Ruben Taelman

Continuous Client-Side Query Evaluation over Dynamic Linked Data

Existing solutions to query dynamic Linked Data sources extend the sparql language, and require continuous server processing for each query. Traditional sparql endpoints already accept highly expressive queries, so extending these endpoints for time-sensitive queries increases the server cost even further. To make continuous querying over dynamic Linked Data more affordable, we extend the low-cost Triple Pattern Fragments (tpf) interface with support for time-sensitive queries. In this paper, we introduce the tpf Query Streamer that allows clients to evaluate sparql queries with continuously updating results. Our experiments indicate that this extension significantly lowers the server complexity, at the expense of an increase in the execution time per query. We prove that by moving the complexity of continuously evaluating queries over dynamic Linked Data to the clients and thus increasing bandwidth usage, the cost at the server side is significantly reduced. Our results show that this solution makes real-time querying more scalable for a large amount of concurrent clients when compared to the alternatives.

Continuously Self-Updating Query Results over Dynamic Heterogeneous Linked Data

Our society is evolving towards massive data consumption from heterogeneous sources, which includes rapidly changing data like public transit delay information. Many applications that depend on dynamic data consumption require highly available server interfaces. Existing interfaces involve substantial costs to publish rapidly changing data with high availability, and are therefore only possible for organisations that can afford such an expensive infrastructure. In my doctoral research, Ii¾źinvestigate how to publish and consume real-time and historical Linked Data on a large scale. To reduce server-side costs for making dynamic data publication affordable, Ii¾źwill examine different possibilities to divide query evaluation between servers and clients. This paper discusses the methods Ii¾źaim to follow together with preliminary results and the steps required to use this solution. An initial prototype achieves significantly lower server processing cost per query, while maintaining reasonable query execution times and client costs. Given these promising results, I feel confident this research direction is a viable solution for offering low-cost dynamic Linked Data interfaces as opposed to the existing high-cost solutions.

OSTRICH : versioned random-access triple store

The Linked Open Data cloud is evergrowing and many datasets are frequently being updated. In order to fully exploit the potential of the information that is available in and over historical dataset versions, such as discovering evolution of taxonomies or diseases in biomedical datasets, we need to be able to store and query the different versions of Linked Datasets efficiently. In this demonstration, we introduce OSTRICH, which is an efficient triple store with supported for versioned query evaluation. We demonstrate the capabilities of OSTRICH using a Web-based graphical user interface in which a store can be opened or created. Using this interface, the user is able to query in, between, and over different versions, ingest new versions, and retrieve summarizing statistics.

PoDiGG: A Public Transport RDF Dataset Generator

A large amount of public transport data is made available by many different providers, which makes RDF a great method for integrating these datasets. Furthermore, this type of data provides a great source of information that combines both geospatial and temporal data. These aspects are currently undertested in RDF data management systems, because of the limited availability of realistic input datasets. In order to bring public transport data to the world of benchmarking, we need to be able to create synthetic variants of this data. In this paper, we introduce a dataset generator with the capability to create realistic public transport data. This dataset generator, and the ability to configure it on different levels, makes it easier to use public transport data for benchmarking with great flexibility.

Declaratively Describing Responses of Hypermedia-Driven Web APIs

While humans browse the Web by following links, these hypermedia links can also be used by machines for browsing. While efforts such as Hydra semantically describe the hypermedia controls on Web interfaces to enable smarter interface-agnostic clients, they are largely limited to the input parameters to interfaces, and clients therefore do not know what response to expect from these interfaces. In order to convey such expectations, interfaces need to declaratively describe the response structure of their parameterized hypermedia controls. We therefore explored techniques to represent this parameterized response structure in a generic but expressive way. In this work, we discuss four different approaches for declaring a response structure, and we compare them based on a model that we introduce. Based on this model, we conclude that a SHACL shape-based approach can be used for declaring such a parameterized response structure, as it conforms to the REST architectural style that has helped shape the Web into its current form.

