Amazon's tech. style:
In the Amazon services architecture, each service is responsible for its own data management, which means that each service team can pick exactly those solutions that are ideally suited for the particular application they are implementing.It allows them to tailor the data management system such that they get maximum reliability and guaranteed performance at the right cost as the system scales up.
So, for the target of "scalability, reliability, performance, and cost-effectiveness":
- Since the Key-Value storage solutions are widely used, they led to the creation of S3.
- Since the simple structured data management systems without complex transactions and relations, without rigid schema is widely used, they led to the creation of SimpleDB.
- But, Amazon found that many applications running in EC2 instances want to use RDBMS, then EBS is launched to provide scalable and reliable storage volume that can be used for persisting the databases.
- To free up users to focus on their applications and business, now RDS is ready. The users need not to maintain their DB and consider who to scale the DB.
- Another case is AWS MapReduce services.
RDS is MySQL in AWS cloud.
Now, AWS users have three methods to use DB:
(2) EC2 AMI+EBS
(3) SimpleDB (no relation model)
We can re-read the good paper "Amazon Cloud Architecture", to understand the strategy of AWS better.
 Amazon Relational Database Service (RDS): http://aws.amazon.com/rds/
 Werner Vogels's blog: http://www.
 Amazon Cloud Architecture: http://jineshvaria.s3.amazonaws.com/public/cloudarchitectures-varia.pdf