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.
docstr: Determine if run_tag exists.
  
  For usage habits of VisualDL, actually call self._exist_in_keys()
  
  Args:
      run: Identity of one tablet.
      tag: Identity of one record in tablet.
  
  Returns:
      True if run_tag exists in buckets.keys, otherwise False.
docstr_tok: ["Determine","if","run_tag","exists",".","For","usage","habits","of","VisualDL",",","actually","call","self._exist_in_keys","(",")","Args",":","run",":","Identity","of","one","tablet",".","tag",":","Identity","of","one","record","in","tablet",".","Returns",":","True","if","run_tag","exists","in","buckets.keys",",","otherwise","False","."]
code: def exist_in_keys(self, run, tag):
      """Determine if run_tag exists.
  
      For usage habits of VisualDL, actually call self._exist_in_keys()
  
      Args:
          run: Identity of one tablet.
          tag: Identity of one record in tablet.
  
      Returns:
          True if run_tag exists in buckets.keys, otherwise False.
      """
      key = run + "/" + tag
      return self._exist_in_keys(key)
  
imports: import collections
  import random
  import threading
label_desc: Use image node_id 4 for calling the Reservoir obj's underlying member method code with example usage: obj.exist_in_keys(run, tag) and returns: self