[Zope-CVS] SVN: Sandbox/zc/extrinsicreference/extrinsicreference.txt Expanded motivation.

[Jim Fulton]

Log message for revision 40061:
  Expanded motivation.
  

Changed:
  U   Sandbox/zc/extrinsicreference/extrinsicreference.txt

-=-
Modified: Sandbox/zc/extrinsicreference/extrinsicreference.txt
===================================================================
--- Sandbox/zc/extrinsicreference/extrinsicreference.txt	2005-11-12 16:30:47 UTC (rev 40060)
+++ Sandbox/zc/extrinsicreference/extrinsicreference.txt	2005-11-12 16:32:40 UTC (rev 40061)
@@ -8,10 +8,27 @@
 reference to it's first chapter would be intrinsic.  Intrinsic
 references are used to satisfy an objects "concerns", the things that
 motivated it's design.  Extrinsic references exist to meet system
-concerns.  We prefer using simple Python references to implement
+concerns.  For examples, a system might use extrinsic references to
+keep track of link dependencies (e.g. backlinks).
+We prefer using simple Python references to implement
 intrinsic references.  This package provides a tool that can be used
 to provide extrinsic references. 
 
+We originally developed this package to manage "shortcuts".  Shortcuts
+are a bit like windows shortcuts or unix symbolic links.  Shortcuts
+hold direct references to their targets.  These were intrinsis to
+shortcuts. It wouldn't make sense to have shortcuts without targets.
+We also wanted to keep track of shortcuts to to objects. That is,
+given an object, we wanted to find all of the shortcuts to it.  This
+was a system/application choice. We used extrinsic references for
+this.
+
+The above example illustrates that it is often useful to use different
+kinds of references for different directions of an association.  We
+often use intrinsic references for one direction and use extrinsic
+references to go the other way.  Of course two-way
+associations/relationships could be build on a mechanism like this.
+
 Extrinsic registries record a key and one or more values to which they
 refer.  The key and all values must be adaptable to 
 zope.app.keyreference.interfaces.IKeyReference.