Add a backend to your extension

Your extension can ship a backend part with which the frontend can interact with. This page provides information on why and how to add a backend.

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

Tip

Check the Quickstart guide and docker extension init <my-extension>. They provide a better base for your extension as it's more up-to-date and related to your install of Docker Desktop.

Why add a backend?

Thanks to the Docker Extensions SDK, most of the time you should be able to do what you need from the Docker CLI directly from the frontend.

Nonetheless, there are some cases where you might need to add a backend to your extension. So far, extension builders have used the backend to:

• Store data in a local database and serve them back with a REST API.
• Store the extension state, for example when a button starts a long-running process, so that if you navigate away from the extension user interface and comes back, the frontend can pick up where it left off.

For more information about extension backends, see Architecture.

Add a backend to the extension

If you created your extension using the docker extension init command, you already have a backend setup. Otherwise, you have to first create a vm directory that contains the code and updates the Dockerfile to containerize it.

Here is the extension folder structure with a backend:

.
├── Dockerfile # (1)
├── Makefile
├── metadata.json
├── ui
    └── index.html
└── vm # (2)
    ├── go.mod
    └── main.go

1. Contains everything required to build the backend and copy it in the extension's container filesystem.
2. The source folder that contains the backend code of the extension.

Although you can start from an empty directory or from the vm-ui extension sample, it is highly recommended that you start from the docker extension init command and change it to suit your needs.

Tip

The docker extension init generates a Go backend. But you can still use it as a starting point for your own extension and use any other language like Node.js, Python, Java, .Net, or any other language and framework.

In this tutorial, the backend service simply exposes one route that returns a JSON payload that says "Hello".

{ "Message": "Hello" }

Important

We recommend that, the frontend and the backend communicate through sockets, and named pipes on Windows, instead of HTTP. This prevents port collision with any other running application or container running on the host. Also, some Docker Desktop users are running in constrained environments where they can't open ports on their machines. When choosing the language and framework for your backend, make sure it supports sockets connection.

package main

import (
	"flag"
	"log"
	"net"
	"net/http"
	"os"

	"github.com/labstack/echo"
	"github.com/sirupsen/logrus"
)

func main() {
	var socketPath string
	flag.StringVar(&socketPath, "socket", "/run/guest/volumes-service.sock", "Unix domain socket to listen on")
	flag.Parse()

	os.RemoveAll(socketPath)

	logrus.New().Infof("Starting listening on %s\n", socketPath)
	router := echo.New()
	router.HideBanner = true

	startURL := ""

	ln, err := listen(socketPath)
	if err != nil {
		log.Fatal(err)
	}
	router.Listener = ln

	router.GET("/hello", hello)

	log.Fatal(router.Start(startURL))
}

func listen(path string) (net.Listener, error) {
	return net.Listen("unix", path)
}

func hello(ctx echo.Context) error {
	return ctx.JSON(http.StatusOK, HTTPMessageBody{Message: "hello world"})
}

type HTTPMessageBody struct {
	Message string
}

Adapt the Dockerfile

Note

When using the docker extension init, it creates a Dockerfile that already contains what is needed for a Go backend.

# syntax=docker/dockerfile:1
FROM node:17.7-alpine3.14 AS client-builder
# ... build frontend application

# Build the Go backend
FROM golang:1.17-alpine AS builder
ENV CGO_ENABLED=0
WORKDIR /backend
COPY vm/go.* .
RUN --mount=type=cache,target=/go/pkg/mod \
    --mount=type=cache,target=/root/.cache/go-build \
    go mod download
COPY vm/. .
RUN --mount=type=cache,target=/go/pkg/mod \
    --mount=type=cache,target=/root/.cache/go-build \
    go build -trimpath -ldflags="-s -w" -o bin/service

FROM alpine:3.15
# ... add labels and copy the frontend application

COPY --from=builder /backend/bin/service /
CMD /service -socket /run/guest-services/extension-allthethings-extension.sock

Configure the metadata file

To start the backend service of your extension inside the VM of Docker Desktop, you have to configure the image name in the vm section of the metadata.json file.

{
  "vm": {
    "image": "${DESKTOP_PLUGIN_IMAGE}"
  },
  "icon": "docker.svg",
  "ui": {
    ...
  }
}

For more information on the vm section of 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.

Invoke the extension backend from your frontend

Using the advanced frontend extension example, we can invoke our extension backend.

Use the Docker Desktop Client object and then invoke the /hello route from the backend service with ddClient. extension.vm.service.get that returns the body of the response.

// ui/src/App.tsx
import React, { useEffect } from 'react';
import { createDockerDesktopClient } from "@docker/extension-api-client";

//obtain docker desktop extension client
const ddClient = createDockerDesktopClient();

export function App() {
  const ddClient = createDockerDesktopClient();
  const [hello, setHello] = useState<string>();

  useEffect(() => {
    const getHello = async () => {
      const result = await ddClient.extension.vm?.service?.get('/hello');
      setHello(JSON.stringify(result));
    }
    getHello()
  }, []);

  return (
    <Typography>{hello}</Typography>
  );
}

Re-build the extension and update it

Since you have modified the configuration of the extension and added a stage in the Dockerfile, you must re-build the extension.

docker build --tag=awesome-inc/my-extension:latest .

Once built, you need to update it, or install it if you haven't already done so.

docker extension update awesome-inc/my-extension:latest

Now you can see the backend service running in the Containers view of the Docker Desktop Dashboard and watch the logs when you need to debug it.

Tip

You may need to turn on the Show system containers option in Settings to see the backend container running. See Show extension containers for more information.

Open the Docker Desktop Dashboard and select the Containers tab. You should see the response from the backend service call displayed.

What's next?

• Learn how to share and publish your extension.
• Learn more about extensions architecture.