1 # Copyright 2013 The Chromium Authors. All rights reserved. 2 # Use of this source code is governed by a BSD-style license that can be 3 # found in the LICENSE file. 4 5 from future import Future 6 from object_store import ObjectStore 7 8 9 class CacheChainObjectStore(ObjectStore): 10 '''Maintains an in-memory cache along with a chain of other object stores to 11 try for the same keys. This is useful for implementing a multi-layered cache. 12 The in-memory cache is inbuilt since it's synchronous, but the object store 13 interface is asynchronous. 14 The rules for the object store chain are: 15 - When setting (or deleting) items, all object stores in the hierarcy will 16 have that item set. 17 - When getting items, the behaviour depends on |start_empty|. 18 - If false, each object store is tried in order. The first object 19 store to find the item will trickle back up, setting it on all object 20 stores higher in the hierarchy. 21 - If true, only the first in-memory cache is checked, as though the store 22 had been initialized with no content as opposed to the union of its 23 delegate stores. 24 ''' 25 def __init__(self, object_stores, start_empty=False): 26 self._object_stores = object_stores 27 self._start_empty = start_empty 28 self._cache = {} 29 30 def SetMulti(self, mapping): 31 self._cache.update(mapping) 32 for object_store in self._object_stores: 33 object_store.SetMulti(mapping) 34 35 def GetMulti(self, keys): 36 missing_keys = list(keys) 37 cached_items = {} 38 for key in keys: 39 if key in self._cache: 40 cached_items[key] = self._cache.get(key) 41 missing_keys.remove(key) 42 if len(missing_keys) == 0 or self._start_empty: 43 return Future(value=cached_items) 44 object_store_futures = [(object_store, object_store.GetMulti(missing_keys)) 45 for object_store in self._object_stores] 46 def resolve(): 47 # Approach: 48 # 49 # Try each object store in order, until there are no more missing keys. 50 # Don't realise the Future value of an object store that we don't need to; 51 # this is important e.g. to avoid querying data store constantly. 52 # 53 # When a value is found, cache it in all object stores further up the 54 # chain, including the object-based cache on CacheChainObjectStore. 55 object_store_updates = [] 56 for object_store, object_store_future in object_store_futures: 57 if len(missing_keys) == 0: 58 break 59 result = object_store_future.Get() 60 for k, v in result.items(): # use items(); changes during iteration 61 if v is None or k not in missing_keys: 62 del result[k] 63 continue 64 self._cache[k] = v 65 cached_items[k] = v 66 missing_keys.remove(k) 67 for _, updates in object_store_updates: 68 updates.update(result) 69 object_store_updates.append((object_store, {})) 70 # Update the caches of all object stores that need it. 71 for object_store, updates in object_store_updates: 72 if updates: 73 object_store.SetMulti(updates) 74 return cached_items 75 return Future(callback=resolve) 76 77 def DelMulti(self, keys): 78 for k in keys: 79 self._cache.pop(k, None) 80 for object_store in self._object_stores: 81 object_store.DelMulti(keys) 82