You are right, a relational database could be used. However, SimpleDB is a very simple system that has a much higher availability than any relational solution t

You are right, a relational database could be used. However, SimpleDB is a very simple system that has a much higher availability than any relational solution that we could think of. In general, it is just there. Next to that, it does not have many features, but it supports just the stuff you would like to do for logs. On top of that, we write many more logs than we read, and we don't care so much for realtime consistency in logs. And it is cheap.