Ruben Verborgh, Ghent University – imec – IDLab
Publishing research on the Web accompanied by machine-readable data is one of the aims of Linked Research. Merely embedding metadata as RDFa in HTML research articles, however, does not solve the problems of accessing and querying that data. Hence, I created a simple ETL pipeline to extract and enrich Linked Data from my personal website, publishing the result in a queryable way through Triple Pattern Fragments. The pipeline is open source, uses existing ontologies, and can be adapted to other websites. In this article, I discuss this pipeline, the resulting data, and its possibilities for query evaluation on the Web. More than 35,000 RDF triples of my data are queryable, even with federated SPARQL queries because of links to external datasets. This proves that researchers do not need to depend on centralized repositories for readily accessible (meta-)data, but instead can—
The World Wide Web continues to shape many domains, and not in the least research. On the one hand, the Web beautifully fulfills its role as a distribution channel of scientific knowledge, for which it was originally invented. This spurs interesting dialogues concerning Open Access  and even piracy  of research articles. On the other hand, the advent of social networking creates new interaction opportunities for researchers, but also forces us to consider our online presence . Various social networks dedicated to research have emerged: Mendeley, ResearchGate, Academia, … They attract millions of researchers, and employ various tactics to keep us there.
A major issue of these social research networks is their lack of mutual complementarity. None of them has become a clear winner in terms of adaption. At first sight, the resulting plurality seems a blessing for diversity, compared to the monoculture of Facebook for social networking in general. Yet whereas other generic social networks such as Twitter and LinkedIn serve complementary professional purposes compared to Facebook, social research networks share nearly identical goals. As an example, a researcher could announce a newly accepted paper on Twitter, discuss its review process on Facebook, and share a photograph of an award on LinkedIn. In contrast, one would typically not exclusively list a specific publication on Mendeley and another on Academia, as neither publication list would be complete.
In practice, this results in constant bookkeeping for researchers who want each of their profiles to correctly represent them—
Online representation is not limited to social networks: scientific publishers also make metadata available about their journals and books. For instance, Springer Nature recently released SciGraph, a Linked Open Data platform that includes scholarly metadata. Accuracy is less of an issue in such cases, as data comes directly from the source. However, quality and usability are still influenced by the way data is modeled and whether or how identifiers are disambiguated. Completeness is not guaranteed, given that authors typically target multiple publishers. Therefore, even such authoritative sources do not provide individual researchers with a correct profile.
In the spirit of decentralized social networking  and Linked Data , several researchers instead started publishing their own data and metadata. I am one of them, since I believe in practicing what we preach  as Linked Data advocates, and because I want my own website to act as the main authority for my data. After all, I can spend more effort on the completeness and accuracy of my publication metadata than most other platforms could reasonably do for me. In general, self-published data typically resides in separate RDF documents  (for which the FOAF vocabulary  is particularly popular ), or inside of HTML documents (using RDFa Lite  or similar formats).
Despite the controllable quality of personally maintained research data and metadata in individual documents on the Web, they are not as visible, findable, and queryable as those of social research networks. I call a dataset interface “queryable” with respect to a given query when a consumer does not need to download the entire dataset in order to evaluate that query over it with full completeness. Unfortunately, hosting advanced search interfaces on a personal website quickly becomes complex and expensive. To mitigate this, I have implemented a simple Extract/
This article describes my use case, which resembles that of many other researchers. I detail the design and implementation of the ETL pipeline, and report on its results. At the end, I list open questions regarding self-publication, before concluding with a reflection on the opportunities for the broader research community.
Like the websites of many researchers, my personal website contains data about the following types of resources:
- people such as colleagues, collaborators, and fellow researchers
- research articles I have co-authored
- blog posts I have written
- courses I teach
- a single RDF document (FOAF profile) containing:
- manually entered data (personal data, affiliations, projects, …)
- automatically generated metadata (publications, blog posts, …)
- an HTML page with RDFa per:
- publication (publication and author metadata)
- blog post (post metadata)
- HTML article (metadata and citations)
- Friend of a Friend (FOAF) (people, documents, …)
- Schema.org (blog posts, articles, courses, …)
- Bibliographic Ontology (BIBO) (publications)
- Citation Typing Ontology (CiTO) (citations)
There is a considerable amount of overlap since much data is available in more than one place, sometimes in different vocabularies. For example, webpages about my publications contain Schema.org markup (to facilitate indexing by search engines), whereas my profile describes the same publications more rigorously using BIBO and FOAF (for more advanced RDF clients). I deliberately reuse the same identifiers for the same resources everywhere, so identification is not an issue.
