Cache image renditions (#4883)

CHANGELOG.txt

@@ -25,6 +25,7 @@ Changelog
  * ListBlocks now call child block `bulk_to_python` if defined (Andy Chosak)
  * Site settings are now identifiable/cachable by request as well as site (Andy Babic)
  * Added `select_related` attribute to site settings to enable more efficient fetching of foreign key values (Andy Babic)
+ * Add caching of image renditions (Tom Dyson, Tim Kamanin)
  * Fix: Added ARIA alert role to live search forms in the admin (Casper Timmers)
  * Fix: Reorder login form elements to match expected tab order (Kjartan Sverrisson)
  * Fix: Re-add 'Close Explorer' button on mobile viewports (Sævar Öfjörð Magnússon)

docs/advanced_topics/performance.rst

@@ -30,6 +30,28 @@ We recommend `Redis <https://redis.io/>`_ as a fast, persistent cache. Install R
     }
 
 
+Caching image renditions
+------------------------
+
+If you define a cache named 'renditions' (typically alongside your 'default' cache),
+Wagtail will cache image rendition lookups, which may improve the performance of pages
+which include many images.
+
+.. code-block:: python
+
+    CACHES = {
+        'default': {...},
+        'renditions': {
+            'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
+            'LOCATION': '127.0.0.1:11211',
+            'TIMEOUT': 600,
+            'OPTIONS': {
+                'MAX_ENTRIES': 1000
+            }
+        }
+    }
+
+
 Search
 ------
 

docs/releases/2.9.rst

@@ -38,6 +38,7 @@ Other features
  * ListBLocks now call child block ``bulk_to_python`` if defined (Andy Chosak)
  * Site settings are now identifiable/cachable by request as well as site (Andy Babic)
  * Added ``select_related`` attribute to site settings to enable more efficient fetching of foreign key values (Andy Babic)
+ * Add caching of image renditions (Tom Dyson, Tim Kamanin)
 
 
 Bug fixes

wagtail/images/models.py

@@ -6,6 +6,7 @@ from io import BytesIO
 
 from django.conf import settings
 from django.core import checks
+from django.core.cache import InvalidCacheBackendError, caches
 from django.core.files import File
 from django.db import models
 from django.forms.utils import flatatt
@@ -277,6 +278,22 @@ class AbstractImage(CollectionMember, index.Indexed, models.Model):
             filter = Filter(spec=filter)
 
         cache_key = filter.get_cache_key(self)
+
+
+        try:
+            rendition_caching = True
+            cache = caches['renditions']
+            rendition_cache_key = "image-{}-{}-{}".format(
+                self.id,
+                cache_key,
+                filter.spec
+            )
+            cached_rendition = cache.get(rendition_cache_key)
+            if cached_rendition:
+                return cached_rendition
+        except InvalidCacheBackendError:
+            rendition_caching = False
+
         Rendition = self.get_rendition_model()
 
         try:
@@ -314,6 +331,9 @@ class AbstractImage(CollectionMember, index.Indexed, models.Model):
                 defaults={'file': File(generated_image.f, name=output_filename)}
             )
 
+        if rendition_caching:
+            cache.set(rendition_cache_key, rendition)
+
         return rendition
 
     def is_portrait(self):

wagtail/images/tests/test_models.py

@@ -2,6 +2,7 @@ import unittest
 
 from django.contrib.auth import get_user_model
 from django.contrib.auth.models import Group, Permission
+from django.core.cache import caches
 from django.core.files.uploadedfile import SimpleUploadedFile
 from django.db.utils import IntegrityError
 from django.test import TestCase
@@ -14,7 +15,6 @@ from wagtail.images.models import Rendition, SourceImageIOError
 from wagtail.images.rect import Rect
 from wagtail.tests.testapp.models import EventPage, EventPageCarouselItem
 from wagtail.tests.utils import WagtailTestUtils
-
 from .utils import Image, get_test_image_file
 
 
@@ -252,6 +252,32 @@ class TestRenditions(TestCase):
         rendition = self.image.get_rendition('width-400')
         self.assertEqual(rendition.alt, "Test image")
 
+    @override_settings(
+        CACHES={
+            'renditions': {
+                'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
+            },
+        },
+    )
+    def test_renditions_cache_backend(self):
+        cache = caches['renditions']
+        rendition = self.image.get_rendition('width-500')
+        rendition_cache_key = "image-{}-{}-{}".format(
+            rendition.image.id,
+            rendition.focal_point_key,
+            rendition.filter_spec
+        )
+
+        # Check rendition is saved to cache
+        self.assertEqual(cache.get(rendition_cache_key), rendition)
+
+        # Mark a rendition to check it comes from cache
+        rendition._from_cache = True
+        cache.set(rendition_cache_key, rendition)
+
+        # Check if get_rendition returns the rendition from cache
+        self.assertEqual(self.image.get_rendition('width-500')._from_cache, True)
+
 
 class TestUsageCount(TestCase):
     fixtures = ['test.json']
@@ -360,6 +386,7 @@ class TestIssue573(TestCase):
     This tests for a bug which causes filename limit on Renditions to be reached
     when the Image has a long original filename and a big focal point key
     """
+
     def test_issue_573(self):
         # Create an image with a big filename and focal point
         image = Image.objects.create(
@@ -500,6 +527,7 @@ class TestFilenameReduction(TestCase):
     This tests for a bug which results in filenames without extensions
     causing an infinite loop
     """
+
     def test_filename_reduction_no_ext(self):
         # Create an image with a big filename and no extension
         image = Image.objects.create(