For example: I've gone searching through the blog posts, github readme, and KONG documentation, but I still have no idea _why_ it needs Cassandra. What does it store in there?
One of the main graphics on the KONG docs shows a Caching plugin (http://getkong.org/assets/images/homepage/diagram-right.png), but the list of available plugins doesn't include such an entry. Is that because caching is built in? Is the cache state stored in Cassandra? Or is the plugin yet to be built?
nginx has a simple in-memory cache, but it can only be shared across workers on the same instance, so in order to scale Kong horizontally by adding more servers there must be a third-party datastore (in this case Cassandra) that stores and serves the data to the cluster.
Kong supports a simple caching mechanism that's basically the one that nginx supports. We are planning to add a more complex Caching plugin that will store data into Cassandra as well, and will make the cached items available across the cluster.