While the publication of structured data as RDF and RDFa is conveniently integrated in the webpage creation process, querying information over the entire website is difficult. For instance, starting from the homepage, obtaining a list of all mentioned people on the website would be non-trivial. In general, SPARQL query execution over Linked Data takes a considerable amount of time, and completeness cannot be guaranteed . So while Linked Data documents are excellent for automated exploration of individual resources, and for aggregators such as search engines that can harvest the entire website, the possibilities of individual automated clients remain limited.
Another problem is the heterogeneity of vocabularies: clients without reasoning capabilities would only find subsets of the information, depending on which vocabulary is present in a given representation. Especially in RDFa, it would be cumbersome to combine every single occurrence of
schema:name with the semantically equivalent
foaf:name. As such, people might have a
foaf:name (because FOAF is common for people), publications a
schema:name (because of
schema:ScholarlyArticle), and neither an
rdfs:label. Depending on the kind of information, queries would thus need different predicates for the concept “label”. Similarly, queries for
schema:CreativeWork would not return results because they are not explicitly mentioned, even though their subclasses
schema:ScholarlyArticle appear frequently.
- Automated clients should be able to evaluate queries with full completeness with respect to the data on the website.
- Semantically equivalent expressions should yield the same query results, regardless of vocabulary with respect to all vocabularies used on the website.
- Queryable data can only involve a limited cost and effort for publishers as well as consumers.
To automate this process, I have developed a simple ETL pipeline. With the exception of a couple of finer points, the pipeline itself is fairly straightforward. What is surprising, however, is the impact such a simple pipeline can have, as discussed hereafter in the Results section. The pipeline consists of the following phases, which will be discussed in the following subsections.
- Extract all triples from the website’s RDF and HTML+RDFa documents.
- Reason over this data and its ontologies to complete gaps.
- Publish the resulting data in a queryable interface.
The source code for the pipeline is available on GitHub. The pipeline can be run periodically, or triggered on website updates as part of a continuous integration process. In order to adapt this to different websites, the default ontology files can be replaced by others that are relevant for a given website.
The pipeline loops through all of the website’s files (either through the local file system or through Web crawling) and makes lists of RDF documents and HTML+RDFa documents. The RDF documents are fed through the Serd parser to verify validity and for conversion into N-Triples , so the rest of the pipeline can assume one triple per line. The RDFa is parsed into N-Triples by the RDFLib library for Python. Surprisingly, this library was the only one I found that correctly parsed RDFa Lite in (valid) HTML5; both Raptor and Apache Any23 seemed to expect a stricter document layout.
In order to fix gaps caused by implicit properties and classes, the pipeline performs reasoning over the extracted data and its ontologies to compute the deductive closure. The choice of ontologies is based on the data, and currently includes FOAF, DBpedia, CiTO, Schema.org, and the Organizations ontology. Additionally, I specified a limited number of custom OWL triples to indicate equivalences that hold on my website, but not necessarily in other contexts.
The pipeline delegates reasoning to the highly performant EYE reasoner , which does not have any RDFS or OWL knowledge built-in. Consequently, relevant RDFS and OWL theories can be selected manually, such that only a practical subset of the entire deductive closure is computed. For instance, my FOAF profile asserts that all resources on my site are different using
owl:AllDifferent; a full deductive closure would result in an undesired combinatorial explosion of
- The ontologies are skolemized  and concatenated into a single ontology file.
- The deductive closure of the joined ontology is computed by passing it to the EYE reasoner with the RDFS and OWL theories.
- The deductive closure of the website’s data is computed by passing it to the EYE reasoner with the RDFS and OWL theories and the deductive closure of the ontology.
- Ontological triples are removed from the data by subtracting triples that also occur in the deductive closure of the ontology.
- Other unnecessary triples are removed, in particular triples with skolemized ontology IRIs, which are meaningless without the ontology.
These steps ensure that only triples directly related to the data are published without any direct or derived triples from its ontologies, which form different datasets. By separating them, ontologies remain published as independent datasets, and users executing queries can explicitly choose which ontologies or datasets to include.
art:publication schema:author rv:me.
dbo:author owl:equivalentProperty schema:author.
schema:author rdfs:range [
owl:unionOf (schema:Organization schema:Person)
art:publication dbo:author rv:me.
art:publication schema:author rv:me.
rv:me rdf:type skolem:b0.
dbo:author owl:equivalentProperty schema:author.
schema:author rdfs:range skolem:b0.
skolem:b0 owl:unionOf skolem:l1.
skolem:l1 a rdf:List.
skolem:l1 rdf:first schema:Organization.
skolem:l1 rdf:rest skolem:l2.
skolem:l2 a rdf:List.
skolem:l2 rdf:first schema:Person.
skolem:l2 rdf:rest rdf:nil.
