Steven Hazel from Sauce Labs on why they switched from CouchDB to MySQL:
Our original product design featured a REST API for storing data on behalf of our customers, and the plan was to drop almost straight through to CouchDB’s already RESTful API. This let us get a prototype up and running in a hurry. It didn’t matter that CouchDB was new and not yet hardened by a lot of real-world usage, we thought, because our database I/O needs were meager, our app was naturally horizontally scalable, and our product was fault-tolerant. We could easily bridge any reliability gap just by keeping replicas and retrying things when they failed. What could go wrong?