# Demonstration: Proving Data Loss, Then Proving Persistence

## OBSERVE

```
$ docker run -d --name no-volume alpine:3.19 sleep 300
$ docker exec no-volume sh -c "echo important-data > /tmp/data.txt"
$ docker exec no-volume cat /tmp/data.txt
important-data
```

Data exists — but is it actually safe?

## HYPOTHESIS

Predict: will `/tmp/data.txt` still exist if this exact container is destroyed and an identical
new one is started?

## INSPECT

```
$ docker rm -f no-volume
$ docker run -d --name no-volume alpine:3.19 sleep 300
$ docker exec no-volume cat /tmp/data.txt
cat: /tmp/data.txt: No such file or directory
```

Gone — proven, not assumed. The "new" container is a completely fresh writable layer, even
though it has the exact same name and image.

## CHANGE

Now with a named volume instead:

```
$ docker rm -f no-volume
$ docker volume create demo-vol
$ docker run -d --name with-volume -v demo-vol:/data alpine:3.19 sleep 300
$ docker exec with-volume sh -c "echo important-data > /data/data.txt"
$ docker rm -f with-volume
$ docker run -d --name with-volume -v demo-vol:/data alpine:3.19 sleep 300
```

## VERIFY

```
$ docker exec with-volume cat /data/data.txt
important-data
```

Proven, not assumed — the exact same destroy-and-recreate sequence, but this time the data
survived, because it lived in the volume, not the container's disposable writable layer.

## Instructor Notes

**Demo goal:** Prove BOTH the loss and the persistence with real destroy/recreate cycles — the
contrast is what makes this lesson land, not either half alone.

**Approximate timing:** 10 minutes live.

**Common mistake to watch for:** stopping (not removing) a container and concluding "data
persists" — `docker stop` doesn't touch the writable layer at all; only `docker rm` (destroying
the container entirely) actually tests real persistence.