Exposing RDF archives using triple pattern fragments

Linked Datasets typically change over time, and knowledge of this historical information can be useful. This makes the storage and querying of Dynamic Linked Open Data an important area of research. With the current versioning solutions, publishing Dynamic Linked Open Data at Web-Scale is possible, but too expensive. We investigate the possibility of using the low-cost Triple Pattern Fragments (tpf) interface to publish versioned Linked Open Data. In this paper, we discuss requirements for supporting versioning in the tpf framework, on the level of the interface, storage and client, and investigate which trade-offs exist. These requirements lay the foundations for further research in the area of low-cost, Web-Scale dynamic Linked Open Data publication and querying.

A Preliminary Open Data Publishing Strategy for Live Data in Flanders

For smart decision making, user agents need live and historic access to open data from sensors installed in the public domain. In contrast to a closed environment, for Open Data and federated query processing algorithms, the data publisher cannot anticipate in advance on specific questions, nor can it deal with a bad cost-efficiency of the server interface when data consumers increase. When publishing observations from sensors, different fragmentation strategies can be thought of depending on how the historic data needs to be queried. Furthermore, both publish/subscribe and polling strategies exist to publish live updates. Each of these strategies come with their own trade-offs regarding cost-efficiency of the server-interface, user-perceived performance and cpu use. A polling strategy where multiple observations are published in a paged collection was tested in a proof of concept for parking spaces availability. In order to understand the different resource trade-offs presented by publish/subscribe and polling publication strategies, we devised an experiment on two machines, for a scalability test. The preliminary results were inconclusive and suggest more large scale tests are needed in order to see a trend. While the large-scale tests will be performed in future work, the proof of concept helped to identify the technical Open Data principles for the 13 biggest cities in Flanders.

Knowledge Representation as Linked Data: Tutorial

The process of extracting, structuring, and organizing knowledge requires processing large and originally heterogeneous data sources. Offering existing data as Linked Data increases its shareability, extensibility, and reusability. However, using Linking Data as a means to represent knowledge can be easier said than done. In this tutorial, we elaborate on how to semantically annotate data, and generate and publish Linked Data. We introduce [R2]RML languages to generate Linked Data. We also show how to easily publish Linked Data on the Web as Triple Pattern Fragments. As a result, participants, independently of their knowledge background, can model, annotate and publish Linked Data on their own.

Versioned Querying with OSTRICH and Comunica in MOCHA 2018

In order to exploit the value of historical information in Linked Datasets, we need to be able to store and query different versions of such datasets efficiently. The 2018 edition of the Mighty Storage Challenge (MOCHA) is organized to discover the efficiency of such Linked Data stores and to detect their bottlenecks. One task in this challenge focuses on the storage and querying of versioned datasets, in which we participated by combining the OSTRICH triple store and the Comunica SPARQL engine. In this article, we briefly introduce our system for the versioning task of this challenge. We present the evaluation results that show that our system achieves fast query times for the supported queries, but not all queries are supported by Comunica at the time of writing. These results of this challenge will serve as a guideline for further improvements to our system.

Generating public transport data based on population distributions for RDF benchmarking

When benchmarking RDF data management systems such as public transport route planners, system evaluation needs to happen under various realistic circumstances, which requires a wide range of datasets with different properties. Real-world datasets are almost ideal, as they offer these realistic circumstances, but they are often hard to obtain and inflexible for testing. For these reasons, synthetic dataset generators are typically preferred over real-world datasets due to their intrinsic flexibility. Unfortunately, many synthetic dataset that are generated within benchmarks are insufficiently realistic, raising questions about the generalizability of benchmark results to real-world scenarios. In order to benchmark geospatial and temporal RDF data management systems such as route planners with sufficient external validity and depth, we designed PODiGG, a highly configurable generation algorithm for synthetic public transport datasets with realistic geospatial and temporal characteristics comparable to those of their real-world variants. The algorithm is inspired by real-world public transit network design and scheduling methodologies. This article discusses the design and implementation of PODiGG and validates the properties of its generated datasets. Our findings show that the generator achieves a sufficient level of realism, based on the existing coherence metric and new metrics we introduce specifically for the public transport domain. Thereby, PODiGG provides a flexible foundation for benchmarking RDF data management systems with geospatial and temporal data.