Re: [ldapext] Sorted attribute values

[Howard Chu <hyc@highlandsun.com>]

Richard Megginson wrote:
> This 
> could possibly solve a problem for many LDAP users - how to store a list 
> of multiple values in order and have the server return them in the same 
> order.

This is a related but different problem, of course. In this case all you 
care about is that a static ordering is preserved. I.e., the client 
stores values in a fixed order and wants them returned in the same 
order. For the "sorted values" problem we're looking for a dynamic 
ordering - the client stores values in an arbitrary order and the server 
rearranges them into a desired order. Another aspect I forgot to mention 
is that for weighted sorting, it's possible for multiple values to have 
the same weight, and you can configure a secondary sort as well (where 
the secondary is one of the other 4 sort types) to handle this case.

Which reminds me, we already have a solution for the static ordering 
problem in OpenLDAP 2.3, adding an "X-ORDERED 'VALUES'" extension to the 
schema definition of the attributeTypes that need it. In this case, we 
also add a {xx} prefix to all of the values of the attribute, but this 
prefix is managed automatically by slapd, is always numbered 
consecutively starting from zero, and is always returned in search 
responses. Pretty sure I posted a full description to this list a few 
months back, but I may be misremembering. The other important aspect of 
this feature is that it allows equality matching of values by prefix. 
Matching by prefix is significant to us because it allows one to 
ldapmodify/Delete specific values by their ordinal position, instead of 
(or in addition to) their actual value. I mainly use this just to avoid 
having to specify long values for deletions, but this also means we can 
delete specific values of multivalued attributes even if they have no 
equality matching rule. Likewise we can provide prefixes on Add 
modifications to insert elements at the specified positions within an 
existing sequence of values. Another (mostly uninteresting) side effect 
of prefix matching is that one can search for e.g. "all attributes that 
have 5 values" by filtering on the prefix.

There is another possibility "X-ORDERED 'SIBLINGS'" that we use as well, 
to preserve the order of peer entries with the same naming attribute. 
This feature is not quite as well-defined; we only use it with 
single-valued RDNs and there are plenty of corner cases I haven't 
thought about.

--
  -- Howard Chu
  Chief Architect, Symas Corp.  http://www.symas.com
  Director, Highland Sun        http://highlandsun.com/hyc
  OpenLDAP Core Team            http://www.openldap.org/project/

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