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Comparison with GraphQL and Other API Formats

Several API formats and architecture including GraphQL, JSON:API and HAL try to solve the under-fetching problem by allowing the client to ask the server to embed related resources in the main HTTP response. Basically, the server will create a big JSON document containing the main requested resource and all its requested relations as nested objects. This solution allows to limit costly round-trip between the client and the server, and is (almost) the only efficient solution when using HTTP/1. However, this hack introduces unnecessary complexity and hurts HTTP cache mechanisms. Also, it isn't necessary anymore with HTTP/2!

By using HTTP/2 Server Push, Vulcain fixes most problems caused by compound documents and sparse fieldsets based formats such as GraphQL and JSON:API:

  • Because each pushed resource is sent in a separate HTTP/2 stream (HTTP/2 multiplexing), related resources can be sent in parallel to the client.
  • While embedding resources is a forced push (the client receive the full JSON documents, even if it already has some parts of it), HTTP/2 Server Push allows the client to cancel the push of resources it already has, saving bandwidth and improving performance.
  • Consequently, clients and network intermediates (such as Varnish cache), can store each resource in a specific cache, while resource embedding only allows to have the full big JSON document in cache, cache invalidation is then more efficient with Vulcain, and can be done at the HTTP level.

Specifically with GraphQL, using cache mechanisms provided by the HTTP protocol isn't easy (POST requests cannot be cached).

Using GraphQL as Query Language for Vulcain

As stated in its name, GraphQL is foremost a convenient Query Language for APIs. Guess what, GraphQL, the query language, is usable as-is with Vulcain servers!

The main idea is to write GraphQL queries client-side, which will be converted in REST requests containing Vulcain headers by a dedicated JavaScript library.

To do so, libraries such as apollo-link-rest can be used. Thanks to apollo-link-rest you can write your request in GraphQL, use all the tools of the frontend ecosystem relying on GraphQL, but let the library send REST requests to the Vulcain server to fulfill the GraphQL query.

This approach also fixes all the problems coming with using GraphQL server-side!

Note: a higher-level library dedicated to Vulcain is being written.

Type System and Introspection

Vulcain focuses on solving the under-fetching and the over-fetching problems. It's out of Vulcain's scope to provide a type system and an introspection mechanism. However, Vulcain has been designed to play very well with existing formats providing these capabilities.

For hypermedia APIs, we strongly recommend to use W3C's JSON-LD along with the Hydra Core Vocabulary. For less advanced non-hypermedia APIs, we recommend OpenAPI (formerly known as Swagger).


Vulcain doesn't provide a way to push new versions of resources in real-time, however Vulcain plays very well with Mercure (created by the same author), a modern and RESTful replacement for WebSockets and GraphQL subscriptions.