Part VI of the DBpedia Growth Hack series (View all)
DBpedia Live is a long term core project of DBpedia that immediately extracts fresh triples from all changed Wikipedia articles. After a long hiatus, fresh and live updated data is available once again, thanks to our former co-worker Lena Schindler whose work we feature in this blog post. Before we dive into Lena’s report, let’s have a look at some general info about DBpedia Live:
Live Enterprise Version
OpenLink Software provides a scalable, dedicated, live Virtuoso instance, built on Lena’s remastering. Kingsley Idehen announced the dedicated business service in our new DBpedia forum. .
On the Databus, we collect publicly shared and business-ready dedicated services in the same place where you can download the data. Databus allows you to download the data, build a service, and offer that service, all in one place. Data up-loaders can also see who builds something with their data
Remastering the DBpedia Live Module
Contribution by Lena Schindler
After developing the DBpedia REST API as part of a student project in 2018, I worked as a student Research Assistant for DBpedia. My task was to analyze and patch severe issues in the DBpedia Live instance. I will shortly describe the purpose of DBpedia Live, the reasons it went out of service, what I did to fix these, and finally, the changes needed to support multi-language abstract extraction.
Overview
The DBpedia Extraction Framework is Scala-based software with numerous features that have evolved around extracting knowledge (as RDF) from Wikis. One part is the DBpedia Live module in the “live-deployed” branch, which is intended to provide a continuously updated version of DBpedia by processing Wikipedia pages on demand, immediately after they have been modified by a user. The backbone of this module is a queue that is filled with recently edited Wikipedia pages, combined with a relational database, called Live Cache, that handles the diff between two consecutive versions of a page. The module that fills the queue, called Feeder, needs some kind of connection to a Wiki instance that reports changes to a Wiki Page. The processing then takes place in four steps:
- A wiki page is taken out of the queue.
- Triples are extracted from the page, with a given set of extractors.
- The new triples from the page are compared to the old triples from the Live Cache.
- The triple sets that have been deleted and added are published as text files, and the Cache is updated.
Background
DBpedia Live has been out of service since May 2018, due to the termination of the Wikimedia RCStream Service, upon which the old DBpedia Live Feeder module relied. This socket-based service provided information about changes to an existing Wikimedia instance and was replaced by the EventStreams service, which runs over a single HTTP connection using chunked transfer encoding, and is following the Server-Sent Event (SSE) protocol. It provides a stream of events, each of which contains information about title, id, language, author, and time of every page edit of all Wikimedia instances.
Fix
Starting in September 2018, my first task was to implement a new Feeder for DBpedia Live that is based on this new Wikimedia EventStreams Service. For the Java world, the Akka framework provides an implementation of a SSE client. Akka is a toolkit developed by Lightbend. It simplifies the construction of concurrent and distributed JVM applications, enabling both Java and Scala access. The Akka SSE client and the Akka Streams module are used in the new EventStreamsFeeder (Akka Helper) to extract and process the data stream. I decided to use Scala instead of Java, because it is a more natural fit to Akka.
After I was able to process events, I had the problem that frequent interruptions in the upstream connection were causing the processing stream to fail. Luckily, Akka provides a fallback mechanism with back-off, similar to the Binary Exponential Backoff of the Ethernet protocol which I could use to restart the stream (called “Graph” in Akka terminology).
Another problem was that in many cases, there were many changes to a page within a short time interval, and if events were processed quickly enough, each change would be processed separately, stressing the Live Instance with unnecessary load. A simple “thread sleep” reduced the number of change-sets being published every hour from thousands to a few hundred.
Multi-language abstracts
The next task was to prepare the Live module for the extraction of abstracts (typically the first paragraph of a page, or the text before the table of contents). The extractors used for this task were re-implemented in 2017. It turned out to be a configuration issue first, and second a candidate for long debugging sessions, fixing issues in the dependencies between the “live” and “core” modules. Then, in order to allow the extraction of abstracts in multiple languages, the “live” module needed many small changes, at places spread across the code-base, and care had to be taken not to slow down the extraction in the single language case, compared to the performance before the change. Deployment was delayed by an issue with the remote management unit of the production server, but was accomplished by May 2019.
Summary
I also collected my knowledge of the Live module in detailed documentation, addressed to developers who want to contribute to the code. This includes an explanation of the architecture as well as installation instructions. After 400 hours of work, DBpedia Live is alive and kicking, and now supports multi-language abstract extraction. Being responsible for many aspects of Software Engineering, like development, documentation, and deployment, I was able to learn a lot about DBpedia and the Semantic Web, hone new skills in database development and administration, and expand my programming experience using Scala and Akka.
“Thanks a lot to the whole DBpedia Team who always provided a warm and supportive environment!”
Thank you Lena, it is people like you who help DBpedia improve and develop further, and help to make data networks a reality.
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