From 3c72e9878de3d8190e3f3a75f9cc0fa72e767c6e Mon Sep 17 00:00:00 2001
From: Joseph Schorr <joseph.schorr@coreos.com>
Date: Thu, 14 Dec 2017 13:36:51 -0500
Subject: [PATCH] Add the concept of a data model cache, for caching of
 Namedtuple objects from the data model

Will be used to cache blobs, thus removing the need to hit the database in most blob requests
---
 data/cache/__init__.py        | 48 +++++++++++++++++++++++++++++++++++
 data/cache/cache_key.py       |  8 ++++++
 data/cache/test/test_cache.py | 16 ++++++++++++
 3 files changed, 72 insertions(+)
 create mode 100644 data/cache/__init__.py
 create mode 100644 data/cache/cache_key.py
 create mode 100644 data/cache/test/test_cache.py

diff --git a/data/cache/__init__.py b/data/cache/__init__.py
new file mode 100644
index 000000000..01c572f38
--- /dev/null
+++ b/data/cache/__init__.py
@@ -0,0 +1,48 @@
+from datetime import datetime
+
+from abc import ABCMeta, abstractmethod
+from six import add_metaclass
+
+from util.expiresdict import ExpiresDict
+from util.timedeltastring import convert_to_timedelta
+
+def is_not_none(value):
+  return value is not None
+
+
+@add_metaclass(ABCMeta)
+class DataModelCache(object):
+  """ Defines an interface for cache storing and returning tuple data model objects. """
+
+  @abstractmethod
+  def retrieve(self, cache_key, loader, should_cache=is_not_none):
+    """ Checks the cache for the specified cache key and returns the value found (if any). If none
+        found, the loader is called to get a result and populate the cache.
+    """
+    pass
+
+
+class NoopDataModelCache(DataModelCache):
+  """ Implementation of the data model cache which does nothing. """
+
+  def retrieve(self, cache_key, loader, should_cache=is_not_none):
+    return loader()
+
+
+class InMemoryDataModelCache(DataModelCache):
+  """ Implementation of the data model cache backed by an in-memory dictionary. """
+  def __init__(self):
+    self.cache = ExpiresDict(rebuilder=lambda: {})
+
+  def retrieve(self, cache_key, loader, should_cache=is_not_none):
+    not_found = [None]
+    result = self.cache.get(cache_key.key, default_value=not_found)
+    if result != not_found:
+      return result
+
+    result = loader()
+    if should_cache(result):
+      expires = convert_to_timedelta(cache_key.expiration) + datetime.now()
+      self.cache.set(cache_key.key, result, expires=expires)
+
+    return result
diff --git a/data/cache/cache_key.py b/data/cache/cache_key.py
new file mode 100644
index 000000000..b0d4d7011
--- /dev/null
+++ b/data/cache/cache_key.py
@@ -0,0 +1,8 @@
+from collections import namedtuple
+
+class CacheKey(namedtuple('CacheKey', ['key', 'expiration'])):
+  """ Defines a key into the data model cache. """
+  pass
+
+def for_repository_blob(namespace_name, repo_name, digest):
+  return CacheKey('repository_blob:%s:%s:%s' % (namespace_name, repo_name, digest), '60s')
diff --git a/data/cache/test/test_cache.py b/data/cache/test/test_cache.py
new file mode 100644
index 000000000..d9e3708d0
--- /dev/null
+++ b/data/cache/test/test_cache.py
@@ -0,0 +1,16 @@
+import pytest
+
+from data.cache import InMemoryDataModelCache, NoopDataModelCache
+from data.cache.cache_key import CacheKey
+
+@pytest.mark.parametrize('cache_type', [
+  (NoopDataModelCache),
+  (InMemoryDataModelCache),
+])
+def test_caching(cache_type):
+  key = CacheKey('foo', '60m')
+  cache = cache_type()
+
+  # Perform two retrievals, and make sure both return.
+  assert cache.retrieve(key, lambda: 1234) == 1234
+  assert cache.retrieve(key, lambda: 1234) == 1234