[JENA-2309] Enhancing Riot for Big Data

• Type: Brainstorming
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: Jena 4.5.0
• Component/s: RIOT
• Assignee: Unassigned
• Reporter: Claus Stadler
• Created: 12/Mar/22 19:40
• Updated: 24/Apr/22 08:59

Description

We have successfully managed to adapt Jena Riot to quite efficiently work within Apache Spark, however we needed to make certain adaption that rely on brittle reflection hacks and APIs that are marked for removal (namely PipedRDFIterator):

In principle, for writing RDF data out, we implemented a mapPartition operation that maps the input RDF to lines of text via StreamRDF which is understood by apache spark's RDD.saveAsText();

However, for use with Big Data we need to

• disable blank node relabeling
• preconfigure the StreamRDF with a given set of prefixes (that is broadcasted to each node)

Furthermore

• The default PrefixMapping implementation is very inefficient when it comes to handling a dump of prefix.cc. I am using 2500 prefixes. Each RDF term in the output results in a scan of the full prefix map
• Even if the PrefixMapping is optimized, the recently added PrefixMap adapter again does scanning - and its a final class so no easy override.

And finally, we have a use case to allow for relative IRIs in the RDF: We are creating DCAT catalogs from directory content as in this file:

DCAT catalog with relative IRIs over directory content: work-in-progress example

If you retrieve the file with a semantic web client (riot, rapper, etc) it will automatically use the download location as the base url and thus giving absolute URLs to the published artifacts - regardless under which URL that directory is hosted.

*IRIxResolver: We rely on IRIProviderJDK which states "do not use in production" however it is the only one the let us achieve the goal. our code

• Prologue: We use reflection to set the resolver and would like the setResolver method our code
• WriterStreamRDFBase: We need to be able to create instances of WriterStreamRDF classes which we can configure with our own PrefixMap instance (e.g. trie-backed), and our own LabelToNode stragegy ("asGiven") - our code
• PrefixMapAdapter: We need an adapter that inherits the performance characteristics of the backing PrefixMapping our code
• PrefixMapping: We need a trie-based implementation for efficiency. We created one based on the trie class in jena which on initial experiments was sufficiently fast. Though we did not benchmark whether e.g. PatriciaTrie from commons collection would be faster. our code
  With PrefixMapTrie the profiler showed that the amout of time spent on abbreviate went from ~100% to 1% - though not totally sure about standard conformance here.
• PipedRDFIterator / AsyncParser: We can read trig as a Splittable format (which is pretty cool) - however this requires being able to start and stop the RDF parser at will for probing. In other words, AsyncParser needs to return ClosableIterators whose close method actually stops the parsing thread. Also when scanning for prefixes we want to be able to create rules such as "as long as the parser emits a prefix with less than e.g. 100 non-prefix events in between keep looking for prefixes" - AsyncParser has the API for it with EltStreamRDF but it is private.

For future-proofness we'd have these use cases to be reflected in jena.
Because we have sorted all the above issues mostly out I'd prefer to address these things with only one or a few PRs (maybe the ClosableIterators on AsyncParsers would be more work because our code only did that for PipedRDFIterator and I haven't looked in detail into the new architecture).

Issue Links

• is related to: JENA-2313 Support parsing fragments of Turtle and TriG