The skolemization in step 1 ensures that blank nodes from ontologies have the same identifier before and after the reasoning runs in steps 2 and 3. Step 2 results in triples 9–17 (note the inferred triples 12 and 15), which are also present in the output of step 3, together with the added triples 6–8 derived from data triple 1. Because of the previous skolemization, triples 9–16 can be removed through a simple line-by-line difference, as they have identical N-Triples representations in the outputs of steps 2 and 3. Finally, step 5 removes triple 8, which is not meaningful as it points to an unreferenceable blank node in the Schema.org ontology. The resulting enriched data is:
art:publication dbo:author rv:me.
art:publication schema:author rv:me.
Thereby, data that was previously only described with Schema.org in RDFa becomes also available with DBpedia. Note that the example triple yields several more triples in the actual pipeline, which uses the full FOAF, Schema.org, and DBpedia ontologies.
Passing the deductive closure of the joined ontology from step 2 to step 3 improves performance, as the derived ontology triples are already materialized. Given that ontologies change slowly, the output of steps 1 and 2 could be cached.
The resulting triples are then published through a Triple Pattern Fragments (TPF)  interface, which allows clients to access a dataset by triple pattern. In essence, the lightweight TPF interface extends Linked Data’s subject-based dereferencing by also providing predicate- and object-based lookup. Through this interface, clients can execute SPARQL queries with full completeness at limited server cost. Because of the simplicity of the interface, various back-ends are possible. For instance, the data from the pipeline can be served from memory by loading the generated N-Triples file, or the pipeline can compress it into a Header Dictionary Triples (HDT)  file.
Special care is taken to make IRIs dereferenceable  during the publication process. While I emphasize IRI reuse, some of my co-authors do not have their own profile, so I had to mint IRIs for them. Resolving such IRIs results in an HTTP
303 redirect to the TPF with data about the concept. For instance, the IRI
https://data.verborgh.org/people/sam_coppens redirects to the TPF of triples with this IRI as subject.
I applied the ETL pipeline to my personal website https://ruben.verborgh.org/ to verify its effectiveness. The data is published at https://data.verborgh.org/
In total, 35,916 triples were generated in under 5 minutes from 6,307 profile triples and 12,564 unique triples from webpages. The table below shows the number of unique triples at each step and the time it took to obtain them. The main bottleneck is not reasoning (≈3,000 triples per second), but rather RDFa extraction (≈100 triples per second), which can fortunately be parallelized more easily.
|step||time (s)||# triples|
|deductive closure ontologies||38.8||144,549|
|deductive closure data and ontologies||61.8||183,282|
|subtract ontological triples||0.9||38,745|
|subtract other triples||1.0||35,916|
While dataset size is not an indicator for quality , the accessibility of the data improves through the completion of inverse predicates and equivalent or subordinate predicates and classes between ontologies. The table below lists the frequency of triples with specific predicates and classes before and after executing the pipeline.
|predicate or class||# pre||# post|
It is important to note that most improvements are solely the result of reasoning on existing ontologies; only 8 custom OWL triples were added (7 for equivalent properties, 1 for a symmetric property).
While computing the deductive closure should not introduce any inconsistencies, the quality of the ontologies directly impacts the result. While inspecting the initial output, I found the following conflicting triples, typing me as a person and a company:
rv:me rdf:type dbo:Person.
rv:me rdf:type dbo:Company.
To find the cause of this inconsistency, I ran the reasoner on the website data and ontologies, but instead of asking for the deductive closure, I asked to prove the second triple. The resulting proof traced the result back to the DBpedia ontology erroneously stating the equivalence of the
dbo:firstPublisher properties. While the former has both people and organisations in its range, the latter is specific to companies—
dbo:Website was deemed equivalent to
schema:WebPage, whereas the latter should be
schema:WebSite. Disjointness constraints in the ontologies would help catch these mistakes. Further validation with RDFUnit  brought up a list of errors, but all of them turned out to be false positives.
Finally, I report on the execution time and number of results for a couple of example SPARQL queries. These were evaluated against the live TPF interface by a TPF client, and against the actual webpages and profile by a Linked Data-traversal-based client (SQUIN ). The intention is not to compare these query engines, as they use different paradigms and query semantics: TPF guarantees 100% completeness with respect to given datasets, whereas SQUIN considers reachable subwebs. The goal is rather to highlight the limits of querying over RDFa pages as practiced today, and to contrast this with the improved dataset resulting from the ETL pipeline.
