Query Operations

Query operations are a key element in comunica for query evaluation. These operations are based on SPARQL Algebra, and use the TypeScript types provided by SPARQLAlgebra.js.

@comunica/bus-query-operation is the bus on which actors can subscribe to perform certain query operations. @comunica/actor-query-operation-quadpattern is for instance responsible for evaluating quad patterns.

Operations take a certain operation as input, together with an optional context. The output is a BindingsStream, a set of variable names that occur in the bindings stream and optional metadata. The input context and output metadata are optional hashes that can contain additional information that can help certain actors. We describe all currently available context and metadata entries hereafter.

Context Entries

Name	Related Actors	Description
sources	@comunica/actor-rdf-resolve-quad-pattern-qpf	A list of source objects and their type, e.g. { type: 'hypermedia', value: 'http://fragments.dbpedia.org/2016-04/en'}

Currently supported types are sparql, hypermedia (for TPF and similar variants), hdt (for a local HDT file) and file (for a local turtle file).

Metadata Entries

Name	Related Actors	Description
searchForms	@comunica/actor-rdf-resolve-quad-pattern-qpf, @comunica/actor-rdf-metadata-extract-hydra-controls	The available Hydra search forms on a certain page.
totalItems	@comunica/actor-rdf-resolve-quad-pattern-qpf, @comunica/actor-rdf-metadata-extract-hydra-count	An estimate of the total number of items in a collection.