What is the state of braid regarding RDF ?

[thhck]

Hello again,

Considering that the Solid Community Server utilizes RDF for data representation, I'm wondering how well does braidjS handle RDF data ?

[toomim]

Sure! I presented a couple ways to Braid RDF at last year's Realtime Solid workshop at the Solid Symposium: https://braid.org/meeting-realtime-solid.

Since Braid just extends HTTP, you need to make the usual decision of how to store RDF on HTTP by choosing your content-type, and then you can additionally choose a representation for how to patch it— the patch-type. Then you put these both into headers.

There already are some patch formats for RDF that you can use if you want, such as N3-Patch. Here is an example scenario where you subscribe to a server and it streams back N3-Patches:

Request:

GET /something
Subscribe: true

Response:

HTTP/1.1 209 Multiresponse
Subscribe: true

200 OK
Patch-Type: application/n3-patch
Content-Type: text/turtle
Content-Length: 191

_:rename a solid:InsertDeletePatch;
  solid:where   { ?person ex:familyName "Garcia". };
  solid:inserts { ?person ex:givenName "Alex". };
  solid:deletes { ?person ex:givenName "Claudia". }.

But there's also a general patch format available in HTTP, that can work for any content-type, that we call a Range Patch, that works by re-using the existing concept of a "Range Request" in HTTP. A range request is how you continue a download with a GET, by saying e.g. "I want the range of this file from bytes 500-10000" if you already have the first 500 bytes. However, it's less-known that you can also do a range request for a PUT. This is sometimes called a "Partial PUT" in HTTP, and the semantics of it are to PUT just that range of the resource — which is the same semantics as you need to patch a resource.

So we tend to use range patches, because they are more general. All you need is a "range unit" that describes the range to be patched as rdf, rather than as bytes. HTTP allows you to define new range units. So we can just define a new range unit called "rdf", and get a nice patch format, which could look like this:

Request:

GET /something-else
Subscribe: true

Response:

HTTP/1.1 209 Multiresponse
Subscribe: true

200 OK
Content-Range: rdf "<Toomim> ?predicate <Toomims-Arm> ?graph.".object[-0:-0]
Content-Type: text/turtle
Content-Length: 7

" very"

I'm trying to remember how RDF works... but I think in this case, our range unit lets us select the quad describing my <Toomim> relationship with my arm <Toomims-Arm>, and within that, look at the .object part of the quad (which is my arm), and then the zero-length subrange [-0:-0] at the end of that string.

Then since the contents of this patch is " very", it means that it is replacing the zero-length region end of that string with the string " very".

I might be getting something wrong, and I forgot the actual semantic scenario/purpose for this example. Maybe you know more about RDF and can make better sense of it.

[thhck]

Thanks for you answer !

TBH I also puzzled by this, I need to dig the theory and put it in practice