redis mutex added.

bookshop/models.py

@@ -1,20 +1,31 @@
 from django.db import models
 from django.core.cache import cache
 import time
+import redis
+
+r = redis.Redis(host='localhost', port=6379, db=0)
 
 
 class Author(models.Model):
     name = models.CharField(max_length=255)
 
+    # _ indicates this function is for internal use only
+    def _get_top_books(self):
+        time.sleep(5)  # make the calculation slower for the sake of argument
+        return list(Book.objects.order_by('-purchase_count')[:10])
+
     def get_top_books(self):
         cache_key = 'Author:get_top_books:{}'.format(self.id)
+        lock_key = 'Lock:{}'.format(cache_key)
         cache_value = cache.get(cache_key)
         if cache_value is not None:
             return list(Book.objects.filter(id__in=cache_value))
+        try:
+            with r.lock(lock_key, timeout=60, blocking_timeout=0):
+                books = self._get_top_books()
+        except redis.exceptions.LockError:
+            raise Exception('ColdCacheException')
 
-        time.sleep(5)  # make the calculation slower for the sake of argument
-        books = list(Book.objects.order_by('-purchase_count')[:10])
-        # cache.set(cache_key, books, 4 * 60 * 60)  # cache for 4 hours
         cache.set(cache_key, [book.id for book in books], 4 * 60 * 60)  # cache for 4 hours
         return books
 

bookshop/views.py

@@ -22,8 +22,8 @@ def author_detail_view(request, pk):
 
 
 def index_view(request):
-    authors = Author.objects.all()
-    books = Book.objects.all()
+    authors = Author.objects.all()[:50]
+    books = Book.objects.all()[:50]
     return render(request, 'bookshop/index.html', {
         'books': books,
         'authors': authors

requirements.txt

@@ -0,0 +1,5 @@
+asgiref==3.2.7
+Django==2.2
+pytz==2020.1
+redis==3.5.2
+sqlparse==0.3.1