Hydra is a lightweight vocabulary that enables the creation of hypermedia-driven web APIs by enriching data with machine-readable semantics. It has two main building blocks - JSON-LD as the serialization format and the Hydra Core Vocabulary, which is a small extension of RDF Schema that describes application domains and API behaviors. The Hydra project provides specifications, implementations, and a community for developing APIs based on this standard.

1. Hydra: Hypermedia-
Driven Web APIs
MARIUS MÖSSMER
APRIL 30, 2014

2. Abstract
 Hydra
 is a lightweight vocabulary to create hypermedia-driven Web
APIs
 it enables the creation of generic API clients
 Authors
 Markus Lanthaler, Graz University of Technology
 Christian Gütl, Curtin University of Technology

3. Basic ideas of Hydra
 With Hydra, a server is able to publish all possible state transitions a client
can perform on the server
 Data is enriched with machine-readable semantics which enable
interaction
 Hydra APIs base upon
 REST architectural style
 HTTP

4. Building blocks of Hydra
 Hydra consists of two fundamental parts:
 JSON-LD
 It’s the serialization format used in the communication between the server and its
clients
 Hydra Core Vocabulary
 represents the shared vocabulary between server and client

5. JSON-LD
 http://www.w3.org/TR/json-ld/
 JSON-LD stands for Javascript Object Notation – Linked Data
 JSON-LD is an extension of JSON
 JSON-LD is a serialization-format
 Use-cases:
 Enables the expression of linked data
 Makes data self-descriptive

6. JSON-LD
Simple JSON-document that represents a user-entity:

7. JSON-LD
Extended JSON-LD-document that represents a user-entity with meaningful
semantics:

8. Hydra Core Vocabulary
 A small vocabulary extending RDF Schema ( http://www.w3.org/TR/rdf-
schema/ )
 Hydra Core Vocabulary is used to
 describe an application-domain with semantic aspects
 express the behaviour of the Web-API

9. Hydra Core Vocabulary
 Simple example: issue-tracking-system
 GET http://example.com/issues/123
What‘s the model of comments
or how to add comments ?

10. Hydra Core
Vocabulary
 GET http://example.com/api/vocab
Model of a comment-entity

11. Hydra Core
Vocabulary
 GET http://example.com/api/vocab
Supported Operations

12. Hydra Core
Vocabulary
Full Overview

13. Hydra Project
 Hydra W3C Community Group (http://www.w3.org/community/hydra/)
 Mailing List
 Wiki
 Chat
 Implementations
 HydraBundle: a bundle for Symfony2 to create Web APIs based on Hydra
 HydraConsole: a generic API console for Hydra-powered Web APIs
 HydraClient: a PHP client library to access Hydra-powered Web APIs

14. Sources
 http://www.hydra-cg.com/spec/latest/core/
 http://www.w3.org/TR/json-ld/
 http://www.w3.org/TR/rdf-schema/
 http://www.markus-lanthaler.com/hydra/
 M. Lanthaler and C. Gütl, “Hydra: A Vocabulary for Hypermedia-Driven
Web APIs”, 2013