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From: rob@agate.Berkeley.EDU (Rob Robertson)
Newsgroups: news.software.b,news.software.nntp,news.software.readers
Subject: FAQ:  Overview database / NOV General Information
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Date: 2 Nov 1993 11:02:06 -0000
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$Header: /var/news/doc/RCS/nov-faq.1,v 1.4 1993/09/08 23:03:08 rob Exp rob $

                          Table Of Contents:
                          ------------------

	What is `threading' and why is it good for you?
	What is the Overview/NOV database?
	What is the format of the NOV database?
	What optional headers should I configure into the NOV database?  
	What is XOVER?
	How do I know if my NNTP server does XOVER?
	What does it buy me?
	How much disc space does the Overview database take up?
	What News Transport Systems support NOV/XOVER and where can I
		get it?
	What Newsreaders Support NOV/XOVER?
	Is there a reference threading implementation for NOV?
	Any tips on configuring the Overview database?
	Contributions to NOV FAQ.

------------------------------

Subject: What is `threading' and why is it good for you?

I put this in first as a background question.

As news volume has grown in the last couple of years, the organization
of the data presented to the user has become more important.  With the
possibility of being presented with several hundred articles to read
in a newsgroup, newer newsreaders have incorporated features that
organize things by subject, and sort them in the order posted, and
`thread' articles in groups according to who replied to who.  Note
there is a difference between subject sorting and date sorting, and
threading.  

This subject sorting and threading can be expensive using normal
NNTP commands, as it involves opening each article and transferring it
over to the client, for a relatively small bit of information.  This
led to newsreader authors extending the NNTP command set so that they
could set up an auxiliary database of just the information they
needed.  The database would be updated as news arrived on the server,
and when a client requested thread information about a particular
group, it would just open one file and send it out, which made things
pretty zippy. 

The downside of this, was that every newsreader had their own
threading database, their own specialized NNTP extension, and it was
customized to their `way' of doing threads.  Trn, NN, and tin are
prime examples.  Each database occupies a fair amount of space, and
in the case of trn and tin, the daemons that maintain the databases
use up a nontrivial amount of system resources.

------------------------------

Subject: What is the Overview/NOV database?

The Overview or News OverView (NOV) database was designed by Geoff
Collyer.  Its a more generalized database, it it contains *no*
threading information, just the information needed to thread.  The
overview database is just a database of files, one for each group,
containing a reference to each article, with seven or eight of the
most popular headers.  Thus, the newsreader client can get most of the
information it needs to thread, but it does the threading the way it
wants to.  Yes, the downside is that now the threading occurs on the
client's CPU, but that's not as expensive as it may seem, if done
properly. 

------------------------------

Subject: What is the format of the NOV database?

[this is stolen from Geoff Collyer's newsoverview(5) man page]

Each newsgroup directory contains a file named  `.overview',
containing  one-line  summaries  of  articles in that group.  Fields
are separated by tabs, and any tabs  or  newlines  in the  original
articles  headers  have  been  replaced  with spaces.  The fields are,
in  order:  article  number  (file name),  subject,  author, date,
message-id, references, byte  count, line count, and optionally other
headers, as arranged  locally  (none  are  supplied  by  the  database
maintenance software, as shipped).  The line-count and references
field may  be  empty.   If the optional other headers are present,
they include their header keyword and  colon;  if  they  are absent
entirely, the tab after the line-count field may also be absent.

ie:

    artid|subject|From|Date|Message-Id|References|bytecount|linecount|\
		Optional-Header: Stuff

where | represents a tab and \ a line continuation.

Remember that header keywords in optional headers should be matched
case insensitively.

------------------------------

Subject: What optional headers should I configure into the NOV database?  

It's nice to configure the pseudo-header Xref: in.  Xref: is a site
specific header that, tells if present where that are article has been
cross posted.  It is useful for newsreaders who want to kill cross
posted articles, you've told them to kill.

------------------------------

Subject: What is XOVER?

XOVER is an NNTP extension for accessing the Overview database.  It
is becoming a defacto standard for accessing bulk header/thread
information via NNTP. 

It is used after establishing a current group with the GROUP command,
and operations on that group.  It's syntax is

	XOVER artnumber1[-artnumber2]

Where artnumber1 is < artnumber2.  If successful, it returns a 224
reply code, and the overview data [as in the Overview database] for
the requested articles.  It is terminated by a "." (dot) on a line by
itself.

