Now that we have a fairly good understanding of what Key / Value stores are, what are they good for? Well, they are really good if you need to lookup things by key :-)
Storing the user’s session is a common example, or the user’s shopping cart. In fact, per user’s data is probably the most common usage for a key value store. The reasons for that are fairly obvious, I would imagine, we have the user’s id already, and we need a well known data. That make it very cheap to access, modify and store at the end of a request.
Other usages include storing already processed data for quick retrieval, consider things like: Favorite Products, Best Of …, etc. The idea here is to have the application access the key / value store using well known keys for those data items, while background processes will update those periodically. It isn’t quite a cache, because there isn’t really a scenario where the value doesn’t exists, but it is a near thing.
One interesting use case that I run into was to store the product catalog in the key/value store, that allowed pulling a particular product’s details in a single call. The key/value store was also used to track how many units were physically in stock, so it was able to tell if the item was available for purchase or not.
Just remember, if you need to do things more complex than just access a bucket of bits using a key, you probably need to look at something else, and the logical next step in the chain in the Document Database.