class_docstr: A map-to-arrays dict, with deterministic Reservoir Sampling.
  
  Store each reservoir bucket by key, and each bucket is a list sampling
  with reservoir algorithm.
code: def get_num_items_index(self, run, tag):
      key = run + "/" + tag
      return self._get_num_items_index(key)
  
imports: import collections
  import random
  import threading
label_desc: Use image node_id 6 for calling the Reservoir obj's underlying member method code with example usage: obj.get_num_items_index(run, tag) and returns: self