[Paper Review] LDQL: A Query Language for the Web of Linked Data (Extended Version)

Olaf Hartig, Jorge Eduardo Pérez Pérez|arXiv (Cornell University)|Jul 16, 2015

Semantic Web and Ontologies2 references3 citations

TL;DR

LDQL is a declarative query language for the Web of Linked Data that separates data source selection (via link path expressions) from query result specification (via SPARQL patterns). It is strictly more expressive than prior formalisms, and the paper provides a syntactic condition ensuring Web-safety—enabling effective evaluation over distributed data sources.

ABSTRACT

The Web of Linked Data is composed of tons of RDF documents interlinked to each other forming a huge repository of distributed semantic data. Effectively querying this distributed data source is an important open problem in the Semantic Web area. In this paper, we propose LDQL, a declarative language to query Linked Data on the Web. One of the novelties of LDQL is that it expresses separately (i) patterns that describe the expected query result, and (ii) Web navigation paths that select the data sources to be used for computing the result. We present a formal syntax and semantics, prove equivalence rules, and study the expressiveness of the language. In particular, we show that LDQL is strictly more expressive than the query formalisms that have been proposed previously for Linked Data on the Web. The high expressiveness allows LDQL to define queries for which a complete execution is not computationally feasible over the Web. We formally study this issue and provide a syntactic sufficient condition to avoid this problem; queries satisfying this condition are ensured to have a procedure to be effectively evaluated over the Web of Linked Data.

Motivation & Objective

• Address the challenge of effectively querying the distributed, globally interconnected Web of Linked Data.
• Overcome limitations of existing query languages that conflate data source selection with query logic.
• Provide a formal foundation for a query language that supports scalable, distributed query execution over RDF data.
• Ensure practical feasibility of query execution by identifying syntactic conditions for Web-safety.
• Demonstrate LDQL's superior expressiveness compared to prior formalisms in the Linked Data query space.

Proposed method

• Propose LDQL as a declarative language with two core components: link path expressions for navigating the Web of Linked Data and SPARQL graph patterns for defining query results.
• Introduce a formal syntax and semantics for LDQL, including support for seed URIs, dynamic URI discovery, and query composition via conjunction, disjunction, and projection.
• Define a notion of Web-safety to identify queries that can be effectively evaluated over the Web, avoiding non-terminating or non-computable executions.
• Establish a sufficient syntactic condition for Web-safety, ensuring that queries can be evaluated in a finite and distributed manner.
• Provide formal proofs of equivalence and expressiveness, showing LDQL subsumes existing formalisms like SPARQL under reachability semantics, NautiLOD, and context-based property paths.
• Design an evaluation algorithm that dynamically discovers URIs and composes results across multiple data sources using compatible solution mappings.

Experimental results

Research questions

• RQ1 How can a query language for the Web of Linked Data effectively separate source selection from result specification?
• RQ2 What formal conditions ensure that LDQL queries can be practically executed over the distributed Web of Linked Data?
• RQ3 How does LDQL's expressiveness compare to existing formalisms for querying Linked Data on the Web?
• RQ4 Can LDQL express queries that are not expressible in prior formalisms such as SPARQL under reachability semantics or NautiLOD?
• RQ5 What syntactic properties guarantee that an LDQL query is computationally feasible to evaluate over the Web?

Key findings

• LDQL is strictly more expressive than SPARQL under reachability-based query semantics, NautiLOD, and SPARQL property path patterns under context-based semantics.
• For every query in the three prior formalisms, an equivalent LDQL query can be constructed, demonstrating its expressive completeness.
• There exist LDQL queries that cannot be expressed in any of the three prior formalisms, proving strict expressiveness superiority.
• The paper identifies a sufficient syntactic condition for Web-safety, ensuring that LDQL queries can be effectively evaluated over the Web without non-terminating or non-computable behavior.
• The evaluation algorithm for LDQL ensures correctness and finite execution by dynamically discovering seed URIs and composing results through compatible solution mappings.
• Formal proofs in the appendix establish the correctness of the semantics, equivalence rules, and Web-safety condition, supporting the language's theoretical and practical viability.