- a Triple Pattern Fragments client (ldf-client 2.0.4) with the pipeline’s TPF interface
- a Linked Data client (SQUIN 20141016) with my homepage as seed
- a Linked Data client (SQUIN 20141016) with my FOAF profile as seed
All clients started with an empty cache for every query, and the query timeout was set to 60 seconds. The waiting period between requests for SQUIN was disabled. For the federated query, the TPF client also accessed DBpedia, which the Linked Data client can find through link traversal. To highlight the impact of the seeds, queries avoid IRIs from my domain by using literals for concepts instead.
|TPF (pipeline)||LD (home)||LD (profile)|
|query||#||t (s)||#||t (s)||#||t (s)|
|people I know ( ||196||2.1||0||5.6||14||60.0|
|people I know ( ||196||2.1||0||3.2||200||60.0|
|publications I wrote||205||4.0||0||10.8||0||10.5|
|my blog posts||43||1.1||40||6.5||40||6.4|
|a colleague’s publications||32||1.1||20||13.9||20||16.3|
|my first-author publications||46||2.7||0||3.8||6||36.2|
|works I cite||33||0.5||0||4.0||0||60.0|
|my interests (federated)||4||0.4||0||4.0||4||1.8|
The first two queries show the influence of ontological equivalences. At the time of writing, my website related me to 196
foaf:Persons through the
foaf:knows predicate. If the query uses only the FOAF vocabulary, with
foaf:name to obtain people’s names, Linked Data traversal finds 14 results. If we use
rdfs:label instead, it even finds additional results on external websites (because of link-traversal query semantics).
A second group of queries reveals the impact of link unidirectionality and inference of subclasses and subproperties in queries for scholarly publications and blog posts. Through traversal, “publications I wrote” (with
foaf:made) does not yield any results, whereas “my publications” (with
schema:author) yields 134, even though both queries are semantically equivalent. Given that my profile actually contained 205 publications, the 71 missing publications are caused by SQUIN’s implementation rather than being an inherent Linked Data limitation. Blog posts are found in all scenarios, even though the traversal client finds 3 fewer posts. Only the TPF client is able to find all articles, because the pipeline generated the inferred type
schema:Article for publications and blog posts. Other more constrained queries for publications yield fewer results through traversal as well. Citations (
cito:cites) are only identified by the TPF client, as articles solely mention its subproperties.
Regarding execution times, the measurements provide positive signals for low-cost infrastructures on the public Web. Note that both clients return results iteratively. With an average arrival rate of 53 results per second for the above queries, the TPF client’s pace exceeds the processing capabilities of people, enabling usage in live applications. Even faster performance could be reached with, for instance, a data dump or a SPARQL endpoint; however, these would involve an added cost for either the data publisher or consumer, and might have difficulties in federated contexts.
A first question is what data should be encoded as Linked Data, and how it should be distributed across resources. In the past, I always had to decide whether to write data directly on the page as HTML+RDFa, whether to place it in my FOAF profile as RDF, whether to do both, or neither. The pipeline partially solves the where problem by gathering all data in a single interface. Even though each page explicitly links to the Linked Data-compatible TPF interface using
void:inDataset—so traversal-based clients can also consume it—
A closely related question is what ontologies should be used on which places. Given that authors have limited time and in order to not make HTML pages too heavy, we should probably limit ourselves to a handful of vocabularies. When inter-vocabulary links are present, the pipeline can then materialize equivalent triples automatically. I have chosen Schema.org for most HTML pages, as this is consumed by several search engines. However, this vocabulary is rather loose and might not fit other clients. Perhaps the FOAF profile is the right place to elaborate, as this is a dedicated RDF document that attracts more specific-purpose clients compared to regular HTML pages.
Even after the above choices have been made, the flexibility of some vocabularies leads to additional decisions. For example, in HTML articles I mark up citations with the CiTO ontology. The domain and range of predicates such as
cito:cites is open to documents, sections, paragraphs, and other units of information. However, choosing to cite an article from a paragraph influences how queries such as “citations in my articles” need to be written. Fortunately, the pipeline can infer the other triples, such that the section and document containing the paragraph also cite the article.
