rfc1913.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                          C. Weider
Request for Comments: 1913                                        Bunyip
Category: Standards Track                                     J. Fullton
                                                                   CNIDR
                                                                S. Spero
                                                                     EIT
                                                           February 1996


               Architecture of the Whois++ Index Service

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   The authors describe an architecture for indexing in distributed
   databases, and apply this to the WHOIS++ protocol.

1. Purpose:

   The WHOIS++ directory service [Deutsch, et al, 1995] is intended to
   provide a simple, extensible directory service predicated on a
   template-based information model and a flexible query language. This
   document describes a general architecture designed for indexing
   distributed databases, and then applys that architecture to link
   together many of these WHOIS++ servers into a distributed, searchable
   wide area directory service.

2. Scope:

   This document details a distributed, easily maintained architecture
   for providing a unified index to a large number of distributed
   WHOIS++ servers. This architecture can be used with systems other
   than WHOIS++ to provide a distributed directory service which is also
   searchable.

3. Motivation and Introduction:

   It seems clear that with the vast amount of directory information
   potentially available on the Internet, it is simply not feasible to
   build a centralized directory to serve all this information. If we
   are to distribute the directory service, the easiest (although not



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RFC 1913       Architecture of the Whois++ Index Service   February 1996


   necessarily the best) way of building the directory service is to
   build a hierarchy of directory information collection agents. In this
   architecture, a directory query is delivered to a certain agent in
   the tree, and then handed up or down, as appropriate, so that the
   query is delivered to the agent which holds the information which
   fills the query.  This approach has been tried before, most notably
   in some implementations of the X.500 standard. However, there are
   number of major flaws with the approach as it has been taken. This
   new Index Service is designed to fix these flaws.

3.1. The search problem

   One of the primary assumptions made by recent implementations of
   distributed directory services is that every entry resides in some
   location in a hierarchical name space. While this arrangement is
   ideal for reading the entry once one knows its location, it is not as
   good when one is searching for the location in the namespace of those
   entries which meet some set of criteria. If the only criteria we know
   about a desired entry are items which do not appear in the namespace,
   we are forced to do a global query. Whenever we issue a global query
   (at the root of the namespace), or a query at the top of a given
   subtree in the namespace, that query is replicated to "all" subtrees
   of the starting point. The replication of the query to all subtrees
   is not necessarily a problem; queries are cheap. However, every
   server to which the query has been replicated must process that
   query, even if it has no entries which match the specified criteria.
   This part of the global query processing is quite expensive. A poorly
   designed namespace or a thin namespace can cause the vast majority of
   queries to be replicated globally, but a very broad namespace can
   cause its own navigation problems. Because of these problems, search
   has been turned off at high levels of the X.500 namespace.

3.2. The location problem

   With global search turned off, one must know in advance how the name
   space is laid out so that one can guide a query to a proper location.
   Also, the layout of the namespace then becomes critical to a user's
   ability to find the desired information. Thus there are endless
   battles about how to lay out the name space to best serve a given set
   of users, and enormous headaches whenever it becomes apparent that
   the current namespace is unsuited to the current usages and must be
   changed (as recently happened in X.500). Also, assuming one does
   impose multiple hierarchies on the entries through use of the
   namespace, the mechanisms to maintain these multiple hierarchies in
   X.500 do not exist yet, and it is possible to move entries out from
   under their pointers.  Also, there is as yet no agreement on how the
   X.500 namespace should look even for the White Pages types of
   information that is currently installed in the X.500 pilot project.



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RFC 1913       Architecture of the Whois++ Index Service   February 1996


3.3. The Yellow Pages problem

   Current implementations of this hierarchical architecture have also
   been unsuited to solving the Yellow Pages problem; that is, the
   problem of easily and flexibly building special-purpose directories
   (say of molecular biologists) and of automatically maintaining these
   directories once they have been built. In particular, the attributes
   appropriate to the new directory must be built into the namespace
   because that is the only way to segregate related entries into a
   place where they can be found without a global search. Also, there is
   a classification problem; how does one adequately specify the proper
   categories so that people other than the creator of the directory can
   find the correct subtree? Additionally, there is the problem of
   actually finding the data to put into the subtree; if one must
   traverse the hierarchy to find the data, we have to look globally for
   the proper entries.

3.4. Solutions

   The problems examined in this section can be addressed by a
   combination of two new techniques: directory meshes and forward
   knowledge.

