Merge branch 'feature-cachebust'

4 files changed, 207 insertions, 0 deletions

diff --git a/docs/api/interfaces.rst b/docs/api/interfaces.rst
index d8d935afd..a62976d8a 100644
--- a/docs/api/interfaces.rst
+++ b/docs/api/interfaces.rst
@@ -86,3 +86,5 @@ Other Interfaces
   .. autointerface:: IResourceURL
      :members:
 
+  .. autointerface:: ICacheBuster
+     :members:
diff --git a/docs/api/static.rst b/docs/api/static.rst
index c28473584..543e526ad 100644
--- a/docs/api/static.rst
+++ b/docs/api/static.rst
@@ -9,3 +9,11 @@
      :members:
      :inherited-members:
 
+  .. autoclass:: PathSegmentMd5CacheBuster
+     :members:
+
+  .. autoclass:: QueryStringMd5CacheBuster
+     :members:
+
+  .. autoclass:: QueryStringConstantCacheBuster
+     :members:
diff --git a/docs/narr/assets.rst b/docs/narr/assets.rst
index b0a8d18b0..95863848b 100644
--- a/docs/narr/assets.rst
+++ b/docs/narr/assets.rst
@@ -287,6 +287,181 @@ suggestion for a pattern; any setting name other than ``media_location``
 could be used.
 
