There may be instances where you'll receive the following alert in your environment 

"Cassandra on CVM x.x.x.31 is now detached from the ring due to Node was down for a long time. Node detach procedure done by x.x.x.41."

Before understanding the alert, let's first understand what is Cassandra and the ring-structure?

 Cassandra stores and manages all of the cluster metadata in a distributed ring-like manner based upon a heavily modified Apache Cassandra. The Paxos algorithm is utilized to enforce strict consistency. This service runs on every node in the cluster. The Cassandra is accessed via an interface called Medusa

To know more about Cassandra and the ring-structure, try going through Nutanix Bible which explains the architecture. . 

Cassandra has a feature called auto-ring repair which helps to prevent taking the cluster down due to multiple node failures at different intervals.

The following article lists more information about the alert, auto-repair feature and how can the node be attached back to the ring 

Cassandra auto-ring-repair feature