Devserver: only populate data during cron jobs, meaning all data from instances

chrome/common/extensions/docs/server2/cache_chain_object_store.py

+# Copyright 2013 The Chromium Authors. All rights reserved.

[cduvall, 2013/04/17 23:30:37]

I like this class

[not at google - send to devlin, 2013/04/18 04:05:41]

:)

+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+from future import Future
+from object_store import ObjectStore
+
+class _GetMultiFuture(object):
+  '''A Future for GetMulti.
+
+  Params:
+  - |toplevel_cache| CacheChainObjectStore's cache.
+  - |object_store_futures| a list of (object store, future) pairs, where future
+    is the result of calling GetMulti on the missing keys for the object store.
+  - |cached_items| a mapping of cache items already in memory.
+  - |missing_keys| the keys that were missing from the GetMulti call
+  '''
+  def __init__(self,
+               toplevel_cache,
+               object_store_futures,
+               cached_items,
+               missing_keys):
+    self._toplevel_cache = toplevel_cache
+    self._object_store_futures = object_store_futures
+    self._results_so_far = cached_items
+    self._missing_keys = missing_keys
+
+  def Get(self):
+    # Approach:
+    #
+    # Try each object store in order, until there are no more missing keys.
+    # Don't realise the Future value of an object store that we don't need to;
+    # this is important e.g. to avoid querying data store constantly.
+    #
+    # When a value is found, cache it in all object stores further up the
+    # chain, including the object-based cache on CacheChainObjectStore.
+    object_store_updates = []
+    for object_store, object_store_future in self._object_store_futures:
+      if len(self._missing_keys) == 0:
+        break
+      result = object_store_future.Get()
+      for k, v in result.items():  # use items(); changes during iteration
+        if v is None or k not in self._missing_keys:
+          del result[k]
+          continue
+        self._toplevel_cache[k] = v
+        self._results_so_far[k] = v
+        self._missing_keys.remove(k)
+      for _, updates in object_store_updates:
+        updates.update(result)
+      object_store_updates.append((object_store, {}))
+    # Update the caches of all object stores that need it.
+    for object_store, updates in object_store_updates:
+      if updates:
+        object_store.SetMulti(updates)
+    return self._results_so_far
+
+class CacheChainObjectStore(ObjectStore):
+  '''Maintains an in-memory cache along with a chain of other object stores to
+  try for the same keys. This is useful for implementing a multi-layered cache.
+  The in-memory cache is inbuilt since it's synchronous, but the object store
+  interface is asynchronous.
+  The rules for the object store chain are:
+    - When setting (or deleting) items, all object stores in the hierarcy will
+      have that item set.
+    - When getting items, each object store is tried in order. The first object
+      store to find the item will trickle back up, setting it on all object
+      stores higher in the hierarchy.
+  '''
+  def __init__(self, object_stores):
+    self._object_stores = object_stores
+    self._cache = {}
+
+  def SetMulti(self, mapping):
+    self._cache.update(mapping)
+    for object_store in self._object_stores:
+      object_store.SetMulti(mapping)
+
+  def GetMulti(self, keys):
+    missing_keys = list(keys)
+    cached_items = {}
+    for key in keys:
+      if key in self._cache:
+        cached_items[key] = self._cache.get(key)
+        missing_keys.remove(key)
+    if len(missing_keys) == 0:
+      return Future(value=cached_items)
+    object_store_futures = [(object_store, object_store.GetMulti(missing_keys))
+                            for object_store in self._object_stores]
+    return Future(delegate=_GetMultiFuture(
+        self._cache, object_store_futures, cached_items, missing_keys))
+
+  def DelMulti(self, keys):
+    for k in keys:
+      self._cache.pop(k, None)
+    for object_store in self._object_stores:
+      object_store.DelMulti(keys)