When marking up data, I noticed that I sometimes attach stronger meaning to concepts than strictly prescribed by their ontologies. Some of these semantics are encoded in my custom OWL triples, whose contents contribute to the reasoning process (but do not appear directly in the output, as this would leak my semantics globally). For instance, I assume equivalence of
foaf:name for my purposes, and treat the
foaf:knows relation as symmetrical (as in its textual—
The reuse of identifiers is another source of debate. I opted as much as possible to reuse URLs for people and publications. The advantage is that this enables Linked Data traversal, so additional RDF triples can be picked up from FOAF profiles and other sources. The main drawback, however, is that the URLs do not dereference to my own datasource, which also contains data about their concepts. As a result, my RDF data contains a mix of URLs that dereference externally (such as http://csarven.ca/#i), URLs that dereference to my website (such as https://ruben.verborgh.org/
owl:sameAs, but then certain results would only be revealed to more complex SPARQL queries that explicitly consider multiple identifiers.
With regard to publishing, I wondered to what extent we should place RDF triples in the default graph on the Web at large. As noted above, inconsistencies can creep in the data; also, some of the things I state might reflect my beliefs rather than general truths. While RDFa does not have a standardized option to place data in named graphs, other types of RDF documents do. By moving my data to a dedicated graph, as is practiced by several datasets, I could create a separate context for these triples. This would also facilitate provenance and other applications, and it would then be up to the data consumer to decide how to treat data graph.
The above questions highlight the need for guidance and examples in addition to specifications and standards. Usage statistics could act as an additional information source. While HTTP logs from the TPF interface do not contain full SPARQL queries, they show the IRIs and triple patterns clients look for. Such behavioral information would not be available from clients or crawlers visiting HTML+RDFa pages.
Finally, when researchers start self-publishing their data in a queryable way at a large scale, we will need a connecting layer to approach the decentralized ecosystem efficiently through a single user interface. While federated query execution over multiple TPF interfaces on the public Web is feasible, as demonstrated above, this mechanism is impractical to query hundreds or thousands of such interfaces. On the one hand, this indicates their will still be room for centralized indexes or aggregators, but their added value then shifts from data to services. On the other hand, research into decentralized technologies might make even such indexes obsolete.
RDFa makes semantic data publication easy for researchers who want to be in control of their online data and metadata. For those who prefer not to work directly on RDFa, or lack the knowledge to do so, annotation tools and editors can help with its production. In this article, I examined the question of how we subsequently can optimize the queryability of researchers’ data on the Web, in order to facilitate their consumption by different kinds of clients.
Simple clients do not possess the capabilities of large-scale aggregators to obtain all Linked Data on a website. They encounter mostly individual HTML+RDFa webpages, which are always incomplete with respect to both the whole of knowledge on a website as well as the ontological constructs to express it. Furthermore, variations in reasoning capabilities make bridging between different ontologies difficult. The proposed ETL pipeline addresses these challenges by publishing a website’s explicit and inferred triples in a queryable interface. The pipeline itself is simple and can be ported to different scenarios. If cost is an issue, the extraction and reasoning steps can run on public infrastructures such as Travis CI, as all involved software is open source. Queryable data need not be expensive either, as proven by free TPF interfaces on GitHub  and by the LOD Laundromat , which provides more than 600,000 TPF interfaces on a single server.
By publishing queryable research data, we contribute to the Linked Research vision: the proposed pipeline increases reusability and improves linking by completing semantic data through reasoning. The possibility to execute live queries—
Even as a researcher who has been publishing RDFa for years, I have often wondered about the significance of adding markup to individual pages. I doubted to what extent the individual pieces of data I created contributed to the larger puzzle of Linked Data on my site and other websites like it, given that they only existed within the confines of a single page. Building the pipeline enabled the execution of complex queries across pages, without significantly changing the maintenance cost of my website. From now on, every piece of data I mark up directly leads to one or more queryable triples, which provides me with a stronger motivation. If others follow the same path, we no longer need centralized data stores. We could execute federated across researchers’ websites, using combinations of Linked Data traversal and more complex query interfaces that can guarantee completeness. Centralized systems can play a crucial role by providing indexing and additional services, yet they should act at most as secondary storage.
Unfortunately, exposing my own data in a queryable way does not relieve me yet of my frustration of synchronizing that data on current social research networks. It does make my data more searchable and useful though, and I deeply hope that one day, these networks will synchronize with my interface instead of the other way round. Most of all, I hope that others will mark up their webpages and make them queryable as well, so we can query research data on the Web instead of in silos. To realize this, we should each contribute our own pieces of data in a way that makes them fit together easily, instead of watching third parties mash our data into an entirely different puzzle altogether.
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