So I was at a project where data was somewhat sharded. We had a customer, which was part of a customer database (accessible by webservice calls), however, we had some additional customer data. This led to the (I think somewhat unfortunate) situation where the application reasoned from their own business object, and, when necessary, retrieved data from the relation system. I feel this is flawed, ideally the entirety of the customer data should be invisible to the business logic of the application. Where I normally would like to have an anemic domain model, this data sharding makes it hard. Let's say you have relation R, and our extended data S. In our application, we want to reason about RS (or at least, not have to make that distinction). So maybe S has some additional attribute which may (or may not) be filled, depending on whether R is available or not. That sounds like a nice idea, but everywhere when R is necessary (and S is available), R needs to be retrieved. And that is ...