Hello I'm

Julián Rojas

I’m a Linked Open Data, Semantic Web and Transportation researcher at IDLab.

A Colombian living in Belgium .


+32 9 331 49 59


Academic and peer-reviewed publications I have contributed to.

  • 2018

    A Preliminary Open Data Publishing Strategy for Live Data in Flanders

    Julián Rojas, Brecht Van de Vyvere, Arne Gevaert, Ruben Taelman, Pieter Colpaert, Ruben Verborgh,

    The Web Conference 2018 - Web Stream Processing Workshop

    Abstract - 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. read less

    Abstract - read more

  • 2018

    Supporting sustainable publishing and consuming of live Linked Time Series Streams

    Julián Rojas, Gayane Sedrakyan, Pieter Colpaert, Miel Vander Sande, Ruben Verborgh,

    ESWC2018 - Poster and Demos

    Abstract - The road to publishing public streaming data on the Web is paved with trade-offs that determine its viability. The cost of unrestricted query answering on top of data streams, may not be affordable for all data publishers. Therefore, public streams need to be funded in a sustainable fashion to remain online. In this paper we present an overview of possible query answering features for live time series in the form of multidimensional interfaces. For example, from a live parking availability data stream, pre-calculated time constrained statistical indicators or geographically classified data can be provided to clients on demand. Furthermore, we demonstrate the initial developments of a Linked Time Series server that supports such features through an extensible modular architecture. Benchmarking the costs associated to each of these features allows to weigh the trade-offs inherent to publishing live time series and establishes the foundations to create a decentralized and sustainable ecosystem for live data streams on the Web. read less

    Abstract - read more

  • 2018

    Decentralized Open Data Publishing for The Public Transport Route Planning Ecosystem

    Julián Rojas, Bert Marcelis, Eveline Vlassenroot, Pieter Colpaert, Mathias Van Compernolle, Ruben Verborgh,

    Open Data Research Symposium

    Abstract - Open data became a fundamental requirement for the route planning application ecosystem in the public transport sector. The way this data is published has a direct influence in the architectural design of route planning applications, which today tends towards centralized solutions (e.g. Google Maps, CityMapper). Linked Connections emerged as a more scalable and cost-efficient decentralized alternative for public transport open data publishing, compared to centralized approaches . However, how this impacts the different actors that belong to the route planning ecosystem is still unknown. In this work, we study and discuss the potential impact of Linked Connections through a set of evaluations that measure the technical and user-perceived performance of route planning applications that run on the client-side, compared to a traditional approach performing the route planning logic on the server-side. Results showed that (i) for some use cases, a Linked Connections based approach outperforms centralized approaches but it heavily depends on the underlying hardware. (ii) More than half of the travellers that participated on the tests preferred the Linked Connections client application due to additional features such as offline querying and privacy safeguarding regardless of the slower performance in some cases. This work provided insights and an initial assessment of the potential effects of implementing a decentralized open data publishing strategy on the public transport route planning ecosystem. The potential benefits of such an approach are aligned with the ideals of open data of fostering innovation, boosting economic growth and providing solutions for more specific necessities. read less

    Abstract - read more

  • 2017

    Providing Reliable Access to Real-Time and Historic Public Transport Data Using Linked Connections

    Julián Rojas, David Chaves Fraga, Pieter Colpaert, Ruben Verborgh, Erik Mannens,

    ISWC2017 - Posters and Demos

    Abstract - Using Linked Data based approaches, public transport companies are able to share their time tables and its updates in an affordable way while allowing user agents to perform multimodal route planning algorithms. Providing time table updates, usually published as data streams, means that data is being constantly modified and if there is a large analytical query, its response might be affected due to the changing data. In this demo we introduce a mechanism to tackle this problem by guaranteeing that a user agent will always receive version based responses, therefore ensuring data consistency. Such mechanism also enables access to historical data that could be used for deep analysis of transport systems. However, how this data shall be archived, in order to keep this ap- proach scalable and inexpensive is still a matter of study. In a demonstrator, we published and query data from the Belgium national train system (SNCB) and Madrid Regional Transport Consortium (CRTM). This paper represents the first step towards establishing an affordable framework to publish reliable transport data. read less

    Abstract - read more

  • 2017

    The tripscore Linked Data client: calculating specific summaries over large time series

    David Chaves Fraga, Julián Rojas, Pieter-Jan Vandenberghe, Pieter Colpaert, Oscar Corcho,

    ISWC2017 - Decentralizing the Semantic Web Workshop

    Abstract - Time series – such as public transport time schedules and their actual departure times – may deliver insights about the public transport network to third parties. Today, however, public transport data is published in a way in which analytical processing is too expensive. In previous work, the Linked Connections(LC) framework was introduced as a cost-efficient publishing alternative to the de-facto GTFS standard and route planning APIs. We study whether this server interface can also be used by Linked Data agents to solve analytical queries over longer periods of time. In this work, we created a serverless Linked Data client in Javascript for the analysis of time series on top of public transport data sources, called tripscore.eu. In this example, it calculates the quality of experience for your journey for the last 5 weeks using the public transport agencies it can discover. We have made the code to this proof of concept available as open source in different reusable components. As the user-perceived performance is quite slow, we formulate opportunities to achieve better response times. We could, on the one hand, suggest the data publisher to publish summaries over longer periods of time. On the other hand, we could also, as reusers, create a private summary of the data on our server and expose this to our user agents. Still an open issue is how this client would discover new public transport agencies reliably, for which we started working on a metadata profile for transport datasets. read less

    Abstract - read more


Current and past projects I have been involved in.

Work Experience

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