@article{VerborghVanderSandeHartigetal.2016,
  author    = {Verborgh, Ruben and Vander Sande, Miel and Hartig, Olaf and Van Herwegen, Joachim and De Vocht, Laurens and De Meester, Ben and Haesendonck, Gerald and Colpaert, Pieter},
  title     = {Triple Pattern Fragments: A low-cost knowledge graph interface for the Web},
  series = {Web semantics : science, services and agents on the World Wide Web},
  volume    = {37-38},
  journal   = {Web semantics : science, services and agents on the World Wide Web},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {1570-8268},
  doi       = {10.1016/j.websem.2016.03.003},
  pages     = {184 -- 206},
  year      = {2016},
  abstract  = {Billions of Linked Data triples exist in thousands of RDF knowledge graphs on the Web, but few of those graphs can be queried live from Web applications. Only a limited number of knowledge graphs are available in a queryable interface, and existing interfaces can be expensive to host at high availability. To mitigate this shortage of live queryable Linked Data, we designed a low-cost Triple Pattern Fragments interface for servers, and a client-side algorithm that evaluates SPARQL queries against this interface. This article describes the Linked Data Fragments framework to analyze Web interfaces to Linked Data and uses this framework as a basis to define Triple Pattern Fragments. We describe client-side querying for single knowledge graphs and federations thereof. Our evaluation verifies that this technique reduces server load and increases caching effectiveness, which leads to lower costs to maintain high server availability. These benefits come at the expense of increased bandwidth and slower, but more stable query execution times. These results substantiate the claim that lightweight interfaces can lower the cost for knowledge publishers compared to more expressive endpoints, while enabling applications to query the publishers' data with the necessary reliability.},
  language  = {en}
}
@article{HartigPirro2017,
  author    = {Hartig, Olaf and Pirr{\`o}, Giuseppe},
  title     = {SPARQL with property paths on the Web},
  series = {Semantic web},
  volume    = {8},
  journal   = {Semantic web},
  number    = {6},
  publisher = {IOS Press},
  address   = {Amsterdam},
  issn      = {1570-0844},
  doi       = {10.3233/SW-160237},
  pages     = {773 -- 795},
  year      = {2017},
  abstract  = {Linked Data on the Web represents an immense source of knowledge suitable to be automatically processed and queried. In this respect, there are different approaches for Linked Data querying that differ on the degree of centralization adopted. On one hand, the SPARQL query language, originally defined for querying single datasets, has been enhanced with features to query federations of datasets; however, this attempt is not sufficient to cope with the distributed nature of data sources available as Linked Data. On the other hand, extensions or variations of SPARQL aim to find trade-offs between centralized and fully distributed querying. The idea is to partially move the computational load from the servers to the clients. Despite the variety and the relative merits of these approaches, as of today, there is no standard language for querying Linked Data on theWeb. A specific requirement for such a language to capture the distributed, graph-like nature of Linked Data sources on the Web is a support of graph navigation. Recently, SPARQL has been extended with a navigational feature called property paths (PPs). However, the semantics of SPARQL restricts the scope of navigation via PPs to single RDF graphs. This restriction limits the applicability of PPs for querying distributed Linked Data sources on the Web. To fill this gap, in this paper we provide formal foundations for evaluating PPs on the Web, thus contributing to the definition of a query language for Linked Data. We first introduce a family of reachability-based query semantics for PPs that distinguish between navigation on the Web and navigation at the data level. Thereafter, we consider another, alternative query semantics that couples Web graph navigation and data level navigation; we call it context-based semantics. Given these semantics, we find that for some PP-based SPARQL queries a complete evaluation on the Web is not possible. To study this phenomenon we introduce a notion of Web-safeness of queries, and prove a decidable syntactic property that enables systems to identify queries that areWeb-safe. In addition to establishing these formal foundations, we conducted an experimental comparison of the context-based semantics and a reachability- based semantics. Our experiments show that when evaluating a PP-based query under the context-based semantics one experiences a significantly smaller number of dereferencing operations, but the computed query result may contain less solutions.},
  language  = {en}
}