  GraphQL is a novel query language proposed by Facebook to implement Web-based
APIs. In this paper, we present a practical study on migrating API clients to
this new technology. First, we conduct a grey literature review to gain an
in-depth understanding on the benefits and key characteristics normally
associated to GraphQL by practitioners. After that, we assess such benefits in
practice, by migrating seven systems to use GraphQL, instead of standard
REST-based APIs. As our key result, we show that GraphQL can reduce the size of
the JSON documents returned by REST APIs in 94% (in number of fields) and in
99% (in number of bytes), both median results.