4. Directory meshes and forward knowledge

   We'll hold off for a moment on describing the actual architecture
   used in our solution to these problems and concentrate on a high
   level description of what solutions are provided by our conceptual
   approach. To begin with, although every entry in WHOIS++ does indeed
   have a unique identifier (resides in a specific location in the
   namespace) the navigational algorithms to reach a specific entry do
   not necessarily depend on the identifier the entry has been assigned.
   The Index Service gets around the namespace and hierarchy problems by
   creating a directory mesh on top of the entries.  Each layer of the
   mesh has a set of 'forward knowledge' which indicates the contents of
   the various servers at the next lower layer of the mesh. Thus when a
   query is received by a server in a given layer of the mesh, it can
   prune the search tree and hand the query off to only those lower
   level servers which have indicated that they might be able to answer
   it. Thus search becomes feasible at all levels of the mesh. In the
   current version of this architecture, we have chosen a certain set of
   information to hand up the mesh as forward knowledge. This may or may
   not be exactly the set of information required to construct a truly
   searchable directory, but the protocol itself doesn't restrict the
   types of information which can be handed around.

   In addition, the protocols designed to maintain the forward knowledge
   will also work perfectly well to provide replication of servers for



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RFC 1913       Architecture of the Whois++ Index Service   February 1996


   redundancy and robustness. In this case, the forward knowledge handed
   around by the protocols is the entire database of entries held by the
   replicated server.

   Another benefit provided by the mesh of index servers is that since
   the entry identification scheme has been decoupled from the
   navigation service, multiple hierarchies can be built and easily
   maintained on top of the existing data. Also, the user does not need
   to know in advance where in the mesh the entry is contained.

   Also, the Yellow Pages problem now becomes tractable, as the index
   servers can pick and choose between information proffered by a given
   server; because we have an architecture that allows for automatic
   polling of data, special purpose directories become easy to construct
   and to maintain.

5. Components of the Index Service:

5.1. WHOIS++ servers

   The whois++ service is described in [Deutsch, et al, 1995]. As that
   service specifies only the query language, the information model, and
   the server responses, whois++ services can be provided by a wide
   variety of databases and directory services. However, to participate
   in the Index Service, that underlying database must also be able to
   generate a 'centroid', or some other type of forward knowledge, for
   the data it serves.

5.2. Centroids as forward knowledge

   The centroid of a server is comprised of a list of the templates and
   attributes used by that server, and a word list for each attribute.
   The word list for a given attribute contains one occurrence of every
   word which appears at least once in that attribute in some record in
   that server's data, and nothing else.

   A word is any token delimited by blank spaces, newlines, or the '@'
   character, in the value of an attribute.

   For example, if a whois++ server contains exactly three records, as
   follows:

   Record 1                        Record 2
   Template: User                  Template: User
   First Name: John                First Name: Joe
   Last Name: Smith                Last Name: Smith
   Favourite Drink: Labatt Beer    Favourite Drink: Molson Beer




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RFC 1913       Architecture of the Whois++ Index Service   February 1996


   Record 3
   Template: Domain
   Domain Name: foo.edu
   Contact Name: Mike Foobar

   the centroid for this server would be

   Template:         User
   First Name:       Joe
                     John
   Last Name:        Smith
   Favourite Drink:  Beer
                     Labatt
                     Molson

   Template:         Domain
   Domain Name:      foo.edu
   Contact Name:     Mike
                     Foobar

   It is this information which is handed up the tree to provide forward
   knowledge.  As we mention above, this may not turn out to be the
   ideal solution for forward knowledge, and we suspect that there may
   be a number of different sets of forward knowledge used in the Index
   Service. However, the directory architecture is in a very real sense
   independent of what types of forward knowledge are handed around, and
   it is entirely possible to build a unified directory which uses many
   types of forward knowledge.

5.3. Index servers and Index server Architecture

   A whois++ index server collects and collates the centroids (or other
   forward knowledge) of either a number of whois++ servers or of a
   number of other index servers. An index server must be able to
   generate a centroid for the information it contains. In addition, an
   index server can index any other server it wishes, which allows one
   base level server (or index server) to participate in many
   hierarchies in the directory mesh.

5.3.1. Queries to index servers

   An index server will take a query in standard whois++ format, search
   its collections of centroids and other forward information, determine
   which servers hold records which may fill that query, and then
   notifies the user's client of the next servers to contact to submit
   the query (referral in the X.500 model). An index server can also
   contain primary data of its own; and thus act a both an index server
   and a base level server. In this case, the index server's response to



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   a query may be a mix of records and referral pointers.

5.3.2. Index server distribution model and centroid propogation

   The diagram on the next page illustrates how a mesh of index servers
   might be created for a set of whois++ servers. Although it looks like
   a hierarchy, the protocols allow (for example) server A to be indexed
   by both server D and by server H.

     whois++               index                   index
     servers               servers                 servers
                           for                     for
                           whois++                 lower-level
                           servers                 index servers