subsystem.clj

(defn find-dependencies
  [system roots]
  (let [graph (component/dependency-graph system roots)
        dependencies (into #{} (mapcat (:dependencies graph) roots))]
    (-> #{}
        (into roots)
        (into dependencies)
        (cond-> (seq dependencies)
          (into (find-dependencies system dependencies))))))

(defn sub-system
  [system roots]
  (let [all (set (keys system))
        dependencies (find-dependencies system roots)
        unnecessary (disj (set/difference all dependencies) :config)]
    (apply dissoc system unnecessary)))

(defn create-system
  [config roots decrypters]
  (let [dependencies (build-dependencies config)]
    (-> (build-system config decrypters)
        (component/system-using dependencies)
        (sub-system roots))))