If you open a summary while unplugged and, Gnus knows from the
group's active range that there are more articles than the
headers currently stored in the Agent, you may see some articles
whose subject looks something like ‘[Undownloaded article #####]’. These
are placeholders for the missing headers. Aside from setting a
mark, there is not much that can be done with one of these
placeholders. When Gnus finally gets a chance to fetch the
group's headers, the placeholders will automatically be replaced
by the actual headers. You can configure the summary buffer's
maneuvering to skip over the placeholders if you care (See
gnus-auto-goto-ignores).
While it may be obvious to all, the only headers and articles available while unplugged are those headers and articles that were fetched into the Agent while previously plugged. To put it another way, “If you forget to fetch something while plugged, you might have a less than satisfying unplugged session”. For this reason, the Agent adds two visual effects to your summary buffer. These effects display the download status of each article so that you always know which articles will be available when unplugged.
The first visual effect is the ‘%O’ spec. If you customize
gnus-summary-line-format to include this specifier,
you will add a single character field that indicates an article's
download status. Articles that have been fetched into either the
Agent or the Cache, will display
gnus-downloaded-mark (defaults to
‘+’). All
other articles will display gnus-undownloaded-mark
(defaults to ‘-’). If you open a group that has not
been agentized, a space (‘ ’) will be displayed.
The second visual effect are the undownloaded faces. The
faces, there are three indicating the article's score (low,
normal, high), seem to result in a love/hate response from many
Gnus users. The problem is that the face selection is controlled
by a list of condition tests and face names (See
gnus-summary-highlight). Each condition is tested in
the order in which it appears in the list so early conditions
have precedence over later conditions. All of this means that, if
you tick an undownloaded article, the article will continue to be
displayed in the undownloaded face rather than the ticked
face.
If you use the Agent as a cache (to avoid downloading the same article each time you visit it or to minimize your connection time), the undownloaded face will probably seem like a good idea. The reason being that you do all of our work (marking, reading, deleting) with downloaded articles so the normal faces always appear. For those users using the agent to improve online performance by caching the NOV database (most users since 5.10.2), the undownloaded faces may appear to be an absolutely horrible idea. The issue being that, since none of their articles have been fetched into the Agent, all of the normal faces will be obscured by the undownloaded faces.
If you would like to use the undownloaded faces, you must
enable the undownloaded faces by setting the
agent-enable-undownloaded-faces group parameter to
t. This parameter, like all other agent parameters,
may be set on an Agent Category (see Agent Categories), a
Group Topic (see Topic Parameters),
or an individual group (see Group
Parameters).
The one problem common to all users using the agent is how
quickly it can consume disk space. If you using the agent on many
groups, it is even more difficult to effectively recover disk
space. One solution is the ‘%F’ format available in
gnus-group-line-format. This format will display the
actual disk space used by articles fetched into both the agent
and cache. By knowing which groups use the most space, users know
where to focus their efforts when “agent expiring”
articles.