Cassandra DB is needed for capturing and storing vCloud Director metrics data so that it can be displayed in portal to end users so that users are aware of VM resource utilization etc.
Prior to vCD v9.0, we needed kairosdb + cassandra together for capturing and storing the metrics data, but things have changed now. VMware has removed the requirement of kairosdb and now metrics data can be sent straight to cassandra database. This metric data in turn can be viewed in tenant UI.
As per vCD 9.0 documentation
… Read MoreCassandra is an open source database that you can use to provide the backing store for a scalable, high-performance solution for collecting time series data like virtual machine metrics. If you want vCloud Director to support retrieval of historic metrics from virtual machines, you must install and configure a Cassandra cluster and use the cell-management-tool to connect the cluster to vCloud Director.