Applies one operator action and appends it to the history. ACKNOWLEDGED accepts the alert as real; ASSIGNED gives it an owner and acknowledges it on the way through; UNASSIGNED gives up ownership without closing; COMMENTED adds a note and changes nothing; CLOSED ends the lifecycle with a resolution; REOPENED puts a closed alert back in the queue. Actions are idempotent where it is sensible for them to be, so a portlet can offer them without reasoning about current state.