riemann-count-distinct-hosts-by-tags.clj

(let [hosts (atom {})]
  (fn [event]
      (let [tag-str (keyword (clojure.string/join "-" (:tags event)))]
        (swap! hosts assoc tag-str (conj (tag-str @hosts #{}) (:host event)))
        (index {:service (str (name tag-str) "-count")
                :time (unix-time)
                :metric (count (tag-str @hosts))}))))