Move cache lookup code to a pluggable backend

ETagCacheLookup is the first example implementation.

rest_framework/cache_lookups.py

@@ -0,0 +1,42 @@
+"""
+Provides a set of pluggable cache policies.
+"""
+from django.core.cache import cache
+
+
+class BaseCacheLookup(object):
+    def get_header(self, obj):
+        return {}
+
+    def resource_unchanged(self, request):
+        """
+        Return `False` if resource has changed, `True` otherwise.
+        """
+        return False
+
+
+class ETagCacheLookup(BaseCacheLookup):
+    """
+    """
+    etag_variable = 'etag'
+
+    @staticmethod
+    def get_cache_key(cls, pk):
+        class_name = cls.__class__.__name__
+        return 'etag-{}-{}'.format(class_name, pk)
+
+    def get_etag(self, obj):
+        return getattr(obj, self.etag_variable)
+
+    def get_header(self, obj):
+        key = self.get_cache_key(obj, 'pk')
+        etag = self.get_etag(obj)
+        cache.set(key, etag)
+        return {'ETag': etag}
+
+    def resource_unchanged(self, request, key):
+        etag = cache.get(key)
+        header = request.META.get('HTTP_IF_NONE_MATCH')
+        if etag is not None and etag == header:
+            return True
+        return False

rest_framework/mixins.py

@@ -8,7 +8,6 @@ from __future__ import unicode_literals
 
 from django.http import Http404
 from rest_framework import status
-from rest_framework.exceptions import PreconditionFailed
 from rest_framework.response import Response
 from rest_framework.request import clone_request
 
@@ -101,11 +100,9 @@ class RetrieveModelMixin(object):
         queryset = self.get_queryset()
         filtered_queryset = self.filter_queryset(queryset)
         self.object = self.get_object(filtered_queryset)
-        headers = {}
-        if getattr(self, 'use_etags', False):
-            if self.get_etag(self.object) == request.META.get('HTTP_IF_NONE_MATCH'):
-                return Response(status=304)
-            headers.update({'ETag': self.get_etag(self.object)})
+
+        headers = self.get_cache_lookup_headers(self.object)
+
         serializer = self.get_serializer(self.object)
         return Response(serializer.data, headers=headers)
 
@@ -127,8 +124,7 @@ class UpdateModelMixin(object):
             created = True
             success_status_code = status.HTTP_201_CREATED
         else:
-            if getattr(self, 'use_etags', False) and self.object.etag != self.etag_header:
-                raise PreconditionFailed
+            self.cache_precondition_check(self.object, request)
             created = False
             success_status_code = status.HTTP_200_OK
 
@@ -172,7 +168,6 @@ class DestroyModelMixin(object):
     """
     def destroy(self, request, *args, **kwargs):
         obj = self.get_object()
-        if self.get_etag(obj) != self.etag_header:
-            raise PreconditionFailed
+        self.cache_precondition_check(obj, request)
         obj.delete()
         return Response(status=status.HTTP_204_NO_CONTENT)

rest_framework/settings.py

@@ -47,6 +47,8 @@ DEFAULTS = {
     ),
     'DEFAULT_THROTTLE_CLASSES': (
     ),
+    'DEFAULT_CACHE_LOOKUP_CLASSES': (
+    ),
 
     'DEFAULT_CONTENT_NEGOTIATION_CLASS':
         'rest_framework.negotiation.DefaultContentNegotiation',

rest_framework/views.py

@@ -60,6 +60,7 @@ class APIView(View):
     throttle_classes = api_settings.DEFAULT_THROTTLE_CLASSES
     permission_classes = api_settings.DEFAULT_PERMISSION_CLASSES
     content_negotiation_class = api_settings.DEFAULT_CONTENT_NEGOTIATION_CLASS
+    cache_lookup_classes = api_settings.DEFAULT_CACHE_LOOKUP_CLASSES
 
     @classmethod
     def as_view(cls, **initkwargs):
@@ -241,6 +242,12 @@ class APIView(View):
             self._negotiator = self.content_negotiation_class()
         return self._negotiator
 
+    def get_cache_lookups(self):
+        """
+        Instantiates and returns the list of cache lookups that this view requires.
+        """
+        return [cache_lookup() for cache_lookup in self.cache_lookup_classes]
+
     # API policy implementation methods
 
     def perform_content_negotiation(self, request, force=False):
@@ -294,6 +301,33 @@ class APIView(View):
             if not throttle.allow_request(request, self):
                 self.throttled(request, throttle.wait())
 
+    def check_preemptive_cache(self, request):
+        for cache_lookup in self.get_cache_lookups():
+            cache_key = cache_lookup.get_cache_key(self.model, self.kwargs['pk'])
+            if cache_lookup.resource_unchanged(request, cache_key):
+                return Response(status=304)
+
+    def get_cache_lookup_headers(self, obj):
+        headers = {}
+        for cache_lookup in self.get_cache_lookups():
+            headers.update(cache_lookup.get_header(obj))
+        return headers
+
+    def check_update_validity(self, request):
+        """
+        """
+        # TODO add setting to cover
+        #  * raise IfMatchMissing when it's missing (if it's there, carry on)
+        #  * continue regardless
+        if request.META.get('HTTP_IF_MATCH') is None:
+            raise exceptions.IfMatchMissing
+
+    def cache_precondition_check(self, obj, request):
+        header = request.META.get('HTTP_IF_MATCH')
+        for cache_lookup in self.get_cache_lookups():
+            if cache_lookup.get_etag(obj) != header:
+                raise exceptions.PreconditionFailed
+
     # Dispatch methods
 
     def initialize_request(self, request, *args, **kargs):
@@ -318,6 +352,7 @@ class APIView(View):
         self.perform_authentication(request)
         self.check_permissions(request)
         self.check_throttles(request)
+        self.check_preemptive_cache(request)
 
         # Perform content negotiation and store the accepted info on the request
         neg = self.perform_content_negotiation(request)
@@ -398,10 +433,9 @@ class APIView(View):
             else:
                 handler = self.http_method_not_allowed
 
-            if getattr(self, 'use_etags', False) and request.method.lower() in ('put', 'delete'):
-                self.etag_header = request.META.get('HTTP_IF_MATCH')
-                if self.etag_header is None:
-                    raise exceptions.IfMatchMissing
+            if request.method.lower() in ('put', 'delete'):
+                # FIXME this method name isn't obvious
+                self.check_update_validity(request)
 
             response = handler(request, *args, **kwargs)
 
@@ -418,5 +452,3 @@ class APIView(View):
         a less useful default implementation.
         """
         return Response(self.metadata(request), status=status.HTTP_200_OK)
-    def get_etag(self, obj):
-        return getattr(obj, self.etag_var)