class Service {
    private Db db;
    private Cache cache;
    Service(Db db, Cache cache) { this.db = db; this.cache = cache; }
    void save(String k) { this.db.write(k); }
    String load(String k) { return this.db.read(k); }
    void put(String k) { this.cache.set(k); }
    void evict(String k) { this.cache.drop(k); }
}
