The Openapi-HyperMedia format (OHM)

Version 0.1.0

This document is licensed under The Apache License, Version 2.0.

Introduction

The Openapi-HyperMedia format (OHM), is a format used to describe REST level 3 APIs. A crucial caracteristic of these APIs is that they use Hypermedia As The Engine Of Application State (HATEOAS) which means that all the controls that allow to go to the next application state must be sent in the server response together with the resource representation. HTML is the most well-known format permitting HATEOAS thanks to links and forms but it is not convenient for machine processing. OHM aims at providing a format readable by machines while exposing HATEOAS controls for dynamic processing and documentation.

The format

OHM relies on 2 other well-defined and well-known formats to describe a REST resource:

• JSON for the representation of the resources
• OpenAPI for the Hypermedia controls

An OHM document is itself a JSON document in the form:

{
    "content": <JSON representation of the resource>,
    "controls": <An OpenAPI Specification (OAS) in JSON format>
}

Example for a Person resource which has a link to its Address resource located at /api/addresses/1

{
    "content": {
        "firstName": "John",
        "lastName": "Doe"
    },
    "controls": {
        "openapi": "3.0.1",
        "info": {
            "title": "rest",
            "version": "0.0.1"
        },
        "paths": {
            "/api/addresses/1": {
                "get": {
                    "summary": "John Doe's address"
                }
            }
        }
    }
}

The media-type for the OHM format shall be application/ohm+json.

OHM Clients/Browsers

Open a swagger-UI based OHM browser

Being a level 3 RESTful format respecting HATEOAS, it is possible to design clients that can work with any OHM API. Since OHM uses OpenAPI for its controls, libraries that can handle this format such as Swagger-Client and even slightly modify the famous Swagger-UI can be used to browse an OHM API.

Demo OHM application

You can find a demo application implementing OHM here (source code). As a truly RESTful application, it can be run from any OHM client without any prior knowledge or specific code in the client about the application. An OHM browser based on a modified Swagger-UI is served by the application for convenience but any other OHM browser would be able to control the application.

Rationale behind OHM format

The OHM format aims at easily transitionning from HTTP-JSON APIs to more RESTful APIs. You can transform your HTTP-JSON to an OHM one by just encapsulating the response body in the content field and use it as is. Then the controls can be added progressively and but the client has no obligation to use them.

OHM has a lot of similarities with other Hypermedia formats. Its advantage is that it is based on OpenAPI, an open standard, widely adopted, with a lot of available resources and libraries.

OpenAPI has proven to be very versatile to describe any type of HTTP operations where other formats only describe safe (GET) operations or have some limitations on what they can describe.

OpenAPI helps with documenting the transitions comprehensively. So the user has better information on how to use them without the need for an external documentation.

By relying on an existing standard it makes the OHM specification very lightweight and familiar when you know OpenAPI.