159 lines
7.1 KiB
Markdown
159 lines
7.1 KiB
Markdown
|
|
In case you didn't notice, our todo list is being wiped clean every single time
|
|
we launch the container. Why is this? Let's dive into how the container is working.
|
|
|
|
## The Container's Filesystem
|
|
|
|
When a container runs, it uses the various layers from an image for its filesystem.
|
|
Each container also gets its own "scratch space" to create/update/remove files. Any
|
|
changes won't be seen in another container, _even if_ they are using the same image.
|
|
|
|
### Seeing this in Practice
|
|
|
|
To see this in action, we're going to start two containers and create a file in each.
|
|
What you'll see is that the files created in one container aren't available in another.
|
|
|
|
1. Start a `ubuntu` container that will create a file named `/data.txt` with a random number
|
|
between 1 and 10000.
|
|
|
|
```bash
|
|
docker run -d ubuntu bash -c "shuf -i 1-10000 -n 1 -o /data.txt && tail -f /dev/null"
|
|
```
|
|
|
|
In case you're curious about the command, we're starting a bash shell and invoking two
|
|
commands (why we have the `&&`). The first portion picks a single random number and writes
|
|
it to `/data.txt`. The second command is simply watching a file to keep the container running.
|
|
|
|
1. Validate we can see the output by `exec`'ing into the container. To do so, open the Dashboard and click the first action of the container that is running the `ubuntu` image.
|
|
|
|
{: style=width:75% }
|
|
{: .text-center }
|
|
|
|
You will see a terminal that is running a shell in the ubuntu container. Run the following command to see the content of the `/data.txt` file. Close this terminal afterwards again.
|
|
|
|
```bash
|
|
cat /data.txt
|
|
```
|
|
|
|
If you prefer the command line you can use the `docker exec` command to do the same. You need to get the
|
|
container's ID (use `docker ps` to get it) and get the content with the following command.
|
|
|
|
```bash
|
|
docker exec <container-id> cat /data.txt
|
|
```
|
|
|
|
You should see a random number!
|
|
|
|
1. Now, let's start another `ubuntu` container (the same image) and we'll see we don't have the same
|
|
file.
|
|
|
|
```bash
|
|
docker run -it ubuntu ls /
|
|
```
|
|
|
|
And look! There's no `data.txt` file there! That's because it was written to the scratch space for
|
|
only the first container.
|
|
|
|
1. Go ahead and remove the first container using the `docker rm -f` command.
|
|
|
|
## Container Volumes
|
|
|
|
With the previous experiment, we saw that each container starts from the image definition each time it starts.
|
|
While containers can create, update, and delete files, those changes are lost when the container is removed
|
|
and all changes are isolated to that container. With volumes, we can change all of this.
|
|
|
|
[Volumes](https://docs.docker.com/storage/volumes/) provide the ability to connect specific filesystem paths of
|
|
the container back to the host machine. If a directory in the container is mounted, changes in that
|
|
directory are also seen on the host machine. If we mount that same directory across container restarts, we'd see
|
|
the same files.
|
|
|
|
There are two main types of volumes. We will eventually use both, but we will start with **named volumes**.
|
|
|
|
## Persisting our Todo Data
|
|
|
|
By default, the todo app stores its data in a [SQLite Database](https://www.sqlite.org/index.html) at
|
|
`/etc/todos/todo.db`. If you're not familiar with SQLite, no worries! It's simply a relational database in
|
|
which all of the data is stored in a single file. While this isn't the best for large-scale applications,
|
|
it works for small demos. We'll talk about switching this to an actual database engine later.
|
|
|
|
With the database being a single file, if we can persist that file on the host and make it available to the
|
|
next container, it should be able to pick up where the last one left off. By creating a volume and attaching
|
|
(often called "mounting") it to the directory the data is stored in, we can persist the data. As our container
|
|
writes to the `todo.db` file, it will be persisted to the host in the volume.
|
|
|
|
As mentioned, we are going to use a **named volume**. Think of a named volume as simply a bucket of data.
|
|
Docker maintains the physical location on the disk and you only need to remember the name of the volume.
|
|
Every time you use the volume, Docker will make sure the correct data is provided.
|
|
|
|
1. Create a volume by using the `docker volume create` command.
|
|
|
|
```bash
|
|
docker volume create todo-db
|
|
```
|
|
|
|
1. Stop the todo app container once again in the Dashboard (or with `docker rm -f <id>`), as it is still running without using the persistent volume.
|
|
|
|
1. Start the todo app container, but add the `-v` flag to specify a volume mount. We will use the named volume and mount
|
|
it to `/etc/todos`, which will capture all files created at the path.
|
|
|
|
```bash
|
|
docker run -dp 3000:3000 -v todo-db:/etc/todos getting-started
|
|
```
|
|
|
|
1. Once the container starts up, open the app and add a few items to your todo list.
|
|
|
|
{: style="width: 55%; " }
|
|
{: .text-center }
|
|
|
|
1. Remove the container for the todo app. Use the Dashboard or `docker ps` to get the ID and then `docker rm -f <id>` to remove it.
|
|
|
|
1. Start a new container using the same command from above.
|
|
|
|
1. Open the app. You should see your items still in your list!
|
|
|
|
1. Go ahead and remove the container when you're done checking out your list.
|
|
|
|
Hooray! You've now learned how to persist data!
|
|
|
|
!!! info "Pro-tip"
|
|
While named volumes and bind mounts (which we'll talk about in a minute) are the two main types of volumes supported
|
|
by a default Docker engine installation, there are many volume driver plugins available to support NFS, SFTP, NetApp,
|
|
and more! This will be especially important once you start running containers on multiple hosts in a clustered
|
|
environment with Swarm, Kubernetes, etc.
|
|
|
|
## Diving into our Volume
|
|
|
|
A lot of people frequently ask "Where is Docker _actually_ storing my data when I use a named volume?" If you want to know,
|
|
you can use the `docker volume inspect` command.
|
|
|
|
```bash
|
|
docker volume inspect todo-db
|
|
[
|
|
{
|
|
"CreatedAt": "2019-09-26T02:18:36Z",
|
|
"Driver": "local",
|
|
"Labels": {},
|
|
"Mountpoint": "/var/lib/docker/volumes/todo-db/_data",
|
|
"Name": "todo-db",
|
|
"Options": {},
|
|
"Scope": "local"
|
|
}
|
|
]
|
|
```
|
|
|
|
The `Mountpoint` is the actual location on the disk where the data is stored. Note that on most machines, you will
|
|
need to have root access to access this directory from the host. But, that's where it is!
|
|
|
|
!!! info "Accessing Volume data directly on Docker Desktop"
|
|
While running in Docker Desktop, the Docker commands are actually running inside a small VM on your machine.
|
|
If you wanted to look at the actual contents of the Mountpoint directory, you would need to first get inside
|
|
of the VM.
|
|
|
|
## Recap
|
|
|
|
At this point, we have a functioning application that can survive restarts! We can show it off to our investors and
|
|
hope they can catch our vision!
|
|
|
|
However, we saw earlier that rebuilding images for every change takes quite a bit of time. There's got to be a better
|
|
way to make changes, right? With bind mounts (which we hinted at earlier), there is a better way! Let's take a look at that now!
|