There isn’t any reason for you to build your own fragment cache, there are several already available as plugins. https://wordpress.org/plugins/search.php?q=fragment+cache. There is one built by me in there, but I won’t plug just that one. If you did want to build you’re own fragment cache many of the plugins in that list wold be a good place to start looking at how others have done it. There are also several good tutorials on creating a fragment cache for WordPress https://www.google.com/search?q=wordpress+fragment+caching&ie=utf-8&oe=utf-8.
