Update: Anatomy of a crash in a new part of Yandex written in Django. Writing to a magic session variable caused an unexpected write into an InnoDB database on every request. Writes took 6-7 seconds because of index rebuilding. Lots of useful details on the sizing of their system, what went wrong, and how they fixed it.
Yandex is a Russian search engine with 3.5 billion pages in their search index. We only know a few fun facts about how they do things, nothing at a detailed architecture level. Hopefully we'll learn more later, but I thought it would still be interesting. From Allen Stern's interview with Yandex's CTO Ilya Segalovich, we learn: