Imagine being able to create an infinitively scalable Database never worrying about running out of disk space or overloading the with too many connections. Welcome to the CloudDB.

First of all what is the "Cloud" According to Wikipedia

Cloud computing is a computing paradigm shift where computing is moved away from personal computers or an individual application server to a “cloud” of computers. Users of the cloud only need to be concerned with the computing service being asked for, as the underlying details of how it is achieved are hidden. This method of distributed computing is done through pooling all computer resources..

So this makes, sense basically a distributed Computing environemnt where applications are round on more then one server or computer.  The big problem with this paradigm is how do you handle storage? The answer is cloud storage.

Cloud storage is a model of networked data storage where data is stored on multiple virtual servers, generally hosted by third parties, rather than being hosted on dedicated servers. Hosting companies operate large data centers; and people who require their data to be hosted buy or lease storage capacity from them and use it for their storage needs. The data center operators, in the background, virtualize the resources according to the requirements of the customer and expose them as virtual servers, which the customers can themselves manage. Physically, the resource may span across multiple servers.

This one is hard to grasp, lets look at some options.

Simple DB provided by Amazon Web Services, is a hosted web service for running queries on structured data in real time. Traditionally, this type of functionality has been accomplished with a clustered relational database that requires a sizable upfront investment, brings more complexity than is typically needed, and often requires a DBA to maintain and administer. In contrast, Amazon SimpleDB is easy to use and provides the core functionality of a database - real-time lookup and simple querying of structured data - without the operational complexity. Amazon SimpleDB requires no schema, automatically indexes your data and provides a simple API for storage and access.

See > aws.amazon.com/simpledb

CouchDB is a free software document-oriented database written in the Erlang programming language. It is designed for extreme scalability and is easily deployed to multi-core or multi-server clusters.

Like other document-oriented database systems, such as Lotus Notes, CouchDB is not a Relational Database Management System. Instead of storing data in rows and columns, the database manages a collection of JSON documents (early versions of CouchDB used XML). As a result, it is often compared with column-oriented datastores like Google's BigTable; however, CouchDB is not a column-oriented store, since the documents in a collection need not share a schema. Aggregate functions and filters are computed in parallel as in MapReduce, rather than at query time (although queries may introduce new aggregate functions, which execute on the existing documents in parallel).

CouchDB exposes a RESTful HTTP API and a large number of pre-client written clients are available. Additionally, a plugin architecture allows for using different computer languages as the internal view server such as Javascript (default), PHP and Ruby and Python. Support for other languages can be easily added.

See > couchdb.org

Any yet another option is ThruDB

Thrudb is a set of simple services built on top of Facebook’s Thrift framework that provides indexing and document storage services for building and scaling websites. Its purpose is to offer web developers flexible, fast and easy-to-use services that can enhance or replace traditional data storage and access layers.

Thrudb - High Level Features:

    * Client libraries for most languages
    * Multi-master replication
    * Incremental backups and redo logging
    * Multiple storage backends (S3 included)
    * Built for horizontal scalability
    * Simple and powerful search api (Lucene)

Thrudb - Services:

    * Thrudoc - Document storage service
    * Thrucene - Indexing and search service
    * Thruqueue - Persistent message queue service
    * Throxy - Partitioning and load balancing service

See > code.google.com/p/thrudb/