Martin Fowler writes .. "If you switch your integration protocol from SQL to HTTP, it now means you can change databases from being IntegrationDatabases to ApplicationDatabases. This change is profound. In the first step it supports a much simpler approach to object-relational mapping - such as the approach taken by Ruby on Rails. But furthermore it breaks the vice-like grip of the relational data model. If you integrate through HTTP it no longer matters how an application stores its own data, which in turn means an application can choose a data model that makes sense for its own needs."

and echoes similar sentiments that I expressed here.

Today's application stack has started taking different views on using the database, particularly relational database, as a persistent store. The driving force is, of course, to reduce the semantic distance between the application model and the data model.

For the case Martin mentions above, the data model for the application need not be relational at all. We are seeing more and more cases where applications need not bolt the data that it operates on, forcibly into the clutches of the relational paradigm. In other words, the data remains much closer to the application domain model. Instead of splitting a domain abstraction into multiple relational tables and using the SQL glue to join them for queries and reports, we can directly operate on a semantically richer persistent abstraction. Storage options have evolved, RAM is the new disk, creation of RAM clusters is now easier than creation of disk clusters. And technologies like Map/Reduce have enabled easy parallelization of data processing on commodity hardware.

I have been hacking around with CouchDB for some time now. This is one platform that promises an HTTP based interface to the entire application stack. It's a server and a database, and the best part of it is that, the database driver is HTTP. JSON based storage of application documents, REST APIs, map/reduce based queries in Javascript - no schema, SQL, no database constraints .. your application data is semantically closer to the domain model. Loosely coupled document storage, multi-version concurrency control, easy replication - try replacing the relational database with this stack if it fits your application model.

Another train of thoughts that positions the database in a new light within an application stack, is the upcoming grid computing platforms on the JVM. Java as a platform is growing fast and grid vendors like Terracotta and Gigaspaces are catching up to this trend. They offer coherently clustered in-memory grid that enables a POJO based programming model without any synchronous interaction with the persistent data store. The application layer can program to the POJO based Network Attached Memory of Terracotta, using standard in-memory data structures. Terracotta offers an interface, which, if you implement, will be called to flush your objects to the database asynchronously in a write-behind thread. One other way to reduce the impedance mismatch of your domain model from the data model.

From http://debasishg.blogspot.com/