From ops mailing list:
There was a restbase/parsoid outage due to a flood of requests for transforming pages on zhwiki.
This was eventually traced back to requests to the /api/rest_v1/feed/featured/yyyy/mm/dd URLs, which seem to cascade (via restbase, mobileapps, restbase again) into making 10+ requests to parsoid to urls like:
These requests are expensive to parse causing the outage.
Here is the grafana diagram for parsoid performance where the spike on language conversion is visible.