Then in MongoDB's case durability is a function of scale, which leads back to the parent's suggestion that it is a technology optimized for performance. Person

Then in MongoDB's case durability is a function of scale, which leads back to the parent's suggestion that it is a technology optimized for performance. Personally I think this is a bad foundation for data that is important. There are probably a lot of use cases where data not being on disk for n seconds (or one minute in the case of MongoDB) is ok. Even when that is the case, I still think that is the most important question to be addressed when choosing MongoDB as a data store. The flexible query API, schema-less document format, secondary indices... those are siren songs of rapid development.