code: public static final void clear(TitanGraph graph) {
          Preconditions.checkNotNull(graph);
          Preconditions.checkArgument(graph instanceof StandardTitanGraph,"Invalid graph instance detected: %s",graph.getClass());
          StandardTitanGraph g = (StandardTitanGraph)graph;
          Preconditions.checkArgument(!g.isOpen(),"Graph needs to be shut down before it can be cleared.");
          final GraphDatabaseConfiguration config = g.getConfiguration();
          BackendOperation.execute(new Callable<Boolean>(){
              @Override
              public Boolean call() throws Exception {
                  config.getBackend().clearStorage();
                  return true;
              }
              @Override
              public String toString() { return "ClearBackend"; }
          }, Duration.ofSeconds(20));
      }
docs: Clears out the entire graph. This will delete ALL of the data stored in this graph and the data will NOT be
  recoverable. This method is intended only for development and testing use.
  
  @param graph
  @throws IllegalArgumentException if the graph has not been shut down
  @throws com.thinkaurelius.titan.core.TitanException if clearing the storage is unsuccessful