private static void emitClusterFeedBlockDiffEvent(PerStateParams params, List<Event> events) {
        // TODO should we emit any events when description changes?
        if (feedBlockStateHasChanged(params)) {
            if (params.feedBlockTo != null) {
                events.add(createClusterEvent(String.format("Cluster feed blocked due to resource exhaustion: %s",
                        params.feedBlockTo.getDescription()), params));
            } else {
                events.add(createClusterEvent("Cluster feed no longer blocked", params));
            }
        }
    }
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