Implementing GraphQL as a Query Language for Deductive Databases in SWI-Prolog Using DCGs, Quasi Quotations, and Dicts

摘要

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