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Versions: (draft-reddy-dasl-requirements) 00

INTERNET-DRAFT                                               Jim Davis
draft-dasl-requirements-01.txt                       Xerox Corporation
Feb 24, 1999                                              Saveen Reddy
Expires August 24, 1999                          Microsoft Corporation
                                                          Judith Slein
                                                     Xerox Corporation

Requirements for DAV Searching and Locating


Status of this Memo

     This document is an Internet-Draft and is in full conformance
     with all provisions of Section 10 of RFC2026.


     Internet-Drafts are working documents of the Internet Engineering
     Task Force (IETF), its areas, and its working groups.  Note that
     other groups may also distribute working documents as
     Internet-Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
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     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
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     This document is a product of the DAV Searching and Locating
     (DASL) Working Group of the IETF. Please send comments to the
     mailing list at:
       www-webdav-dasl@w3.org
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       www-webdav-dasl-request@w3.org

     Discussions of the list are archived at:
       http://www.w3.org/pub/WWW/Archives/Public/www-webdav-dasl


Abstract


The Distributed Authoring and Versioning protocol [WEBDAV] defines
simple mechanisms to assign and retrieve values for properties. This
document presents requirements for a WebDAV extension to support
efficient searching for resources based on WEBDAV properties and
content. These requirements are intended to be the basis for the DAV
Searching and Location (DASL) protocol.


1. Introduction


Motivation for DASL

WEBDAV and HTTP provide support for client-side search, but not server-
side search. The GET method defined in [HTTP] allows clients to
retrieve a resource's content; the PROPFIND method defined in [WEBDAV]
allows clients to retrieve a resource's properties. Having retrieved a
resource's properties and/or content, the client can compare them to
its search criteria to determine whether the resource is of interest.
Although this client-side searching is logically sufficient, and
requires no modifications to the server, it comes at a significant
cost, because it makes inefficient use of network resources. A client
must retrieve properties and content for each resource under
consideration. Furthermore, it does not take advantage of server
intelligence. Servers capable of searching can use sophisticated
mechanisms to generate results: internal caching of intermediate search
results, content-indexing, etc.

Even simple, common queries may expose these limitations. Consider the
query "find all text files modified during the last week." When such a
query is extended to a large number of clients searching against a
single server, the limitations become more apparent. Client-side
searching has difficulties scaling in these cases.

DASL allows for server-side searching. Server-side searching allows the
client to formulate a query and have the server perform task of
selecting the resources that fit the criteria. This overcomes both of
the limitations of client-side searching described above. The benefit
is a searching solution that scales; the cost is that the server
software becomes more complex.

This document presents requirements for any protocol that might be
proposed for DASL. These requirements come from considerations of the
scenarios presented in [SCENARIOS], from the need to support the WebDAV
object model, the use of HTTP, and general IETF rules. We provide
rationale for those requirements whose justification is not obvious.
We assign each requirement a priority, one or two, where one is higher.
The significance of the number is that priority one requirements are
those that any protocol must define to be considered successful, where
priority two requirements are those that are desirable but not
necessary. There are no priority three requirements at present.


2. Terminology


scope
        a set of resources to be searched.
criteria
        an expression against which each resource in the search scope
        is evaluated.
result set
        a set of records, one for each resource for which the search
        criteria evaluated to True.
record
        a description of a resource. A result record is a set of
        properties, and possibly other descriptive information
result
        A result is a result set, optionally augmented with other
        information describing the search as a whole.
result record definition
        a specification of the set of properties to be returned in the
        result record
sort specification
        a specification of an ordering on the result records in the
        result set.
search modifier
        an instruction that governs the execution of the query but is
        not part of the search scope, result record definition, the
        search criteria, or the sort specification. An example of a
        search modifier is one that controls how much time the server
        can spend on the query before giving a response.
query
        A query is a combination of a search scope, search criteria,
        result record definition, sort specification, and a search
        modifier.
query grammar
        a set of definitions of XML elements, attributes, and
        constraints on their relations and values that defines a set of
        queries and the intended semantics.
schema
        a listing, for any given grammar and scope, of the properties
        and operators that may be used in a query with that grammar and
        scope.
Hit highlighting
        is a specification of the location(s) within a resource
        containing text that matched a content-query. It allows clients
        to provide visual cues to a user to identify segments in a text
        resource that cause them to match content-based queries.
paged results
        allows a client to request that the server return a subset of
        the result set rather than the entire set. In subsequent calls
        to the server, additional results from the same query can be
        requested. Paged results are intended to improve the
        performance and manageability of search results.

In addition to the terms defined above, this document uses terminology
consistent with [HTTP] and [WEBDAV].

Requirements are divided into five categories, and numbered within each
category. The categories are Scope, Criteria, Record Definition, Other
and Discovery.


3. Requirements: Scope


S1: It is possible to specify at least one resource in the scope (P1).
It is possible to specify a set of distinct, unrelated resources in the
scope (P2).
        As this is the first requirement in the document, we explain
        the notation. S1 means this is the requirement one in the Scope
        section, P1 means that the requirement to have at least one
        resource in scope is essential, and P2 means that allowing more
        than one is nice but not required.

