title: Auto updating cache with Spray-caching
body: <p>i'm really noob in scala and spray caching and i need to know how it work. 
  i have db and i want cache some tables and update their every hour for exmaple. i can`t understand how it realise in spray-caching library. i think write something look like,</p>
  
  <pre><code>val cache: Cache[Double] = LruCache()
  def cachedOp[T](key: T): Future[Double] = cache(key) {
  //some big function which download table in json array
    getTableFromDb()
  }
  </code></pre>
  
  <p>but have not any idea how it work. please help me, i want learn and understand. Sorry for stupid question and bad language.</p>