Member extraction algorithm and blank node graphs

[pietercolpaert]

Blank node graphs are a neat trick. For example:

_:b0 {
 <a> <b> <c> .
}

When having parsed the page containing the data, we can be certain we got all triples possible within _:b0, as the scope of this blank node is limited to this page. We can also be certain that this graph will never conflict with a graph from another source.

This means we could use blank node graphs as a package as part of a member and give a specific purpose to parts of our graph. I.e.:

ex:LDES a ldes:EventStream ;
    rdfs:comment "An LDES with per member: an ActivityStreams update, the payload and the signature of the payload";
    tree:view <> ;
    tree:member <A> .

## Imagine we would like to create a transactional profile of LDES, including things like policies, transactions, policies, ways to upsert/remove sets of triples, etc.
<A> a patch:Event ;
    patch:processingMethod patch:Upsert ;
    patch:upsertKey <https://example.org/Dataset1> ;
    patch:transaction ex:Transaction1 ;
    patch:upsertPayload _:b0 ;
    patch:sequence 1 ;
    patch:time "2024-09-09T13:27:33.681Z";
    patch:provenance _:b1 ;
    patch:signature _:b2 ;
    patch:policy _:b3 .

_:b0 {
     ## The (updated) representation of this particular dataset
     ## ...
     <https://example.org/Dataset1> a dcat:Dataset ;

}

_:b1 {
     <https://example.org/Dataset1#Event1> a as:Create, prov:Activity ;
       as:object <https://example.org/Dataset1> ;
       as:published "2023-10-01T12:00:00Z"^^xsd:dateTime .
}

_:b2 {
     # Signature
     [] a ex:DataIntegrityProof;
        ...
ex:signature "rCWNBuxBK1In93X8dvuK1ss91LK0rMiA2KzvsNaEhdGt7PTD5aQ0X58TzbvnTOhvl9t5bRGoOHnxfys52Q9bWjnmD4GoljEWVWFSrBnORsLBOLwcAnLRfEtTvz4t0EYV";
        ex:target (_:b0 _:b1 _:b3) ;
}

_:b3 {
  ## We want to indicate that this specific member must be removed after 1 month
  [] a ex:Policy ;
      ex:target <https://example.org/Dataset1> ;
      ex:duty [
             a ex:RemovalDuty ;
             ex:after "P1M" 
     ]
}

Then we must be able to have a smarter way of dealing with blank node graphs, which at this moment is not specified at all in the member extraction algorithm.

Proposal:

• Extend the spec that shapes and CBD never look into blank node graphs
• From the moment a blank node is encountered, we should check whether there is a blank node graph for it. If there is, include that blank node graph’s quads in the member.

[pietercolpaert]

Additionally:

The main goal of this entire spec is to be able to package triples. If there’s one system, I believe we probably should not mix it with other systems (this is a design choice I guess). So we could change the algorithm in a way which makes sure that

1. Only shape topologies can trigger an HTTP request to out of band members: the package here is the shape topology, but also the external resource in a way. Shape topologies only trigger when no named member graphs or blank node graphs are used.
2. From the moment you use a named graph as a member, we limit the metadata to CBD, and only further include data from the named graph. Here the package is one particular named graph that, because it is a member of a collection, is through convention going to remain the same everywhere at this particular moment. The CBD algorithm excludes blank node graphs and member graphs.
3. A check needs to happen that checks whether blank nodes in the page are directly related to a member, which means that an advanced packaging technique is used in this case to form the member.

[pietercolpaert]

• This also becomes related to Clear out tree:shape definition #75 as tree:shape would become less important to trigger an HTTP request
• We should take a look a describing neighbourhoods using shapes that is a paper that formalizes the same ideas we had with Shape Topologies: https://www.researchgate.net/publication/384049733_What's_in_a_Neighborhood_Describing_Nodes_in_RDF_Graphs_Using_Shapes
• We should introduce a note on triple terms
• And of course also take into account the Blank Node Graphs as pitched above