Re: namespaces in subtree filtering

[Phil Shafer <phil@juniper.net>]

Martin Bjorklund writes:
>Andy noted at the meeting that subtree filtering can't be used to
>retreive elements from mulitple namespaces.
>
>The text from rfc4741, section 6.2.1 says:
>
>   If namespaces are used, then the filter output will only include
>   elements from the specified namespace.  A namespace is considered to
>   match (for filter purposes) if the content of the 'xmlns' attributes
>   are the same in the filter and the underlying data model.
>
>This is somewhat difficult to interpret.  What exactly does "the
>'xmlns' attributes" mean?  Note the plural.

I interpret this as an element by element rule, meaning that
the filter will only match elements which match the name and
namespace of element in the filter.  This rule is applied
recusively, so that if your data looks like:

  <a:a>
    <b:b>
      <c:c>4</c:c>
    </b:b>
  </a:a>

and you use the filter:

  <a:a>
    <b:b/>
  </b:b>

your output would include <c:c>.  If you used the filter:

  <a>
    <b/>
  <a>

you would get nothing, since the namespace don't match.

>Suppose we have this in the database:
>
>   <if:interfaces xmlns:if="http://example.com/interface";>
>     <if:interface>
>       <if:ifIndex>1</if:ifIndex>
>       <if:ifType>ds0</if:ifType>
>       <ds0:circuitIdentifier xmlns:ds0="http://example.com/ds0";>foo</ds0:circuitIdentif
>ier>
>     </if:interface>
>   </if:interfaces>
> 
>Accordning to the example in 6.2.1, the following filter:
>
>   <get>
>     <filter>
>       <if:interfaces xmlns:if="http://example.com/interface"/>
>     </filter>
>   </get>
>
>would return
>
>   <if:interfaces xmlns:if="http://example.com/interface";>
>     <if:interface>
>       <if:ifIndex>1</if:ifIndex>
>       <if:ifType>ds0</if:ifType>
>     </if:interface>
>   </if:interfaces>

I would rather see all of <if:interface> emitted.  If you use
this strict interpretation, your application will need to send
every possible namespace to get a full tree, which is impossible.
Given that this full fetch is a normal and useful scenario, it
shouldn't be impossible, so we shouldn't do the strict interpretation.

>What about this filter:
>
>   <get>
>     <filter>
>       <if:interfaces xmlns:if="http://example.com/interface";
>                      xmlns:ds0="http://example.com/ds0"/>
>     </filter>
>   </get>
>
>Here we have two xmlns attributes.  Would we get the ds0 elements in
>this case?

Remember that the use of xmlns is only an encoding issue.  The XML
information model defines the element as belonging to a namespace,
and the prefix (and the xmlns attribute that defines the prefix
mapping) is an encoding issue, not a semantic one.

This means that an unused prefix is meaningless, so trying to
use 'xmlns:ds0' like this won't work.

Thanks,
 Phil

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