So as the question says, I was wondering the goal / purpose of the Radix testnetwork, Cassandra. I only know that Dan Hughes, the Founder of Radix works on Cassandra exclusively.
But… Is there a connection between Dan’s work and Radix Team work? Or are they completely separated?
Thanks for the help.
If you want to learn more about Cassandra, I recommend you to give a look at this article.
In brief, 
[…] Cassandra** is a project and test network being run within Radix Labs to research, test, and demonstrate various implementations of Cerberus, Radix’s cross-shard consensus protocol. Fully sharded Cerberus is scheduled to go live in the Radix Xi’an release, and Cassandra is exploring some of the outer reaches of the form the implementation at Xi’an might take.
Regarding your second question,
[…] Cassandra is being undertaken in collaboration with the Radix Community, who run Cassandra nodes with Dan live on his Twitch channel, as well as the University of California Davis’ ExpoLab consensus experts.
To explain briefly, it’s a testnet, to test all the theory which will be useful to implement Xi’an. Before Cassandra, Dan only had a lot of things in ideas/theory, but by Cassandra he is able to actually test in the real world.
Dan is definitely a genius but some implementation risk does remain. But yeah, it’s getting minimized through Cassandra.