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Containers Are Ephemeral (Volumes and Data)

A container is not a server. A container is a process. When it dies, its writable layer is disposable.

If students miss this, they lose data and panic.

Demonstration: data disappears​

Run a container and create a file inside it:

docker run -it --name temp alpine:latest sh

Inside container:

echo "important" > /data.txt
ls -la /data.txt
exit

Remove the container:

docker rm temp

Start a new container:

docker run -it alpine:latest sh
ls -la /data.txt

What you see:

  • file is missing

What this means:

  • container writable layer is not durable
  • new container starts fresh

Two ways to persist data​

  1. Volumes (managed by Docker)
  2. Bind mounts (map a host folder into the container)

Use volumes for most cases. Use bind mounts for local development.

Create a volume:

docker volume create appdata

List volumes:

docker volume ls

Inspect a volume:

docker volume inspect appdata

Sample output (short):

[
  {
    "Name": "appdata",
    "Mountpoint": "/var/lib/docker/volumes/appdata/_data"
  }
]

Use a volume with a container​

Write data into the volume:

docker run --rm -it -v appdata:/data alpine:latest sh

Inside container:

mkdir -p /data/demo
echo "persisted" > /data/demo/value.txt
ls -la /data/demo
exit

Start a new container with the same volume:

docker run --rm -it -v appdata:/data alpine:latest sh
cat /data/demo/value.txt
exit

What you see:

  • the file still exists

Bind mounts (for local development)​

Create a host folder and file:

mkdir -p host-data
echo "from host" > host-data/hello.txt

Run container with bind mount:

docker run --rm -it -v "$(pwd)/host-data":/data alpine:latest sh

Inside container:

ls -la /data
cat /data/hello.txt
echo "from container" > /data/container.txt
exit

Back on host:

ls -la host-data
cat host-data/container.txt

Volume vs bind mount (simple rule)​

Volume:

  • Docker manages location
  • good for production
  • avoids hardcoding host paths

Bind mount:

  • exact host path
  • best for development
  • permissions can bite on Linux

Cleaning volumes​

Delete a specific volume:

docker volume rm appdata

Remove unused volumes:

docker volume prune

Be careful:

  • prune deletes volumes not used by any container

Inspect mounts on a container​

Start a container:

docker run -d --name voltest -v appdata:/data nginx:alpine

Inspect mounts:

docker inspect voltest -f '{{json .Mounts}}'

Stop and remove:

docker rm -f voltest

Data in the volume remains unless you delete the volume.