# Working on the Engine API

The Engine API is an HTTP API used by the command-line client to communicate with the daemon. It can also be used by third-party software to control the daemon.

It consists of various components in this repository:

- `api/swagger.yaml` A Swagger definition of the API.
- `api/types/` Types shared by both the client and server, representing various objects, options, responses, etc. Most are written manually, but some are automatically generated from the Swagger definition. See [#27919](https://github.com/docker/docker/issues/27919) for progress on this.
- `cli/` The command-line client.
- `client/` The Go client used by the command-line client. It can also be used by third-party Go programs.
- `daemon/` The daemon, which serves the API.

## Swagger definition

The API is defined by the [Swagger](http://swagger.io/specification/) definition in `api/swagger.yaml`. This definition can be used to:

1. Automatically generate documentation.
2. Automatically generate the Go server and client. (A work-in-progress.)
3. Provide a machine readable version of the API for introspecting what it can do, automatically generating clients for other languages, etc.

## Updating the API documentation

The API documentation is generated entirely from `api/swagger.yaml`. If you make updates to the API, edit this file to represent the change in the documentation.

The file is split into two main sections:

- `definitions`, which defines re-usable objects used in requests and responses
- `paths`, which defines the API endpoints (and some inline objects which don't need to be reusable)

To make an edit, first look for the endpoint you want to edit under `paths`, then make the required edits. Endpoints may reference reusable objects with `$ref`, which can be found in the `definitions` section.

There is hopefully enough example material in the file for you to copy a similar pattern from elsewhere in the file (e.g. adding new fields or endpoints), but for the full reference, see the [Swagger specification](https://github.com/docker/docker/issues/27919).

`swagger.yaml` is validated by `hack/validate/swagger` to ensure it is a valid Swagger definition. This is useful when making edits to ensure you are doing the right thing.

## Viewing the API documentation

When you make edits to `swagger.yaml`, you may want to check the generated API documentation to ensure it renders correctly.

Run `make swagger-docs` and a preview will be running at `http://localhost:9000`. Some of the styling may be incorrect, but you'll be able to ensure that it is generating the correct documentation.

The production documentation is generated by vendoring `swagger.yaml` into [docker/docker.github.io](https://github.com/docker/docker.github.io).