Major Description
Add an appender for Cassandra. This should be able to follow the same pattern as the other NoSQL appenders, or possibly the JDBC style appenders as Cassandra does normally use a SQL-like language called CQL.
There are numerous Java drivers available for Cassandra: https://cassandra.apache.org/doc/latest/getting_started/drivers.html#java
The main driver I'm familiar with is the Datastax driver as it's the one used by spring-data, but it may be worth comparing the various drivers to see if any of them would fit better into the architecture of log4j already.