private static ClusterEvent createClusterEvent(String description, PerStateParams params) {
        return new ClusterEvent(ClusterEvent.Type.SYSTEMSTATE, description, params.currentTime);
    }
Responsible for inferring the difference between two cluster states and their
state annotations and producing a set of events that describe the changes between
the two. Diffing the states directly provides a clear picture of _what_ has changed,
while the annotations are generally required to explain _why_ the changes happened
in the first place.

Events are primarily used for administrative/user visibility into what's happening
in the cluster and are output to the Vespa log as well as kept in a circular history
buffer per node and for the cluster as a whole.
clustercontroller-core/src/main/java/com/yahoo/vespa/clustercontroller/core/EventDiffCalculator.java