What do you mean by "nearest"? Is this the geographical distance b/n two 
nodes? I dont see this reflected in the XML document.

Cheers,
Dimitre Novatchev

"Nick Leverton" <nick@l...> wrote in message 
news:6psr1cF9n7siU1@m......
>I have an application which attempts to describe a tree of TCP subnets,
> which in essence are not fully accessible from each other.  I have a
> description of the network in XML as shown in the excerpt below.
>
> The application is actually trying to optimise delivery of large files
> to multiple destinations over expensive links, so it's not just a matter
> of opening up firewalls and adding a bit of NATting.  To avoid duplicated
> transfers I need to know what is the nearest machine which leads onto
> the ultimate destination for the file I am currently handling.
>
> So for instance files destined for units 26 and 27 are first delivered
> to node V9990 which then delivers them to V9991, to which 26 and 27
> are directly attached.  The distinction between nodes and units isn't
> important for this part of the task.  The ID attribute defines the
> ultimate destination which I am trying to reach and each ID is unique,
> so there is only one "nearest" IP address corresponding to each ID.
>
> <?xml version="1.0"?>
> <nodes>
>  <node id="V9990" ip="1.1.1.1">
>    <unit id="23" ip="10.10.10.10"/>
>    <unit id="24" ip="10.10.10.11"/>
>    <node id="V9991" ip="10.10.10.12">
>      <unit id="26" ip="192.168.0.1"/>
>      <unit id="27" ip="192.168.0.2"/>
>    </node>
>  </node>
>  <node id="V9992" ip="2.2.2.2">
>    <node id="V9993" ip="10.10.10.10">
>      <unit id="21"/>
>      <unit id="22"/>
>    </node>
>  </node>
> </nodes>
>
> To simplify network maintenance I would like to use the same config file
> on all the "nodes", and to modify the XPath query with extra terms on
> the sub-nodes.  In other words, on the "root" machine a query for id=26
> will return ip=1.1.1.1, but on node V9990 a query for id=26 will return
> ip=10.10.10.12
>
> In summary, what I want to do is to retrieve the nearest ip attribute
> in the document which has a given id attribute as a descendant.  I am
> currently using the following XPath:
>
> Querying from the root:
>  descendant-or-self::*[@ip and 
> descendant-or-self::*[@id="26"]][last()]/@ip
>
>  I used descendant-or-self as the first term here rather than //*
>  because I don't want XPath to descend the doc and return all matches,
>  only the node which matches nearest the root of the XML document.
>
> Querying from a sub-node:
>  //*[@id="V9990"]/*[@ip and descendant-or-self::*[@id="26"]][last()]/@ip
>
>  Here I establish a context node first and then work on that with
>  predicates.
>
> First question - these two work, but are probably not ideal since I'm
> not yet very familiar with XPath.  In particular I don't understand
> why I need to use [last()] predicate rather than [1], as I thought the
> descendant axis should work downwards in document order not upwards.
>
> Secondly, I now have a requirement to retrieve all the "nearest" ip
> attributes for polling/reporting purposes. In other words, querying
> from the root I would want to return 1.1.1.1 and 2.2.2.2.  Or querying
> from node V9990 I would want to return 10.10.10.10, 10.10.10.11 and
> 10.10.10.12.  I don't mind about getting multiple instances of the same
> attribute back as de-duping is simple.  But I cannot figure out how to
> arrange the predicates so as to return the "topmost" ip attribute only,
> neither for the root case nor for the sub-context case.
>
> Am I bending XPath a step too far here ?  I was hoping not to have to
> introduce an extra processing step but I am thinking maybe the sub-nodes
> need to extract their "local" view of the network and only to work
> on that.  Any advice would be very helpful.
>
> I'm working in perl XML::XPath in case it makes a difference.
>
> Thankyou
>
> Nick
> -- 
> Serendipity: http://www.leverton.org/blosxom (last update 19th September 
> 2008)
>        "The Internet, a sort of ersatz counterfeit of real life"
>                -- Janet Street-Porter, BBC2, 19th March 1996