Article placeholders are displayed on a dynamic special page (Special:AboutTopic/Q\d+) that serves Wikidata's content in a human readable way. This special page is right now being linked to on Special:Search (it appears in search results per default). The special page can also be accessed with arbitrary item id manually given. When there already is page on a wiki linked with a given item, we redirect to the local page instead (so if there already is an article about Berlin on a wiki, the article placeholder for the Berlin item will redirect to that article).
Special:AboutTopic basically just parses a Wikitext snippet which transcludes a template which then invokes a Scribunto/Lua module. That module does most of the heavy lifting, by using Wikibase's functionality for Wikidata data access.
- It's planned to have these placeholders indexed by search engines (only notable ones: 3,451,555, as of 2016-08-14, minus the ones where articles already exists at the individual wiki). This will mean that we will probably get occasional requests to all of these pages on all wikis with AP enabled.
- It's also planned to have article placeholders on more wikis and also on wikis with more traffic. The placeholders need to handle the additional traffic this will cause.
In order to be able to implement the changes suggested above, we will need to find a strategy for caching article placeholders and probably also to invalidate that caching in case something relevant on Wikidata changes.
The easiest solution to this would be to implement T109458: [Story] CDN cache article placeholders which suggests to cache the placeholders for a certain amount of time, without any invalidation strategy.
As a limited trial, we could make placeholders indexable on a single wiki only, or maybe even a subset of the notable placeholders on one or two wikis.
Article placeholders are currently enabled on a few low-traffic wikis, right now (as of 2016-08-14): cywiki, eowiki, guwiki, htwiki, knwiki, lvwiki, napwiki, nnwiki, orwiki, testwiki, test2wiki, testwikidatawiki (wmgUseArticlePlaceholder). Current traffic statistics for the placeholders can be found on Grafana.
: On wikidata: SELECT COUNT(*) FROM wb_entity_per_page INNER JOIN page_props AS pp1 ON pp1.pp_page = epp_page_id AND pp1.pp_propname = 'wb-claims' INNER JOIN page_props AS pp2 ON pp2.pp_page = epp_page_id AND pp2.pp_propname = 'wb-sitelinks' WHERE epp_redirect_target IS NULL AND pp1.pp_value > 2 AND pp2.pp_value > 2;