linkcache.txt

The LinkCache class maintains a list of article titles and
the information about whether or not the article exists in
the database. This is used to mark up links when displaying
a page. If the same link appears more than once on any page,
then it only has to be looked up once.

In practice, what happens is that the global cache object
$wgLinkCache is consulted and updated every time the function
getArticleID() from Title is called.

This has a side benefit that we take advantage of. We have
tables "links" and "brokenlinks" which we use to do things
like the Orphans page and Whatlinkshere page. It just so
happens that after we update a page, we display it--and as
we're displaying it, we look up all the links on that page,
causing them to be put into the cache. That information is
exactly what we need to update those two tables. So, we do
something tricky when we update pages: just after the update
and before we display, we clear the cache. Then we display
the updated page. Finally, we put a LinksUpdate object onto
the deferred updates list, which fetches its information from
the cache.

There's a minor complication: displaying a page also looks up
a few things like the talk page link in the quick bar and the
date links. Since we don't want those in the link tables, we
must take care to suspend the cache while we look those up.
Skin.php does exactly that--see dateLink(), for example.

