title: Best way to provide `delta` of changes in database record
body: <p>We have a REST API which is used by a mobile application. As a distilled version, let's assume the API provides list of books and their contents such as author's name, publisher, year and content of pages.</p>
  
  <p>The mobile app needs to cache the last 10 books in local storage so it always has 10 books available offline. The books might be updated at the server and the app must sync with the latest changes.</p>
  
  <ul>
  <li>A new book is added</li>
  <li>A books is updated</li>
  <li>A book is deleted</li>
  <li>etc</li>
  </ul>
  
  <p>We need a way, at the server, to only provide the changes that have been made since the last time mobile was synced. So instead of requesting the whole latest 10 books they only request what has changed since the last fetch.</p>
  
  <p>This is something that has been implemented in version management systems like Git but we are looking for a very simple way to do it with database records. </p>
  
  <p>What is a very very very simple way to implement such <code>delta</code>?</p>