[Culprit-Finder] Seperate gae related part in cache_decorator and gitile repository to gae_libs/

appengine/findit/lib/cache.py

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+
+class Cache(object):
+  """An interface to cache and retrieve data.
+
+  Subclasses should implement the Get/Set functions.
+  TODO: Add a Delete function (default to no-op) if needed later.

[wrengr, 2016/12/06 21:52:57]

I'd consider settable(-only) caches to be a different interface from
settable-and-deletable caches. The particular subclasses we have so far all
support deletion, but that needn't be true in general.

[Sharu Jiang, 2016/12/06 23:58:17]

Acknowledged.

+  """
+  def Get(self, key):
+    """Returns the cached data for the given key if available.

[wrengr, 2016/12/06 21:52:57]

There should be more description about what the expected/required behavior of
subclasses is. For example, would an LRU cache be an allowed subclass? That is,
is it required that whenever something has been set it must always be available
for getting, or not?

[Sharu Jiang, 2016/12/06 23:58:17]

Good question, but I think this function just have very little requirements for
its sub classes. 

+
+    Args:
+      key (str): The key to identify the cached data.
+    """
+    raise NotImplementedError()
+
+  def Set(self, key, data, expire_time=0):
+    """Cache the given data which is identified by the given key.
+
+    Args:
+      key (str): The key to identify the cached data.
+      data (object): The python object to be cached.
+      expire_time (int): Number of seconds from current time (up to 1 month).
+    """
+    raise NotImplementedError()