Extension Backend Details

## The backend Alongside a frontend application, extensions can also contain one or many backend services. In most cases, the Extension does not need a backend, and features can be implemented just by invoking docker commands through the SDK. However, there are some cases when an extension requires a backend service, for example: - To run long-running processes that must outlive the frontend - To store data in a local database and serve them back with a REST API - To store the extension state, like when a button starts a long-running process, so that if you navigate away from the extension and come back, the frontend can pick up where it left off - To access specific resources in the Docker Desktop VM, for example by mounting folders in the compose file > [!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. Usually, the backend is made of one container that runs within the Docker Desktop VM. Internally, Docker Desktop creates a Docker Compose project, creates the container from the `image` option of the `vm` section of the `metadata.json`, and attaches it to the Compose project. See the [ui metadata section](metadata.md#vm-section) for more details. In some cases, a `compose.yaml` file can be used instead of an `image`. This is useful when the backend container needs more specific options, such as mounting volumes or requesting [capabilities](https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities) that can't be expressed just with a Docker image. The `compose.yaml` file can also be used to add multiple containers needed by the extension, like a database or a message broker. Note that, if the Compose file defines many services, the SDK can only contact the first of them. > [!NOTE] > > In some cases, it is useful to also interact with the Docker engine from the backend. > See [How to use the Docker socket](../guides/use-docker-socket-from-backend.md) from the backend. To communicate with the backend, the Extension SDK provides [functions](../dev/api/backend.md#get) to make `GET`, `POST`, `PUT`, `HEAD`, and `DELETE` requests from the frontend. Under the hood, the communication is done through a socket

Extensions can include a backend service, which is typically a container running within the Docker Desktop VM. While extensions can often function without a backend by using Docker commands through the SDK, backends are useful for long-running processes, data storage, maintaining extension state, and accessing resources in the Docker Desktop VM. Docker Desktop creates a Docker Compose project for the backend container, based on either an image or a `compose.yaml` file. The Extension SDK provides functions for the frontend to communicate with the backend via HTTP requests.