Why Range doesn't work for LDP "paging" (cf 2NN Contents-of-Related)

[Sandro Hawke <[e-mail]>]

Earlier today the LDP Working Group discussed the matter of whether we 
could use range headers instead of separate page URIs.  Use of Range 
headers was suggested on this list recently.

Our conclusion was still "no", for the following reasons.  Please let us 
know if you see a good solution to any/all of them:

1.  We don't know how the server would initiate use of Range.   With our 
current separate-page design, the server can do a 303 redirect to the 
first page if it determines the representation of the entire resource is 
too big.   The question here is what to do when the client didn't 
anticipate this possibility.  True, the 303 isn't a great solution 
either, since unprepared clients might not handle it well either.   
Perhaps one should give a 4xx or 5xx when the client asks for a giant 
resource without a range header...?   But there's no "representation too 
big" code defined.

2.  We don't know how we could do safe changes.  With our current 
design, it's possible for the resource to change while paging is 
happening, and the client ends up with a representation whose inaccuracy 
is bounded by the extent of the change.  The data is thus still usually 
perfectly usable.  (If such a change is not acceptable, the client can 
of course detect the change using etags and restart.)   This bounded 
inaccuracy a simple and practical concept with RDF (in a way it isn't 
with arbitrary byte strings). Just using Range, a deletion would often 
result in data unrelated to the change being dropped from what the 
client sees.    I suppose perhaps one could use some kind of tombstones 
to avoid this problem, not closing in gaps from deletion.  Basically, a 
client might ask for triples 0-9 and only get 3 triples because the 
others were deleted?  Does that make sense with Range?   Is it okay to 
not have the elements be contiguous?

3.  Many of our usual RDF data systems don't support retrieval of ranges 
by integer sequence numbers.   While some database systems have an 
internal integer row number in every table that could be used for Range, 
many others do not, and we don't know of a straightforward and 
appropriate way to add it.

4.  Finally, there was some question as to whether the Web 
infrastructure has any useful support for non-byte ranges.   This is 
perhaps not an objection, but it came up during the discussion, and we'd 
be interested in any data people have on this.

Bottom line is we still think just using rel=first/last/next/prev, among 
distinct resources, is a pretty reasonable design.   And if we're doing 
that, it'd be nice to have 2nn Contents-of-Related.

      -- Sandro