I worked on the AdWords MySQL system while at Google around 2003-2005. We put a lot of work into it and with the right partitioning and caching strategies MySQL

I worked on the AdWords MySQL system while at Google around 2003-2005. We put a lot of work into it and with the right partitioning and caching strategies MySQL held up pretty well. I'm a little surprised it took so long for Google to migrate away from it, given all the systems expertise the company has. But most of Google's engineering efforts are for enormously large data where a little data loss and slow consistency is OK. AdWords is more like a traditional ACID application than, say, a search engine. A particular advantage of MySQL over the current crop of NoSQL systems or a home-grown system is there's a lot of people who understand how MySQL fails and how to tune it. It's predictable. I suspect that's part of why it had such a long life in AdWords.