Hi all!

tl;dr: How to best handle the situation of an old parser cache entry not
containing all the info expected by a newly deployed version of code?


We are currently working to improve our usage of the parser cache for
Wikibase/Wikidata. E.g., We are attaching additional information related to
languagelinks the to ParserOutput, so we can use it in the skin when generating
the sidebar.

However, when we change what gets stored in the parser cache, we still need to
deal with old cache entries that do not yet have the desired information
attached. Here's a few options we have if the expected info isn't in the cached
ParserOutput:

1) ...then generate it on the fly. On every page view, until the parser cache is
purged. This seems bad especially if generating the required info means hitting
the database.

2) ...then invalidate the parser cache for this page, and then a) just live with
this request missing a bit of output, or b) generate on the fly c) trigger a
self-redirect.

3) ...then generated it, attach it to the ParserOutput, and push the updated
ParserOutput object back into the cache. This seems nice, but I'm not sure how
to do that.

4) ...then force a full re-rendering and re-caching of the page, then continue.
I'm not sure how to do this cleanly.


So, the simplest solution seems to be 2, but it means that we invalidate the
parser cache of *every* page on the wiki potentially (though we will not hit the
long tail of rarely viewed pages immediately). It effectively means that any
such change requires all pages to be re-rendered eventually. Is that acceptable?

Solution 3 seems nice and surgical, just injecting the new info into the cached
object. Is there a nice and clean way to *update* a parser cache entry like
that, without re-generating it in full? Do you see any issues with this
approach? Is it worth the trouble?


Any input would be great!

Thanks,
daniel

--
Daniel Kinzler
Senior Software Developer

Wikimedia Deutschland
Gesellschaft zur Förderung Freien Wissens e.V.

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