What instantiates a CacheBackendInterface? I am working on upgrading a module and there is a use statement

use Drupal\Core\Cache\CacheBackendInterface;


What is the point of using an interface like that?  Interfaces dont have actual code and this .module file doesnt inherit  from that interface.

Now when the module is in enabled i get

Error: Cannot instantiate interface Drupal\Core\Cache\CacheBackendInterface in /srv/www/mydev/current/core/lib/Drupal/Component/DependencyInjection/Container.php, line 265


and the only lines I can find relating to the cache interface are

\Drupal::cache('libraries')-&gt;deleteAll();
\Drupal::cache('libraries')-&gt;set($name, $library);


So I am confused as to why this is supposed to clear the cache?  

Or is it that it is not supposed work and the latest version of the module is not complete yet?
You're using an old version of libraries module it seems. There was a core change that changed how the cache backends are defined.

Update it, clear caches and it should work.