        Rationale: Supporting multiple resources in scope could be
        difficult to define, because distinct resources may have
        different sets of metadata, support different operators, or
        have different access rights.

S2 It is possible to specify a WebDAV collection as a scope (P1).

S3: It is possible to specify other types of resources in a scope (P2).
        Rationale: A client might wish to determine whether a given
        resource was of interest without transferring it.

S4: When the scope is a collection, it is possible to specify the depth
(P1).
        Users often intend to scope their searches either to the
        immediate children of a container or to extend the search
        recursively to the container's children. Furthermore, depth
        control is needed to prevent servers from performing
        unnecessary work.


4. Requirements: Criteria


Criteria generalities

C1: It is possible to search properties in a query (P1). It is possible
to search both DAV-defined and application-defined properties in a
query (P1).

        Further requirements for properties are below.

C2: It is possible to search content in a query (P1).
        Note that at this writing, unlike property searches, there is
        no single widely accepted semantics for content-based queries.
        Further requirements for content criteria are below.

C3: It is possible to search both properties and content in a single
query.

C4: It is possible to combine criteria with Boolean operators (i.e.
and, or, not) (P1).

Criteria for properties

C5: It is possible to include undefined properties in a query without
error (P1).
        Rationale:. This arises from the property model of DAV. Unlike
        the more familiar relational model, DAV does not define tables
        or schema for resources, hence there is no guarantee that all
        properties will be defined for all resources. Moreover, DAV
        allows an client to store arbitrary properties on arbitrary
        resources. Therefore DASL must support queries that use
        properties that are not defined on all resources in the scope.
        If such a query failed, there would be no way to locate the
        desired resources.

C5.1: It is possible to test whether a property is defined (P1).

C6.1: It is possible to compare a property value to a constant
value (P1).

C6.2.1: It is possible to compare property values to other properties
of the same resource (P2).

C6.2.2: It is possible to compare property values to other properties
of other resources (P2).

        Note that this may involve a "join". We do not expect the first
        version of the DASL protocol to meet this requirements.

C6.3: It is possible to compare property values to results of
expressions (P2).

C6.4: It is possible to match property values with string-ending
wildcards (P1). It is possible to match property values with pattern
matching operators similar to the SQL "like" operator or regular
expressions (P2).

        The minimum is necessary to enable DASL to locate resources by
        content type, e.g. to locate all image files by comparison with
        "image/*". More powerful comparisons are useful when strings
        encode structured data such as times or lists. Note that these
        are constraints on what the protocol must define, not on what
        servers must necessarily implement.

C6.5: It is possible to compare property values taking into account
their structure (P2).

        Explanation: Some WebDAV properties are defined to contain
        strings (e.g. DAV:getcontenttype), but others contain
        structured values (e.g., DAV:resourcetype, DAV:lockdiscovery).
        Support for structured value criteria is needed, for example,
        to locate resources locked in a certain manner by a certain
        principal. The working group consensus is that this feature,
        while undeniably very useful, is so difficult to define that it
        is better for DASL to proceed than attempt to define it. Also,
        there is much activity in the W3C to define an XML query
        language, and it was felt better to wait for this to complete
        than to define a competing standard.

C7.1: The protocol defines an equality operator (P1).

C7.2: The protocol defines relative operators (P1).

C8: The protocol defines means to specify case sensitivity (P1).

        Note this does not say that all DASL servers must support both
        case-sensitive and case-insensitive comparisons, but only that
        the protocol must be able to express a client's preference, and
        define behavior in the case where the server cannot support
        that preference.

C9: The protocol supports language-specific definitions for string
comparison and sorting (P1).

        Different cultures define different rules for string
        comparison, e.g. for collating sequence and for significance of
        diacritics. Cross-language comparison is out of scope for DASL,
        but comparisons within the same language must be done with the
        appropriate semantics.

        Requirements: Criteria for content searches

C10: It is possible to search content of any text media type (P1). The
definition of "searching content" for DASL means locating sequences of
characters in the contents of the resource.

        DASL defines no requirements for searching for structure within
        text media types (e.g. for finding character strings only
        within certain HTML tags.) This functionality is too
        complicated to specify at the present time.

        DASL defines no requirements for searching other media types
        that might contain text (e.g. subtypes of application).
        Searching non-text media types (e.g.images, audio) is out of
        scope for DASL.

C11.1: It is possible to search for words that are within a specified
number of words (or, for some languages, characters) of each
other (P1).

        This is often called 'near' search. It is used to locate
        concepts that can be expressed in more than one way using the
        same set of words, e.g. one might locate both "the President's
        impeachment" and "the impeachment of the President".

C11.2: It is possible to search for words that occur within the same
grammatical context, e.g. same phrase, sentence, or paragraph (P2).

        This is sometimes called 'in' search.

C12.1: It is possible for a client to control whether content searches
does or does not use a stemming comparison (P2).

C12.2: It is possible for a client to request comparisons using
phonetic similarity (e.g. soundex) (P2)

C12.3: It is possible for the client to request keyword expansion
(thesaurus expansion) (P2).

