Issue #3329824: Define a PermanentBackendInterface

modules/pcb_memcache/src/Cache/PermanentMemcacheBackend.php

@@ -3,19 +3,14 @@
 namespace Drupal\pcb_memcache\Cache;
 
 use Drupal\memcache\MemcacheBackend;
+use Drupal\pcb\Cache\PermanentBackendInterface;
 
 /**
  * Defines a permanent memcache cache implementation.
  *
- * This cache implementation can be used for data like
- * stock which don't really need to be cleared during normal
- * cache rebuilds and need to be cleared . It uses the database to
- * store cached data. Each cache bin corresponds to a database
- * table by the same name.
- *
- * @ingroup cache
+ * {@inheritdoc}
  */
-class PermanentMemcacheBackend extends MemcacheBackend {
+class PermanentMemcacheBackend extends MemcacheBackend implements PermanentBackendInterface {
 
   /**
    * {@inheritdoc}