code: public void init(IPersistentSubscriptionStore storageService) {
          Log.debug("init invoked");
  
          m_storageService = storageService;
  
          //reload any subscriptions persisted
          if (Log.DEBUG) {
              Log.debug("Reloading all stored subscriptions...subscription tree before {}", dumpTree());
          }
          
          for (Subscription subscription : m_storageService.retrieveAllSubscriptions()) {
              Log.debug("Re-subscribing {} to topic {}", subscription.getClientId(), subscription.getTopic());
              addDirect(subscription);
          }
          if (Log.DEBUG) {
              Log.debug("Finished loading. Subscription tree after {}", dumpTree());
          }
      }
docs: Initialize basic store structures, like the FS storage to maintain
  client's topics subscriptions