 .. index::
+   single: Cache Busting
+
+.. _cache_busting:
+
+Cache Busting
+-------------
+
+.. versionadded:: 1.6
+
+In order to maximize performance of a web application, you generally want to 
+limit the number of times a particular client requests the same static asset.
+Ideally a client would cache a particular static asset "forever", requiring 
+it to be sent to the client a single time.  The HTTP protocol allows you to 
+send headers with an HTTP response that can instruct a client to cache a 
+particular asset for an amount of time.  As long as the client has a copy of 
+the asset in its cache and that cache hasn't expired, the client will use the
+cached copy rather than request a new copy from the server.  The drawback to 
+sending cache headers to the client for a static asset is that at some point
+the static asset may change, and then you'll want the client to load a new copy
+of the asset.  Under normal circumstances you'd just need to wait for the 
+client's cached copy to expire before they get the new version of the static 
+resource.
+
+A commonly used workaround to this problem is a technique known as "cache 
+busting".  Cache busting schemes generally involve generating a URL for a 
+static asset that changes when the static asset changes.  This way headers can
+be sent along with the static asset instructing the client to cache the asset
+for a very long time.  When a static asset is changed, the URL used to refer to
+it in a web page also changes, so the client sees it as a new resource and 
+requests a copy, regardless of any caching policy set for the resource's old 
+URL.
+
+:app:`Pyramid` can be configured to produce cache busting URLs for static 
+assets by passing the optional argument, ``cachebust`` to 
+:meth:`~pyramid.config.Configurator.add_static_view`:
+
+.. code-block:: python
+   :linenos:
+
+   # config is an instance of pyramid.config.Configurator
+   config.add_static_view(name='static', path='mypackage:folder/static',
+                          cachebust=True)
+
+Setting the ``cachebust`` argument instructs :app:`Pyramid` to use a cache 
+busting scheme which adds the md5 checksum for a static asset as a path segment
+in the asset's URL:
+
+.. code-block:: python
+   :linenos:
+
+   js_url = request.static_url('mypackage:folder/static/js/myapp.js')
+   # Returns: 'http://www.example.com/static/c9658b3c0a314a1ca21e5988e662a09e/js/myapp.js`
+
+When the asset changes, so will its md5 checksum, and therefore so will its
+URL.  Supplying the ``cachebust`` argument also causes the static view to set
+headers instructing clients to cache the asset for ten years, unless the
+``max_cache_age`` argument is also passed, in which case that value is used.
+
+.. note:: 
+
+   md5 checksums are cached in RAM so if you change a static resource without
+   restarting your application, you may still generate URLs with a stale md5
+   checksum. 
+
+Disabling the Cache Buster
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It can be useful in some situations (e.g. development) to globally disable all
+configured cache busters without changing calls to
+:meth:`~pyramid.config.Configurator.add_static_view`.  To do this set the 
+``PYRAMID_PREVENT_CACHEBUST`` environment variable or the 
+``pyramid.prevent_cachebust`` configuration value to a true value.
+
+Customizing the Cache Buster
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Revisiting from the previous section:
+
+.. code-block:: python
+   :linenos:
+
+   # config is an instance of pyramid.config.Configurator
+   config.add_static_view(name='static', path='mypackage:folder/static',
+                          cachebust=True)
+
+Setting ``cachebust`` to ``True`` instructs :app:`Pyramid` to use a default
+cache busting implementation that should work for many situations.  The 
+``cachebust`` may be set to any object that implements the interface,
+:class:`~pyramid.interfaces.ICacheBuster`.  The above configuration is exactly
+equivalent to:
+
+.. code-block:: python
+   :linenos:
+
+   from pyramid.static import PathSegmentMd5CacheBuster
+
+   # config is an instance of pyramid.config.Configurator
+   config.add_static_view(name='static', path='mypackage:folder/static',
+                          cachebust=PathSegmentMd5CacheBuster())
+
+:app:`Pyramid` includes a handful of ready to use cache buster implementations:
+:class:`~pyramid.static.PathSegmentMd5CacheBuster`, which inserts an md5
+checksum token in the path portion of the asset's URL,
+:class:`~pyramid.static.QueryStringMd5CacheBuster`, which adds an md5 checksum
+token to the query string of the asset's URL, and
+:class:`~pyramid.static.QueryStringConstantCacheBuster`, which adds an
+arbitrary token you provide to the query string of the asset's URL.  
+
+In order to implement your own cache buster, you can write your own class from
+scratch which implements the :class:`~pyramid.interfaces.ICacheBuster`
+interface.  Alternatively you may choose to subclass one of the existing
+implementations.  One of the most likely scenarios is you'd want to change the
+way the asset token is generated.  To do this just subclass an existing
+implementation and replace the :meth:`~pyramid.interfaces.ICacheBuster.token`
+method.  Here is an example which just uses Git to get the hash of the 
+currently checked out code:
+
+.. code-block:: python
+   :linenos:
+
+   import os
+   import subprocess
+   from pyramid.static import PathSegmentMd5CacheBuster
+
+   class GitCacheBuster(PathSegmentMd5CacheBuster):
+       """
+       Assuming your code is installed as a Git checkout, as opposed to as an
+       egg from an egg repository like PYPI, you can use this cachebuster to
+       get the current commit's SHA1 to use as the cache bust token.
+       """
+       def __init__(self):
+           here = os.path.dirname(os.path.abspath(__file__))
+           self.sha1 = subprocess.check_output(
+               ['git', 'rev-parse', 'HEAD'],
+               cwd=here).strip()
+
+       def token(self, pathspec):
+           return self.sha1
+   
+Choosing a Cache Buster
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The default cache buster implementation, 
+:class:`~pyramid.static.PathSegmentMd5CacheBuster`, works very well assuming 
+that you're using :app:`Pyramid` to serve your static assets.  The md5 checksum
+is fine grained enough that browsers should only request new versions of 
+specific assets that have changed.  Many caching HTTP proxies will fail to 
+cache a resource if the URL contains a query string.  In general, therefore, 
+you should prefer a cache busting strategy which modifies the path segment to
+a strategy which adds a query string.  
+
+It is possible, however, that your static assets are being served by another
+web server or externally on a CDN.  In these cases modifying the path segment
+for a static asset URL would cause the external service to fail to find the
+asset, causing your customer to get a 404.  In these cases you would need to 
+fall back to a cache buster which adds a query string.  It is even possible 
+that there isn't a copy of your static assets available to the :app:`Pyramid`
+application, so a cache busting implementation that generates md5 checksums
+would fail since it can't access the assets.  In such a case, 
+:class:`~pyramid.static.QueryStringConstantCacheBuster` is a reasonable 
+fallback.  The following code would set up a cachebuster that just uses the 
+time at start up as a cachebust token:
+
+.. code-block:: python
+   :linenos:
+
+   import time
+   from pyramid.static import QueryStringConstantCacheBuster
+
+   config.add_static_view(
+       name='http://mycdn.example.com/', 
+       path='mypackage:static',
+       cachebust=QueryStringConstantCacheBuster(str(time.time())))
+
+.. index::
    single: static assets view
 
 .. _advanced_static:
diff --git a/docs/narr/environment.rst b/docs/narr/environment.rst
index 7bac12ea7..0b06fb80b 100644
--- a/docs/narr/environment.rst
+++ b/docs/narr/environment.rst
@@ -157,6 +157,28 @@ feature when this is true.
 |                                 |                                  |
 +---------------------------------+----------------------------------+
 
+Preventing Cache Busting
+------------------------
+
+Prevent the ``cachebust`` static view configuration argument from having any
+effect globally in this process when this value is true.  No cache buster will
+be configured or used when this is true.  
+
+.. versionadded:: 1.6
+
+.. seealso::
+
+    See also :ref:`cache_busting`.
+
++---------------------------------+----------------------------------+
+| Environment Variable Name       | Config File Setting Name         |
++=================================+==================================+
+| ``PYRAMID_PREVENT_CACHEBUST``   |  ``pyramid.prevent_cachebust``   |
+|                                 |  or ``prevent_cachebust``        |
+|                                 |                                  |
+|                                 |                                  |
++---------------------------------+----------------------------------+
+
 Debugging All
 -------------