Amazon SimpleDB is a web service that enables you to store semi-structured data within Amazon's data centers. The service provides automatic, geographically diverse data replication and internal routing around failed storage nodes. It offers high availability and enables horizontal scalability. The service allows you to offload hardware maintenance and database management tasks.
You can use SimpleDB as a distributed key-value store using the GetAttributes, PutAttributes, and DeleteAttributes API calls. You also have the option to query for your data along any of its attributes using the Select API call. SimpleDB is not a relational database, so there are no joins, foreign keys, schema definitions, or relational constraints that you can specify. SimpleDB also has limited support for transactions, and updates propagate between replicas in the background. SimpleDB supports strong consistency, where read operations immediately reflect the results of all completed and eventual consistency, where storage nodes are updated asynchronously in the background.
The normal window of time for all storage nodes to reach consistency in the background is typically small. During a server or network failure, consistency may not be reached for longer periods of time, but eventually all updates will propagate. SimpleDB is best used by applications able to deal with eventual consistency and benefit from the ability to remain available in the midst of a failure.