[ndemoor]

Well, the downside of using multiple DB stores is that the logic of keeping everything consistent is in the hands of the developer. So you have to make sure that everything is written correctly.

For instance, if you write to MySQL and Mongo, but your Mongo is down, you'll either have to queue the data item somewhere for a write once the system is back up, or you have a migration system in place that gets everything from MySQL since the downtime and writes it back to Mongo.

Depending on the type of data we have a few easing factors: for some data stores it is not that big of a deal if it doesn't get written to it's 2nd layer (eg. cache) as we can rewrite it the next time it is requested in layer 1.