Documenting APIs


Further guidance on using the Project Open Data Metadata Schema guidelines to document application programming interfaces (APIs) in data.json files for hosting at


Agencies should list private, internal, and public-facing web APIs as part of their Enterprise Data Inventory and Public Data Listing data.json files. The original v1.0 schema accommodated the listing of APIs with a separate webServices field, but with the v1.1 schema, APIs should now be listed as a separate distribution within a dataset. Since an API provides indirect access to a dataset and the primary URL that a user needs in order to access the API is the API documentation, all web APIs should be referenced using the primary API documentation URL as an accessURL with the format specified as “API”. The downloadURL and mediaType fields should be left empty for a distribution that describes an API, but if the raw data download that powers the API is also available (as it should be) then that should be specified using the downloadURL and mediaType of another distribution entry on the same dataset.

Machine Readable API Documentation

For APIs that also have machine readable documentation (like Swagger, RAML, API Blueprint, HAL, Hydra, etc) it can be specified with describedBy and describedByType. The URL for the machine readable documentation should be specified by describedBy and describedByType should be a media type that identifies the format of the machine readable documentation.

Media Types for Machine Readable API Documentation

These media types should be used for the describedByType field along with the URL to the machine readable documentation itself using the describedBy field. This should not be confused with the format field which should be “API” for an API and mediaType which should be blank for an API.

  • The media type for Swagger has been proposed as application/swagger+json
  • The media type for RAML has been defined as application/raml+yaml
  • The media type for API Blueprint has been defined as application/vnd.apiblueprint.ast in abstract syntax tree form, plus others depending on the serialization
  • The media type for HAL has been defined as application/hal+json and application/hal+xml for the JSON and XML variants.
  • The media type for Hydra has been proposed as application/ld+json with rel="" included in the Link HTTP header.

Reference Standardized API Specifications

Some APIs may implement a common standard such as WMS, WFS, or Open311. If this is an established standard, use the canonical URI for the standard as the value for the conformsTo field in the same distribution object where the API is listed.

Example of an API listing in a data.json file

"distribution": [
        "accessURL": "",
        "description": "A fully queryable REST API with JSON and XML output",
        "describedBy": "",
        "describedByType": "application/swagger+json",
        "format": "API",
        "title": "Vegetables REST API"