Docker - persistent database (data volume)

Every time the container is started, the previously added data will disappear

Start the getting-started container image, browser access, and add a few items to it.Let's refresh the page, or visit another browser, the items added above are still there

But if you stop the container, restart a container, and access it with a browser, you will find that the newly added items are gone

The container's filesystem

When a container runs, it uses the layers in the image as its filesystem.Each container also has its own "scratch space" to create/update/delete files.No changes will be seen in the other container, even if they use the same image.

Data Volume

Each container starts with an image definition every time it starts.While a container can create, update, and delete files, that data is lost when a container is removed and all data changes are isolated from that container.

Data volumes provide the ability to connect a container's specific filesystem path back to the host.If a directory in the container is mounted, changes in that directory will also be seen on the host.If we mount the same directory when the container restarts, we see the same files.

Data volumes are of two types: Named volumesnamed volumes, bind mounts bind mounts

Data Volume is a special directory available to one or more containers that bypasses UFS and provides many useful features:

• Data volumes can be shared and reused between containers
• Data volume changes will take effect immediately
• The modification of data volume will not affect the image
• Data volumes persist by default, even if the container is deleted

Persistent data

The data for the todo items is stored in a SQLite database in the container file system, directory: /etc/todos/todo.db

Since the database is 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 it left off last.We can persist data by creating a data volume and attaching (often called "mounting") it to the directory where the data is stored.When our container writes the todo.db file, it will be persisted to the host in the data volume.

1. Create a data volume.

$ docker volume create todo-db

2. Stop and delete the todo container again because it is still running and not using the data volume.

docker psdocker rm -f 

3. Start the To-Do project container and use -v to specify the data volume to mount.We will use this named volume and mount it to /etc/todos, which will capture all files created at that path.

You can also use the -v flag to mount multiple data volumes multiple times in a docker run command

If you don't run the docker volume create command, but use the following command directly, docker will automatically create one for us

$ docker run -dp 3000:3000 -v todo-db:/etc/todos getting-started

4. After the container starts, open the application and add some items to the to-do list.

5. Stop and delete the container of to-do items.

docker psdocker rm -f 

[External link image transfer failed, the origin site may have anti-leech mechanism, it is recommended to save the image and upload it directly (img-YGTtfRFI-1659341392848) (https://raw.githubusercontent.com/yuyun-21/uplod-images/hyh/images/Flowable/image-20220617115819573.png)]

6. Start a new container using the same command as above.

$ docker run -dp 3000:3000 -v todo-db:/etc/todos getting-started

7. Open the browser and you should see the items added earlier in the list

View all data volumes

docker volume ls

View the specified data volume

docker volume inspect todo-db

• Mountpoint is the actual location on disk where the data is stored
• Most devices require root access to access this directory from the host