NNTP servers implementing the XOVER command should try to ensure that
the data is as consistant as possible, they should avoid handing out
XOVER information for articles that don't exist, and should generate
XOVER information on the fly for articles that do exist, but aren't
found in the Overview database for some reason.  While the job of
XOVER is to provide access to the Overview database, validating the
Overview data is best done in the NNTP server, rather than having the
NNTP client go through contortions to do it.

------------------------------

Subject: How do I know if my NNTP server does XOVER?

By initially trying it, and looking at the response code.  Before
doing a GROUP in your session, you should get back a 500 (no such
command) response if the server doesn't implement XOVER, and a 412
(not in a newsgroup) response, if it does.

------------------------------

Subject: What does it buy me?

Speed, if you have newsreader clients that can take advantage of this, 
disc space if you are currently supporting more than one version of a
threading database.

If used by the NNTP daemon, XHDR can be speed up by *orders* of
magnitude by using the Overview database.

------------------------------

Subject: How much disc space does the Overview database take up?

About 10% of what your news spool space is.

------------------------------

Subject: What News Transport Systems support NOV/XOVER and where can I
	 get it? 

INN 1.4 supports the Overview database and XOVER right out of the box.

C-News has an add-on patch kit that allows it to support NOV.  It's
available via ftp from world.std.com:/src/news/nov.dist.tar.Z.  That
gets you to the Overview stage, if you want to use the XOVER
extension, you need Wayne Davison's version of Stan Barber's NNTP
reference package  (via ftp via ftp.uu.net in
/news/readers/trn/nntp-t4.tar.gz).  This is labeled as BETA at the
moment, but for not much longer.    

The next version of C-News promises to integrate all this.

------------------------------

Subject: What Newsreaders Support NOV/XOVER?

Among `officially released' newsreaders trn3.3 and tin-1.21 support
both the Overview database and XOVER. 

Win-vn 0.8 for MS-Windows claims to support XOVER [Alan Thew].

Geoff has modified `proof of concept' newsreaders in
world.std.com:/src/news/ that support the Overview database (*not*
XOVER).

There is a modified version of NN (read unofficial) that supports
XOVER in agate.berkeley.edu:/pub/usenet/NN-6.4P18+xover.tar.Z.  The
official NN maintainer has been saying that that he's going to 
issue a release that's going to support XOVER `soon' for a bunch o
months now. 

There are some excellent patches by Felix Lee that allow GNUS to
support XOVER (plus some kool async transfer mods).  Perhaps he'll
post them again soon.  Perhaps they will be integrated into GNUS.

------------------------------

Subject:  Is there a reference threading implementation for NOV data? 

Yes, in Geoff Collyer's package at
world.std.com:/src/news/nov.dist.tar.Z.  

------------------------------

Subject: Any tips on configuring the Overview database?

It's probably a good idea to configure in the optional pseudo header
Xref:, as many newsreaders like to see it.

If newsreading is totally limited to NNTP, you might want to consider
putting your Overview database on another partition on another disc
(if you have space).  This buys you more room for news on your spool
disc, and also spreads the disc accesses out across multiple drives,
reducing the beating your spool partition is going to take.  Currently
INN can do this easily, by changing the OVERVIEWDIR variable.  C-news
is more problematic.

It would be nice if news news transport authors would not assume that 
the Overview database directory and the news spool directory are the 
same, so that others that read via NFS or locally can enjoy the same
performance win....

------------------------------

Subject: Contributions to NOV FAQ.

Thanks to the following for contributions, additions, corrections, and
updates: 

    Geoff Collyer <geoff@world.std.com>
    Wayne Davison <davison@borland.com>
    Christopher Davis <ckd@eff.org>
    Jason Haar <j.haar@cantva.canterbury.ac.nz>
    Damon Hart-Davis <d@exnet.co.uk>
    Iain Lea <iain@anl433.erlm.siemens.de>
    Mark Linimon <linimon@nominil.lonesome.com>
    Doug Sewell <doug@cc.ysu.edu>
    News.Admin@liverpool.ac.uk (Alan Thew)

This posting, like much of Usenet, is maintained on a purely volunteer
basis.  I welcome reactions, additions, and corrections via email at
rob@agate.berkeley.edu.
