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Subject: Re: [xri] Delegation Terminology / Linked XRD

Forgot to reply to this earlier in the week, but we do need to wrap up  
this discussion and decide how to move forward...

I agree that this is an absolutely valid model, and was actually the  
first one I envisioned.  One thing to take into consideration when  
flattening these XRDs is what to do with the Link priorities.  What if  
XRD1 has two related resources, an OpenID provider with priority of 5,  
and another XRD with priority of 10.  That linked XRD, XRD2, also  
lists an OpenID provider with a priority of 1.  So which OpenID  
provider should be used first?  The one with priority of 1, because it  
has the lowest priority value?  But the linked XRD had a priority of  
10... is that taken into consideration when determining the "effective  
priority" of resources linked from that XRD?  There are a couple of  
different ways to answer it, but my point is that you will probably  
want to be explicit in how applications should handle this to ensure  
consistent behavior.

The second problem with this approach is that you end up fetching  
every XRD for the resource every time, even if the highest priority  
<Link> ends up being in the first document.  I don't anticipate people  
going too crazy with linked XRDs, but it is certainly a practical  
deployment issue we should consider.

With the proposed approach of sorting by priority first, and then  
fetching linked XRDs as necessary, it solves both the question of how  
to handle "effective priority" as well as "just in time fetching" of  
linked XRDs.  It certainly changes how we need to word the priority  
attribute section, since it assumes you're sorting a pre-filtered set  
of elements.


On Jul 4, 2009, at 10:46 AM, Eran Hammer-Lahav wrote:

> My point is that if you draw a graph of the resource and all its  
> links from the various sources and XRDs, it looks like you should  
> flatten the content of XRDs, not search them in order...
>> -----Original Message-----
>> From: Will Norris [mailto:will@willnorris.com]
>> Sent: Friday, July 03, 2009 11:25 PM
>> To: XRI TC
>> Subject: Re: [xri] Delegation Terminology / Linked XRD
>> On Jul 3, 2009, at 5:11 PM, Eran Hammer-Lahav wrote:
>>>> Well there is a precedence.  Even though a XRD-linked XRD shares  
>>>> the
>>>> semantics as one linked from an HTTP header, we can't ignore the
>> fact
>>>> that <Link> elements do in fact have a priority.  I remember that
>>>> LRDD
>>>> specifically stated that determining which one of multiple
>>>> describedby
>>>> links should be used was application specific.  And that seems to
>>>> make
>>>> sense because there is no good deterministic way to order them.  We
>>>> can order <Link> elements though, so it makes sense to respect  
>>>> their
>>>> order.
>>> I agree that Link priority must take precedence but that's the easy
>>> case. What happens when all priorities are equal.
>> If a linked XRD is really just another linked resource, then the spec
>> instructs consuming applications to choose randomly.
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