term. Use volumes for anything important!