UPDATE: This post is almost two years old. Some of the things I mentioned in this post might no longer be true. Cloud is very dynamic and things keep changing. So, please do visit Azure’s official documentation to understand the current state.
When you mention Azure Cosmos DB in any architecture/solutioning discussion, a likely question you get is “Isn’t Azure Cosmos DB expensive?”. Like any other thing in this world, the right but not so useful answer is that it depends. First of all, let us understand how Cosmos DB is priced. Cosmos DB is not priced based on the usage. Of course, Cosmos DB is more than a simple NoSQL database but like other NoSQL databases, the price is based on what you reserve. So, the analogy is renting a car rather than hailing a cab – you pay regardless of you used the rented car or not. So, what you reserve with Cosmos is the capacity – what Microsoft calls Request Units (RU) per second. You pay for the RU as well as the space (GB).