An interesting post from lunar logic guys about using MySQL and MongoDB for their Kanban product, how that get there and the tools they are using.
As a personal note, I thought how this system would be characterized in terms of CAP. It should be quite clear that we cannot speak about consistency over the two systems as MongoDB doesn’t really support transactions (you can check these notes on MongoDB for more details). So, in case their system would be using master-master MySQL replication and replica-pairs for MongoDB, and the internal tools would know how to work with this setup, we could probably say that we have an AP system. But if any of these preconditions are not fulfilled, I’d say both A and P are lost.