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Subject: Re: [xdi] proposal for binary data literal node

Mike, the question you raised actually highlights a number of the key reasons for the literal context proposal. Let me explain.

First, per Markus' last point in his message, one primary purpose of literal contexts is so an XDI processor always knows the type of the literal node it is dealing with by the type of the literal context. To use Mike's example, if "*logo" is defined as a literal context containing a binary value (which has several semantic issues -- see below), then @!1111*logo always identifies a single binary value.

Secondly, it's a misunderstanding of literal contexts that there could be two literal arcs. The whole idea of a literal context is that it has a single literal arc. If the literal context is defined as containing binary data, then that literal arc points to binary data. If the literal context is defined as containing a number, then the literal arc points to a number. And so on.

These are all controlled by XDI dictionary definitions, which is why I think the XDI Dictionary spec is moving up in priority.

Now, the semantic issues. First, @!1111*logo is semantically wrong because uses a *name to identify a literal context defined in a dictionary, and * is not the dictionary namespace. The dictionary namespace is + (with the exceptions of a handful of special $words defined by the XDI TC).

So it should be @!1111+logo. The second semantic issue is that +logo should not be a literal context because it is not a single defined datatype. +logo should be a supertype that has defined subtypes. In other words, the dictionary defintion should say +logo/$is$a/+image, and (among other options) +$image/$a/$a$xsd$jpeg. So if the binary you wanted was a JPEG, then the address would be:
  1. @!1111+logo$a$xsd$jpeg  <== canonical JPEG version of the logo (zero-or-one)
  2. @!1111+logo/$a$xsd$jpeg  <== (note the slash) relational arc pointing to all the JPEF versions of the logo (zero-or-more)

So it is the dictionary-defined  $jpeg, and not +logo (and definitely not *logo) that is the XRI literal context storing the binary data. And because the $dictionary will define $jpeg as a JPEG binary, any time an XDI processor sees $jpeg as the literal context, it knows the value is a JPEG.

I'll put this on the agenda for tomorrow's call (even though Mike won't be there).


On Thu, Mar 1, 2012 at 4:35 PM, Michael Schwartz <mike@gluu.org> wrote:


Thanks for the quick response!

1. Yes, you could have both a ! and !! arc, why not?  It doesn't seem inherently problematic to me.

2. yes, you're right I forgot the / for the literal arc :)

3. !! was just an idea... it could be something else.

4. Its useful to be able to differentiate binary data from other types of
  data because there persistence strategy varies greatly.

- Mike

1. Could you have BOTH a binary and a non-binary literal in a context? I.e.
have a ! arc and a !! arc.
If not, will the server throw an error if you already have one literal and
try to add the other one?

2. In your example, wouldn't the XRI actually be @!1111*logo/!! instead of
@!1111*logo!!  ?

3. Something is bothering me about the fact that !! is two subsegments. I
am worried it will result in ambiguity in one way or the other.

4. I don't understand why you can't load the metadata together with the
literal, to figure out whether it is binary or not.
Wasn't this the whole point of what Drummond called "literal contexts"?


On Thu, Mar 1, 2012 at 9:49 PM, Michael Schwartz <mike@gluu.org> wrote:


I'm attaching a proposal to represent binary literal values as "!!"

So, for example, the XRI for a my company's logo might be :

The fact that the image is a binary can't be represented in the metadata,
because it is not known which will be loaded first, the literal node, or
the metadata about the literal node.

This is a critical issue to optimize persistence of binary objects, which
need to be indexed differently.

I'd like to fast track this because we need to store binaries as part of
the oxPlus demo :)



- Mike

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