The OSI Model has been around for several decades now. It remains especially relevant when extending the concepts of n-tiered application design. The application layer of the OSI model, can be expanded into:
- The App Presentation Layer
- The App Web Services Layer
- The App Business Logic Layer
- The App Database Layer
As database systems have evolved rapidly over the last decade, we see database systems providing features like foreign key enforcement, indexing, view, triggers, data transformation, fulltext indexing, spatial capabilities, and more.
The problem here that databases start getting bloated, and they no longer focus on the key value that they provide. Data storage and retrieval.
So it stands to reason why Amazons Web Services have offered SimpleDB has its key database offering for Cloud services. Of course they also offer other relational database services.
So why does Amazons prefer SimpleDB? Scalability, and lower costs/GB of data stored.