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Re: [p2-dev] URLs, URIs, and IDs (oh my) 
Hi Doug,

Schaefer, Doug wrote:

Hmm, I thought the U meant Universal. Maybe not so?
Actually, my understanding is it means Uniform: http://en.wikipedia.org/wiki/Uniform_Resource_Identifier
RE: URI...notwithstanding my obvious interests in representing ECF's value in a broader community...I agree that URI can be used quite successfully and naturally for representing identity of 'resources' (hence the 'R' ;-)...that's our (ECF's) intention. But as per my previous comments, it can be both too much (lots of functionality that's inappropriate for some identifiers) and too little (limited/difficult extensibility) for some use cases that are important...to us at least (e.g. connections to xmpp servers, or irc servers, or multicast groups, named asynchronous data channels, etc., etc., etc). If supporting those use cases are not that important for a connection framework (although I would argue they should be supported in such a framework), or for p2 (which obviously seems to me to be much more resource focussed in it's use of URLs for identifying repos), then by all means it would make sense to use emf URI alone. In bringing this up I was simply trying to identify (no pun intended) what I consider a weakness of using URI for cross-process identity...and offering an alternative at the level where generality and extensibility seem pretty important to me: Equinox, p2, e4 (i.e. Platform in whatever form). 

Scott
Doug. 
I don't have anything more useful to say other than I'm watching this
thread in consideration for how we'll deal with Resource identification
for e4. URI seemed to be the best answer. But then we're being
influenced by EMF guys too :).


-----Original Message-----
From: p2-dev-bounces@xxxxxxxxxxx [mailto:p2-dev-bounces@xxxxxxxxxxx] On Behalf Of Scott Lewis 
Sent: Tuesday, September 30, 2008 12:40 PM
To: P2 developer discussions
Subject: Re: [p2-dev] URLs, URIs, and IDs (oh my)

Hi Jeff,

Jeff McAffer wrote:

Scott Lewis wrote:
RE: having one URI class used everywhere...I agree it would be attractive, but extension is difficult given URI class is
final (to
guarantee standards compliance among other reasons)...as in many cases means writing wrappers (anyway) to have new constructors, override other methods, etc.
I'm not sure what extension is needed. can you elaborate
or give an 
example?
Sure. One example needed extension is in URI construction. Many IDs have specific syntax requirements beyond the URI syntax spec...as a concrete example: xmppids. So it's desirable/necessary to be able to run custom String parsing code on construction.
ID: Extensible using OSGi/Equinox mechanisms...e.g. extension registry, adapters URI: Impl has much functionality (already...yay), Standards compliant
I tihnk there are some extension registry things in the EMF
URI world. 
Not sure what all they do.  Perhaps Ed can elaborate.

Cons:

ID:  Another abstraction so has some impl cost
URI: Won't satisfy all use cases, so clients will still be creating/need to create own ID types (at least for some
things like 
ECF connections)
API breakage aside, would it make sense to rephrase ECF API
in terms 
of URIs?

I don't think so.   Even with the 'loosest' URI structure...e.g.

<scheme>:<scheme specific part>
there are cases where unique identifiers (for some protocols) would have to be forced into being a URI. And for many other cases the URI class is overkill. For example...xmppids don't have any path (there's a resource identifier, but it's not got any internal structure), and so all of the path, query, etc stuff that is part of URI is cruft.
One thing that I did not get about the EMF stuff was the
URIConverter
class. Here is some relevant JavaDoc 
http://download.eclipse.org/modeling/emf/emf/javadoc/2.4.1/org/eclipse

/emf/common/util/URI.html



http://download.eclipse.org/modeling/emf/emf/javadoc/2.4.1/org/eclipse

/emf/ecore/resource/URIConverter.html
the EMF approach has people creating URIConverter to do
whatever they
want with the various URIs. So URIs are the ids (doh) and the converters make them "real".
Yeah, I think (for good reason), most of the usage of URIs is for resource IDs...i.e. things that generally have a path/name/ext etc. 
One thought that I've had is that a new interface could be created
IResourceID:

public interface IResourceID extends ID {
org.eclipse.emf.common.util.URI getURI(); 
}

Then IResourceID could be used in appropriate places within p2 (and/or
e4) along with URIConverter, etc. This would, I think, be both an easy and useful way to go...as it would still be using emf.URI for implementation, but gain the extensibility benefits of using the namespace extension point. The main cost to the programmer would be calling (e.g.)
resourceID.getURI().getPath() rather than resourceURI.getPath() ...i.e. one level of redirection.
Just a thought. Even if URI is used directly in p2 we will certainly do this ourselves if emf URI is added somewhere in Equinox. 

Scott


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