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revised proposal for query schema discovery

From: Jim Davis <jdavis@parc.xerox.com>
Date: Sat, 20 Jun 1998 18:38:35 PDT
Message-Id: <3.0.3.32.19980620183835.007fb6a0@mailback.parc.xerox.com>
To: www-webdav-dasl@w3.org
The current protocol draft proposes that a client discover the schema for a
given query grammar on a given resource by doing a PROPFIND on the resource
where the property name is the name of the grammar.  Thus if one is using
the dav:simplesearch grammar on the resource nosy.com/finder, you send
nosy.com a  PROPFIND on /finder for the dav:simplesearch property.

Judy Slein pointed out weeks ago that this can't work, because a query
schema depends not just on the arbiter and grammar, but also on the scope.

I propose the following replacement.  Like the current scheme, it uses
PROPFIND on the arbiter.  Unlike the current scheme, the property to be
fetched is not the URI of the grammar, rather it is a *parameterized*
property, e.g. something like
<prop>
  <queryschema>
    <grammar>URI</grammar>
    <scope>scope1</scope>
    <scope>scope2</scope>
  </queryschema>
</prop>

(This is not the exact syntax I propose, but is simpler, to show the idea.)

As far as I can tell, nothing in WebDAV mandates that the tags within the
prop element be empty.  I've sent WebDAV a letter to ask.

For those who want the full details, here's the language I would propose to
add to the protocol draft. I suggest you read it only if you think the
basic idea is sound.  If you think the whole concept is bogus, skip it, but
please come up with a better idea, because the only alternative I can think
of is to define a new method.

DETAILS:

4.2 Query Grammar Schema Discovery

Servers MAY support the discovery of the schema for a query grammar.

The DASL response header provides means for clients to discover the set of
query grammars supported by a resource.  This alone is not sufficient
information for a client to generate a query.  For example, the
DAV:simplesearch grammar defines a set of queries consisting of a set of
operators applied to a set of properties and values, but the grammar itself
does not specify which properties may be used in the query.  While WebDAV
defines a small set of properties that are mandatory (e.g.
DAV:getcontentlength) or  suggested, but optional (e.g., DAV:displayname)
it does not (and can not) mandate that any of  these properties be used in
DASL queries.  In addition, it is likely that some servers will define
additional properties, e.g. for metadata such as author name, or subject.
Query Grammar Schema Discovery allows a client to discover the set of
properties that are searchable, selectable, and sortable, and to discover
their underlying datatypes.  Moreover, while the DAV:simplesearch grammar
defines a minimal set of operators, it is possible that a resource might
support additional operators in a query.  Query Grammar Schema Discovery
allows a client to discover these operators, their syntax, and the
underlying datatypes they  work with.

The schema for a given query grammar depends on both the resource (the
arbiter) and the scope.  A given resource might have access to one set of
properties for one potential scope, and another set for a different scope.
For example, consider a server able to search two distinct collections, one
holding cooking recipes, the other design documents for nuclear weapons.
While both collections might support properties such as author, title, and
date, the first might also define properties such as calories and
preparation time, while the second defined properties such as yield and
applicable patents.  Two distinct resources indexing the same collection
might have access to different properties.  For example, the recipe
collection mentioned above might also indexed by a value-added server that
also stored the names of chefs who had tested the recipe.  Note also that
the available query schema might also depend on other factors, such as the
identity of the principal conducting the search, but these factors are not
exposed in this protocol.

A client obtains the schema for a given query grammar on an arbiter
resource by invoking the PROPFIND method on the dav:queryschema property of
that resource.  Unlike other properties used within the dav:prop element of
dav:propfind, the dav:queryschema property is not empty, instead it
contains elements that indicate the grammar and scope to be used.  This
allows a server to define potentially different query schema for multiple
grammars and scopes on the same arbiter resource.

Each query grammar supported by DASL defines its own syntax for expressing
the possible query schema.  The query schema for DAV:simplesearch is
defined in section 7.18

4.2.1 DAV:queryschema property

<!ELEMENT queryschema (grammar, from) >
<!ELEMENT grammar (#PCDATA) > ; URI of the grammar
;from is defined in section 7.3

4.2.2 Example

PROPFIND / HTTP/1.1
Host: recipes.com
Content-Type: text/xml
Connection: Close
Content-Length: xxxx

<?xml version="1.0"?>
<?xml:namespace ns="DAV:" prefix="D"?>
<D:propfind>
  <D:prop>
    <D:queryschema>
      <D:grammar>DAV:simplesearch</D:grammar>
      <D:from>
        <D:scope>
          <D:href>/recipes/</D:href>
        </D:scope>
        <D:scope>
          <D:href>http://lanl.gov/bombs</D:href>
          <D:depth>1</D:depth>
        </D:scope>
      </D:from>
    </D:queryschema>
  </D:prop>
<D:propfind>

An example response is shown below in section 7.18.
Received on Saturday, 20 June 1998 21:56:03 GMT

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