class_docstr: Router provides mapping from external address to internal address.
code: def _cache(self) -> Dict[Tuple[str, Any], Client]:
      try:
          return self._cache_local.cache
      except AttributeError:
          cache = self._cache_local.cache = dict()
          return cache
  
file_path: mars/mars/oscar/backends/router.py
imports: import threading
  from typing import Dict, List, Tuple, Type, Any, Optional
  from .communication import get_client_type, Client
label_desc: Use image node_id 5 for calling the Router obj's underlying member method code with example usage: obj._cache() and returns: self, cache