  The methods to access large relational databases in a distributed system are
well established: the relational query language SQL often serves as a language
for data access and manipulation, and in addition public interfaces are exposed
using communication protocols like REST. Similarly to REST, GraphQL is the
query protocol of an application layer developed by Facebook. It provides a
unified interface between the client and the server for data fetching and
manipulation. Using GraphQL's type system, it is possible to specify data
handling of various sources and to combine, e.g., relational with NoSQL
databases. In contrast to REST, GraphQL provides a single API endpoint and
supports flexible queries over linked data.
  GraphQL can also be used as an interface for deductive databases. In this
paper, we give an introduction of GraphQL and a comparison to REST. Using
language features recently added to SWI-Prolog 7, we have developed the Prolog
library GraphQL.pl, which implements the GraphQL type system and query syntax
as a domain-specific language with the help of definite clause grammars (DCG),
quasi quotations, and dicts. Using our library, the type system created for a
deductive database can be validated, while the query system provides a unified
interface for data access and introspection.
