SimpleDB comes with some scalability limitations, but sharding can be done to increase scalability. It has higher latency than MongoDB and Cassandra but has a throughput limit and it is more costly than other options. Amazon SimpleDB is a database service by Amazon, where the data stored in the Amazon Cloud. It is a popular database that comes with a key-value store model.
AmazonDB supports the following programming languages:
If you have a high read rate and you need wider query options, MongoDB is better. It is one of the most popular data stores. But MongoDB is less durable, for a big amount of data at least 2 MongoDB server instances are used as master/slave. It works on server operating systems such as Linux, OS X, Solaris, Windows. It has got manual Scalability and is much faster as compared to SimpleDB.
Cassandra has weak query options but is as durable as PostgreSQL. It is as fast as MongoDB plus it performs even faster on higher data size. In Cassandra write operations are much faster as compared to read operations. If you are looking to store terabytes of data Cassandra is the best fit for you as it is designed as a distributed database and there is no need to share your data to improve scalability.
If you want to know more about Hadoop, you can refer to the following video: