Overview of RESTful API Description Languages

RESTful (REpresentational State Transfer) API (Application Programming Interface) DLs (Description Languages) are formal languages designed to provide a structured description of a RESTful web API that is useful both to a human and for automated machine processing. API Description Languages are sometimes called interface description languages (IDLs). The structured description might be used to generate documentation for human programmers; such documentation may be easier to read than free-form documentation, since all documentation generated by the same tool follows the same formatting conventions. Additionally, the description language is usually precise enough to allow automated generation of various software artifacts, like libraries, to access the API from various programming languages, which takes the burden of manually creating them off the programmers.

History

There are two previous major description languages: WSDL2.0 (Web Services Description Language) and WADL (Web Application Description Language). Neither is widely adopted in the industry for describing RESTful APIs, citing poor human readability of both and WADL being actually unable to fully describe a RESTful API.^[1]

Alternatives

Hypertext-driven APIs

An alternative approach to building RESTful APIs is known under the acronym HATEOAS (Hypermedia as the Engine of Application State). In this approach, the client software is not written to a static interface description shared through documentation. Instead, the client is given a set of entry points and the API is discovered dynamically through interaction with these endpoints. HATEOAS was introduced in Roy Fielding's doctoral thesis Architectural Styles and the Design of Network-based Software Architectures. HATEOAS has been the original vision for RESTful APIs which distinguished them from RPC mechanisms.^[2]

List of RESTful API DLs

• Web Services Description Language (WSDL)
• Web Application Description Language (WADL)
• CloudRail
  • URL: http://cloudrail.com/
  • developer: licobo GmbH
• Google Cloud Endpoints
  • URL: https://cloud.google.com/endpoints/, https://developers.google.com/discovery
  • developer: Google
• Open Data Protocol (OData)
  • OASIS standard^[3]
  • URL: http://www.odata.org/
  • developer: Microsoft
• OpenAPI Specification
  • URL: https://openapis.org/
  • developer: Open API Initiative (OAI)
• RESTful Service Description Language (RSDL)
  • URL: http://www.balisage.net/Proceedings/vol10/html/Robie01/BalisageVol10-Robie01.html
  • developer: Michael Pasternak
• RESTful API Modeling Language (RAML)
  • URL: http://raml.org/
  • developer: Mulesoft, http://www.mulesoft.com/
• Hypermedia
• API Blueprint
  • URL: https://apiblueprint.org/
  • developer Apiary, https://apiary.io/company
• I/O Docs
  • URL: https://github.com/mashery/iodocs
  • developer: Mashery, http://www.mashery.com/
• Apache Avro
• Barrister
  • URL: http://barrister.bitmechanic.com/
  • developer: James Cooper^[4]

List of data description languages

A significant part of RESTful API description is the specification of returned data structures. The IDL might either specify its own format or use an existing data description format. A notable example which many RESTful API DLs use is JSON Schema.

• json:api
  • http://jsonapi.org/
  • Started as REST adapter for Ember Data
• JSON Schema
  • used by OpenAPI, Google APIs Discovery,^[5] I/O Docs
• Apache Avro
  • https://avro.apache.org/
  • both Interface Description Language and data description language
• JSON-RPC 2.0
  • used by Barrister

Comparison of RESTful API DLs

The community around RESTful API DLs is vibrant and the landscape is still changing. According to a presentation by Akana, the most active projects in this area are OpenAPI, RAML and API Blueprint.^[1]

	Sponsor	Initial commit	Latest stable release	Stable release date	Software license[6]	Format	Open Source	Code generation (client)	Code generation (server)
RAML	MuleSoft	September, 2013				YAML	Yes	limited	Yes
API Blueprint	Apiary	April, 2013				Markdown	Yes		Yes
OpenAPI	Open API Initiative (OAI)	July, 2011				JSON	Yes	Yes	Yes

Frameworks

Many server frameworks interoperate with one or more IDLs.

• Gugamarket REST API framework
  • IDLs: OpenAPI
  • URL: https://pliik.github.io/gugamarket/
  • developer: https://github.com/pliik

References

1. http://www.slideshare.net/SOA_Software/api-description-languages
2. Fielding, Roy. "REST APIs must be hypertext-driven". Retrieved 4 November 2015.
3. "OASIS Open Data Protocol (OData) TC - OASIS".
4. "Barrister RPC - About".
5. https://developers.google.com/discovery/v1/reference/apis
6. Licenses here are a summary, and are not taken to be complete statements of the licenses. Some packages may use libraries under different licenses.

External links

• Investigating Api Developer Tooling