C13: It is possible for a client to conduct a relevance search (P2). In
such a search, the query consists of a set of words (perhaps an entire
resource), and the result is a list of resources whose contents most
closely resemble the query, sorted in decreasing order of resemblance.


5. Requirements: Results


R1: It is possible to specify a sorting for the result set (P1).

R2: It is possible to specify a set of properties to be returned in the
result records, distinct from the properties in criteria (P1).

        For example, a query might ask for "the authors of those
        documents under 10K in size". In this case, the criterion
        relates only to the size, but the desired result record
        contains only the author.

R3: It is possible for a client to request limits on the resources
consumed in creating of transmitting in the result set (P1).

        Some queries can potentially return very large result sets.
        Clients that are good citizens will voluntarily limit the size
        of such results. In addition, some servers may charge money for
        queries.

R3.1: It is possible for a client to limit the number of records in the
result set (P1).

        This is the most meaningful unit of resource consumption to the
        client.

R4: It is possible for the server to return fewer result records than
match the criteria (P1).

        "Client proposes, server disposes".

R5: It is possible to a client to request paged results (P1).

        Paged retrieval is necessary if result sets are very large and
        if clients must also present a responsive interface to a user.
        Note that this requirement is silent about whether a server
        implements paged results by storing results from a query or
        recalculating them as needed.


6. Requirements: Other


O1: It is possible to support multiple query grammars (P1).

        Rationale: A particular query grammar may not expose all the
        useful searching functionality of a server. Clients should be
        allowed to query a server using any grammar that takes
        advantage of those special server capabilities. This
        requirement also allows DASL to define an initial limited query
        grammar which meets all the mandatory requirements without
        needing to address all the desirable, but non-mandatory
        requirements.

O2: It is possible to extend the basic grammar defined by DASL (P1).

03: It is possible for the server to redirect a query (P1).

        This is useful when a server is not able to search a given
        scope, but can refer the client to another server which is able
        to search the scope.

O4: It is possible for the client to request hit highlighting (P2).


7. Requirements: Discovery


D1: It is possible for a client to discover the set of query grammars
supported by a server (P1).

        Without this, it is not very useful for servers to support
        multiple grammars.

D2: It is possible for a client to discover the schema supported by a
server for a particular grammar with a particular scope (P1).

        Note that the schema may differ depending on the scope. Query
        schema discovery allows a client to use optional properties and
        operators supported by a server.

D3: It is possible for a client to determine information about the
properties within a scope (P2).

        This information can enable a user interface to help a user to
        construct a valid query, for example by providing meaningful
        names for properties, constraints on values, hints about data
        type, and so on, or information about expected performance, for
        example whether a property is indexed (and hence more quickly
        searched).


8. External Requirements


DASL must describe how to perform searches on internationalized content
and properties. This is in keeping with IETF policy.

Information intended for user comprehension must conform to the IETF
Character Set Policy [CHAR].

The WebDAV working group is currently addressing the standardization of
mechanisms for authors to submit variants and version of resources, or
for means of exposing access control. DASL should provide mechanisms
that can query for variants, versions, and access control but can not
do so until they are defined. Likewise, DASL may contribute
requirements to access control (e.g. control over querying).


9. Related Work


Z39.50: "Information Retrieval (Z39.50): Application Service Definition
and Protocol Specification".
http://lcweb.loc.gov/z3950/agency/

Z39.50 Profile for Simple Distributed Search and Ranked Retrieval
http://lcweb.loc.gov/z3950/agency/profiles/zdsr.html

The STARTS Protocol
http://www-db.stanford.edu/~gravano/starts.html

The Harvest Information Discovery and Access System
http://mordor.transarc.com/afs/transarc.com/public/trg/Harvest/


10. References


[CHAR]      H.T. Alvestrand, "IETF Policy on Character Sets and
            Languages", June 1997, internet-draft, work-in-progress,
            draft-alvestrand-charset-policy-02.txt.

[HTTP]      R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, and
            T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
            RFC 2068, U.C. Irvine, DEC, MIT/LCS, January 1997.

[SCENARIOS] Henderson, R. et al Scenarios for DAV Searching and
            Locating. Work in progress.
            draft-henderson-dasl-scenarios-00.html, September 18, 1998
            (Expires Mar 23, 1999)

[WEBDAV]    Y. Y. Goland, E. J. Whitehead, Jr., A. Faizi, S. R. Carter,
            D. Jensen, "Extensions for Distributed Authoring and
            Versioning on the World Wide Web", IETF Proposed Standard,
            RFC 2518


11. Authors' Addresses


        Jim Davis
        Xerox Corporation
        3333 Coyote Hill Road
        Palo Alto, CA 94304
        Email: jdavis@parc.xerox.com

        Saveen Reddy
        Microsoft Corporation
        One Microsoft Way
        Redmond WA, 9085-6933
        email: saveenr@microsoft.com

        Judith Slein
        Xerox Corporation
        800 Phillips Road 105-50C
        Webster, NY 14580
        Email: slein@wrc.xerox.com


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