Merge pull request #175 from boxine/cached-feature-flags

Allow FeatureFlags to be more resource-efficient

README.md

@@ -148,11 +148,11 @@ Feature flags: https://github.com/boxine/bx_django_utils/blob/master/bx_django_u
 
 #### bx_django_utils.feature_flags.admin_views
 
-* [`ManageFeatureFlagsBaseView()`](https://github.com/boxine/bx_django_utils/blob/master/bx_django_utils/feature_flags/admin_views.py#L31-L101) - Base admin extra view to manage all existing feature flags in admin.
+* [`ManageFeatureFlagsBaseView()`](https://github.com/boxine/bx_django_utils/blob/master/bx_django_utils/feature_flags/admin_views.py#L31-L105) - Base admin extra view to manage all existing feature flags in admin.
 
 #### bx_django_utils.feature_flags.data_classes
 
-* [`FeatureFlag()`](https://github.com/boxine/bx_django_utils/blob/master/bx_django_utils/feature_flags/data_classes.py#L18-L176) - A feature flag that persistent the state into django cache/database.
+* [`FeatureFlag()`](https://github.com/boxine/bx_django_utils/blob/master/bx_django_utils/feature_flags/data_classes.py#L21-L205) - A feature flag that persistent the state into django cache/database.
 
 #### bx_django_utils.feature_flags.test_utils
 

bx_django_utils/feature_flags/README.md

@@ -72,5 +72,7 @@ class ManageFeatureFlagsAdminExtraView(ManageFeatureFlagsBaseView):
     pass
 ```
 
+## Performance considerations
 
-
+By default, each time the flags state is evaluated (e.g. when calling `foo_feature_flag.is_enabled()`), the flag state is fetched from the underlying storage. This may cause poor performance in hot code paths.
+You can limit this evaluation to once per n seconds by passing the `cache_duration=timedelta(seconds=n)` argument to the `FeatureFlag` constructor.

bx_django_utils/feature_flags/admin_views.py

@@ -84,6 +84,10 @@ def form_valid(self, form):
             feature_flag.set_state(new_state)
             change_message = f'Set "{feature_flag.human_name}" to {feature_flag.state.name}'
 
+            if hasattr(feature_flag, '_cache_duration'):
+                eta = int(feature_flag._cache_duration.total_seconds())
+                change_message += f' (will take up to {eta} seconds to take full effect)'
+
             # Create a LogEntry for this action:
             content_type_id = ContentType.objects.get_for_model(FeatureFlagModel).id
             log_entry = LogEntry.objects.log_action(

bx_django_utils/feature_flags/data_classes.py

@@ -1,7 +1,9 @@
+import datetime
 import logging
+import time
 from collections.abc import Iterable, Iterator
 from contextlib import contextmanager
-from typing import Optional
+from typing import Callable, Optional
 
 from django.core.cache import cache
 
@@ -11,9 +13,10 @@
 from bx_django_utils.feature_flags.utils import validate_cache_key
 from bx_django_utils.models.manipulate import create_or_update2
 
-
 logger = logging.getLogger(__name__)
 
+DEFAULT_DURATION = datetime.timedelta(seconds=0)
+
 
 class FeatureFlag:
     """
@@ -30,7 +33,11 @@ def __init__(
         initial_enabled: bool,
         description: Optional[str] = None,
         cache_key_prefix: str = 'feature-flags',
+        cache_duration: datetime.timedelta = DEFAULT_DURATION,
     ):
+        """
+        :param cache_duration: how long the state of the flag should be cached in-process
+        """
         self.human_name = human_name
         self.description = description
 
@@ -50,6 +57,12 @@ def __init__(
         else:
             self.initial_state = State.DISABLED
 
+        if cache_duration:
+            self._cache_duration: datetime.timedelta = cache_duration
+            self._cache_time_func: Callable[[], float] = time.monotonic
+            self._cache_from: float = self._cache_time_func()
+            self._cache_value: State = self.initial_state
+
     def enable(self) -> bool:
         return self.set_state(new_state=State.ENABLED)
 
@@ -82,6 +95,22 @@ def reset(self) -> None:
 
     @property
     def is_enabled(self) -> bool:
+        if not hasattr(self, '_cache_duration'):  # caching is disabled
+            return self._compute_is_enabled()
+
+        elapsed = self._cache_time_func() - self._cache_from
+
+        # cache is still valid
+        if elapsed <= self._cache_duration.total_seconds():
+            return bool(self._cache_value.value)
+
+        # cache is invalid -> recompute
+        state = self._compute_is_enabled()
+        self._cache_from = self._cache_time_func()
+        self._cache_value = State(state)
+        return state
+
+    def _compute_is_enabled(self) -> bool:
         try:
             raw_value = cache.get(self.cache_key)
         except Exception as err:

bx_django_utils/feature_flags/tests/test_feature_flags.py

@@ -1,3 +1,4 @@
+import datetime
 import logging
 from unittest.mock import patch
 
@@ -249,6 +250,27 @@ def increment():
             increment()
         self.assertEqual(some_var, 1)
 
+    def test_cache(self):
+        duration = datetime.timedelta(seconds=60)
+        ff = FeatureFlag(
+            cache_key='cache-test',
+            human_name='Cache Test',
+            initial_enabled=True,
+            cache_duration=duration,
+        )
+        self.assertTrue(ff)
+
+        ff.disable()
+        self.assertTrue(ff)  # still enabled due to caching!
+
+        orig_time_func = ff._cache_time_func
+
+        def future():
+            return orig_time_func() + duration.total_seconds() + 1
+
+        ff._cache_time_func = future
+        self.assertFalse(ff)  # now it's disabled due to cache expiration
+
 
 class IsolatedFeatureFlagsTestCase(FeatureFlagTestCaseMixin, TestCase):
     """"""  # noqa - Don't add to README