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(gnus)Asynchronous Fetching

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Asynchronous Article Fetching
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   If you read your news from an NNTP server that's far away, the
network latencies may make reading articles a chore.  You have to wait
for a while after pressing `n' to go to the next article before the
article appears.  Why can't Gnus just go ahead and fetch the article
while you are reading the previous one?  Why not, indeed.

   First, some caveats.  There are some pitfalls to using asynchronous
article fetching, especially the way Gnus does it.

   Let's say you are reading article 1, which is short, and article 2 is
quite long, and you are not interested in reading that.  Gnus does not
know this, so it goes ahead and fetches article 2.  You decide to read
article 3, but since Gnus is in the process of fetching article 2, the
connection is blocked.

   To avoid these situations, Gnus will open two (count 'em two)
connections to the server.  Some people may think this isn't a very nice
thing to do, but I don't see any real alternatives.  Setting up that
extra connection takes some time, so Gnus startup will be slower.

   Gnus will fetch more articles than you will read.  This will mean
that the link between your machine and the NNTP server will become more
loaded than if you didn't use article pre-fetch.  The server itself will
also become more loaded--both with the extra article requests, and the
extra connection.

   Ok, so now you know that you shouldn't really use this thing...
unless you really want to.

   Here's how:  Set `gnus-asynchronous' to `t'.  The rest should happen
automatically.

   You can control how many articles are to be pre-fetched by setting
`gnus-use-article-prefetch'.  This is 30 by default, which means that
when you read an article in the group, the back end will pre-fetch the
next 30 articles.  If this variable is `t', the back end will pre-fetch
all the articles it can without bound.  If it is `nil', no pre-fetching
will be done.

   There are probably some articles that you don't want to
pre-fetch--read articles, for instance.  The
`gnus-async-prefetch-article-p' variable controls whether an article is
to be pre-fetched.  This function should return non-`nil' when the
article in question is to be pre-fetched.  The default is
`gnus-async-read-p', which returns `nil' on read articles.  The
function is called with an article data structure as the only parameter.

   If, for instance, you wish to pre-fetch only unread articles shorter
than 100 lines, you could say something like:

     (defun my-async-short-unread-p (data)
       "Return non-nil for short, unread articles."
       (and (gnus-data-unread-p data)
            (< (mail-header-lines (gnus-data-header data))
               100)))
     
     (setq gnus-async-prefetch-article-p 'my-async-short-unread-p)

   These functions will be called many, many times, so they should
preferably be short and sweet to avoid slowing down Gnus too much.
It's probably a good idea to byte-compile things like this.

   Articles have to be removed from the asynch buffer sooner or later.
The `gnus-prefetched-article-deletion-strategy' says when to remove
articles.  This is a list that may contain the following elements:

`read'
     Remove articles when they are read.

`exit'
     Remove articles when exiting the group.

   The default value is `(read exit)'.