Rsine Getting Notified on Linked Data Changes

With the growing amount of content available on the Web of Data it becomes increasingly difficult for human users to track changes of resources they are interested in. This even holds true for “local” use cases where, e.g., contributors are working on a dataset in a collaborative way, linking and annotating each other's resources.

For example, contributors who develop controlled vocabularies, typically want to know whenever the meaning of a concept is fundamentally changed. This is because the concept might have been used for indexing documents and the changed meaning impairs search precision. However, with increasing frequency of change and size of the curated information resources, pull-based approaches do not scale anymore.

In this section we introduce the Resource SubscrIption and Notification sErvice (rsine), a framework that notifies subscribers whenever assertions to resources they are interested in are created, updated or removed. It is based on the W3C standards RDF and SPARQL and designed to be used alongside with existing triple storage solutions supporting these technologies. Multiple instances of the framework can communicate dataset changes also among each other. This allows to subscribe for changes of resources that are created or modified in other datasets on the Web that are managed by rsine.

An application of rsine is for instance the support of integrated quality management in controlled vocabulary development. We have shown in our previous work [8, 9] that potential quality problems in controlled Web vocabularies can be detected from patterns (“quality issues”) in the underlying RDF graph. We believe that immediate notification of the responsible contributors after such quality issues have been introduced will lead to faster development and higher quality of the created vocabularies.

Related Work

SparqlPuSH [13] is a subscription/notification framework that allows for “proactive notification of data updates in RDF stores”. Users express the resources they are interested in as SPARQL queries which are used by the service to create RSS or Atom feeds. These feeds are published on “hubs” using the PubSubHubbub protocol which handles the dissemination of notifications. Our approach is closely related to SparqlPuSH but is designed to operate on a more general level. In particular, creation and subscription to feeds, as proposed in SparqlPuSH, is only one of the possible options for notifying subscribers in rsine. Furthermore, SparqlPuSH only relies on the extensiveness of the data contained in the underlying RDF store. Thus, it is not possible to, e.g., find out about all resources deleted in a certain period of time. Rsine supports these scenarios by using a standard ontology for storing changeset metadata.

SDShare[1] is a protocol for the distribution of changes to resources that are represented in RDF. A server that exposes data provides four different Atom feeds that provide information about the state of the data and update information. The protocol is designed to support replications of linked data sources and relies on clients actively monitoring the provided feeds. Furthermore, clients only get information about the updated resource URIs and are expected to fetch the actual changes of resources themselves.

In the course of the REWERSE [10] project, a “general framework for evolution and reactivity in the Semantic Web” has been proposed that is based on Event-Condition-Action (ECA) rules. The framework is designed to be independent from the languages used to specify define events, conditions and actions. We stick to this approach but utilize a custom RDF ontology to express the ECA rules. Additionally we decided to use SPARQL for definitions of both events and conditions because of its wide acceptance and our focus on RDF data. This results in a light-weight approach, eliminating the need for custom event matchers and detection engines in favour of SPARQL endpoints and incremental RDF changesets. Actions are represented in our rsine ECA rules by specifying one or multiple notifiers.

  • [1] Final Draft:sdshare.org/spec/sdshare-20120710.html
 
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