@@ -1,5 +1,42 @@

-This directory contains code pertaining to the Docker API:

+# Working on the Engine API

- - Used by the docker client when communicating with the docker daemon

+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.

- - Used by third party tools wishing to interface with the docker 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. To automatically generate documentation.

+2. To 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, you'll need to 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 for when you are 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.

+

+All the documentation generation is done in the documentation repository, [docker/docker.github.io](https://github.com/docker/docker.github.io). The Swagger definition is vendored periodically into this repository, but you can manually copy over the Swagger definition to test changes.

+

+Copy `api/swagger.yaml` in this repository to `engine/api/[VERSION_NUMBER]/swagger.yaml` in the documentation repository, overwriting what is already there. Then, run `docker-compose up` in the documentation repository and browse to [http://localhost:4000/engine/api/](http://localhost:4000/engine/api/) when it finishes rendering.