[Mediawiki-api] Possible API load increase from upcoming Pending Changes release

[Rob Lanphier]

Hi everyone,

As you may know, we're working on an update to Pending Changes for
next month.  One thing we're trying to do is to improve the perceived
performance:
https://bugzilla.wikimedia.org/show_bug.cgi?id=25289
("Make review load faster by speeding up display of old revisions")

The problem, in a nutshell, is that the diff page includes two parts:
the diff itself, and a parsed version of the later revision.  If the
later revision is not the latest revision, that revision isn't cached.
 For complicated pages, parsing can take 20 seconds or longer.

Priyanka is the dev working on this, and the strategy she is pursuing
now is to switch from immediately parsing the later revision, and
instead calling the API to get a parsed version of the latest
revision.  What that means is that we can display the diff
immediately, and then inject the diff via Javascript.

In talking to Sam this morning about this, a couple things became clear:
1.  This requires action=parse, which is already on the list of APIs
that generate healthy load
2.  While this move should be net-neutral in CPU load, it does shift
the load from general purpose Apaches to the API servers, the latter
of which are more heavily loaded

Additionally, it's theoretically possible that this will actually
generate more load, since it'll be easier for people to skim the
history, unintentionally generating lots of (never completed) API
calls.

How big of a problem is this?

Rob