I'm just now starting to go deeper into the world of cloud hosting and databases, and am getting overwhelmed by how deep this information goes. It's all a little too much to consume in a short amount of time. I get a lot of pricing information, but I'm unable to determine what that means to me. I'm making what you might compare to an email app. Users can send messages to one another. I just don't understand, out of the several options, what would be ideal for an app like this, where users would be constantly sending and receiving text data. With Amazon DynamoDB, I have to specify a pre-defined throughput with number of reads and writes per second. Sure I can just type 50, but I'm not exactly sure what 50 writes per second represents. I'm trying to determine what would be the most cost efficient solution, and I want to know what a throughput of 50 reads/writes/second compares to. Is that a high number? What is a good throughput number for a message sending app with say 50,000 daily users?

I'm just providing specific numbers so I can understand what these throughput numbers represent. 100 transactions/second to me seems like a small number since I'm not familiar with this stuff, so I'm just looking to bring everything in context. What would 100 read/write/second be useful for? Are there any average example values available?

And I'm not sure what each service is good for. For a message sending app, is there any reason I'd want to choose say Amazon DynamoDB over Google App Engine?

Any insight would be greatly appreciated.