Set up a minimal backend extension

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To start creating your extension, you first need a directory with files which range from the extension’s source code to the required extension-specific files. This page provides information on how to set up a simple Docker Extension that runs CLI commands in the backend.

For extensions with a backend service running REST services over sockets or named pipes, see the vm-ui extension sample.

Note

Before you start, make sure you have installed the latest version of Docker Desktop.

Note

If you want to start a codebase for your new extension, our Quickstart guide and docker extension init <my-extension> provides a better base for your extension as it is more up-to-date and related to your install of Docker Desktop.

Extension folder structure

In the minimal-backend sample folder, you can find a ready-to-go example that represents a UI extension built on HTML that runs a backend service. We will go through this code example in this tutorial.

Although you can start from an empty directory, it is highly recommended that you start from the template below and change it accordingly to suit your needs.

.
├── Dockerfile # (1)
├── Makefile
├── client # (2)
│   └── src
│       ├── App.tsx
│       └── ... React aplication
├── hello.sh # (3)
└── metadata.json # (4)
└── ui # (5)
    └── index.html
  1. Contains everything required to build the extension and run it in Docker Desktop.
  2. The source folder that contains the UI application. In this example we use a React frontend, the main part of th extension is an App.tsx.
  3. The script that runs inside the container.
  4. A file that provides information about the extension such as the name, description, and version.
  5. The source folder that contains all your HTML, CSS and JS files. There can also be other static assets such as logos and icons. For more information and guidelines on building the UI, see the Design and UI styling section.

If you want to set up user authentication for the extension, see Authentication.

Invoke the extension backend from your javascript code

Using the React extension example, we can invoke our extension backend from the App.tsx file.

Use the Docker Desktop Client object and then invoke a binary provided in our backend container (that lives inside the Docker Desktop VM) with ddClient.docker.extension.vm.cli.exec(). In our example, our hello.sh script returns a string as result, we obtain it with result?.stdout.

const ddClient = createDockerDesktopClient();
const [backendInfo, setBackendInfo] = useState<string | undefined>();

async function runExtensionBackend(inputText: string) {
  const result = await ddClient.extension.vm?.cli.exec("./hello.sh", [
    inputText,
  ]);
  setBackendInfo(result?.stdout);
}

Create a Dockerfile

At minimum, your Dockerfile needs:

  • Labels which provide extra information about the extension.
  • The source code which in this case is an index.html that sits within the ui folder. index.html refers to javascript code in script.js.
  • The metadata.json file.
FROM node:17.7-alpine3.14 AS client-builder
# ... build React application

FROM alpine:3.15

LABEL org.opencontainers.image.title="HelloBackend" \
    org.opencontainers.image.description="A sample extension that runs a shell script inside a container's Desktop VM." \
    org.opencontainers.image.vendor="Docker Inc." \
    com.docker.desktop.extension.api.version="1.0.0-beta.1" \
    com.docker.desktop.extension.icon="https://www.docker.com/wp-content/uploads/2022/03/Moby-logo.png"

COPY hello.sh .
COPY metadata.json .
COPY --from=client-builder /app/client/dist ui

CMD [ "sleep", "infinity" ]

Configure the metadata file

A metadata.json file is required at the root of the image filesystem.

{
  "vm": {
    "image": "${DESKTOP_PLUGIN_IMAGE}"
  },
  "ui": {
    "dashboard-tab": {
      "title": "Hello Backend Extension",
      "root": "/ui",
      "src": "index.html"
    }
  }
}

For more information on the metadata.json, see Metadata.

Warning

Do not replace the ${DESKTOP_PLUGIN_IMAGE} placeholder in the metadata.json file. The placeholder is replaced automatically with the correct image name when the extension is installed.

What’s next?

Learn how to build and install your extension.

Docker, extensions, sdk, build