move fos httpcache bundle tip up to beginning of invalidation section

dbu · dbu · commit 979034a10a60 · 2014-12-25T18:07:22.000+01:00
diff --git a/book/http_cache.rst b/book/http_cache.rst
@@ -401,8 +401,8 @@ header when none is set by the developer by following these rules:
 
 .. _http-expiration-validation:
 
-HTTP Expiration and Validation
-------------------------------
+HTTP Expiration, Validation and Invalidation
+--------------------------------------------
 
 The HTTP specification defines two caching models:
 
@@ -419,7 +419,9 @@ The HTTP specification defines two caching models:
   header) to check if the page has changed since being cached.
 
 The goal of both models is to never generate the same response twice by relying
-on a cache to store and return "fresh" responses.
+on a cache to store and return "fresh" responses. To achieve long caching times
+but still provide updated content immediately, *cache invalidation* is
+sometimes used.
 
 .. sidebar:: Reading the HTTP Specification
 
@@ -819,7 +821,7 @@ cache lifetimes, but to actively notify the gateway cache when content
 changes. Reverse proxies usually provide a channel to receive such
 notifications, typically through special HTTP requests.
 
-.. tip::
+.. warning::
 
     While cache invalidation is powerful, avoid it when possible. If you fail
     to invalidate something, outdated caches will be served for a potentially
@@ -836,6 +838,13 @@ explicitly invalidating. For invalidation, your application needs to detect
 when content changes and tell the cache to remove the URLs which contain
 that data from its cache.
 
+.. tip::
+
+    If you want to use cache invalidation, have a look at the
+    `FOSHttpCacheBundle`_. This bundle provides services to help with various
+    cache invalidation concepts, and also documents the configuration for the
+    a couple of common caching proxies.
+
 If one content corresponds to one URL, the ``PURGE`` model works well.
 You send a request to the cache proxy with the HTTP method ``PURGE`` instead
 of ``GET`` and make the cache proxy detect this and remove the data from the
@@ -894,10 +903,6 @@ different URLs. More flexible concepts exist for those cases:
 * **Cache tagging** lets you add a tag for each content used in a response
   so that you can invalidate all URLs containing a certain content.
 
-If you need such features, you should use the `FOSHttpCacheBundle`_. This
-bundle documents the configuration for the caching proxy and provides
-services to send invalidation requests based on URLs and Symfony routes.
-
 .. index::
 single: Cache; ESI
   single: ESI
