On Monday we detected an issue with our indexing systems that affected Google search results. Once the issue was identified, it was promptly fixed by our Site Reliability Engineers and by now it has been mitigated. Thank you for your patience!
The indexing system, Caffeine, does multiple things: 1. ingests fetchlogs, 2. renders and converts fetched data, 3. extracts links, meta and structured data, 4. extracts and computes some signals, 5. schedules new crawls, 6. and builds the index that is pushed to serving.
If something goes wrong with most of the things that it's supposed to do, that will show downstream in some way. If scheduling goes awry, crawling may slow down. If rendering goes wrong, we may misunderstand the pages. If index building goes bad, ranking & serving may be affected
Don't oversimplify search for it's not simple at all: thousands of interconnected systems working together to provide users high quality and relevant results. Throw a grain of sand in the machinery and we have an outage like yesterday.