===========================
Conditional View Processing
===========================

HTTP clients can send a number of headers to tell the server about copies of a
resource that they have already seen. This is commonly used when retrieving a
Web page (using an HTTP ``GET`` request) to avoid sending all the data for
something the client has already retrieved. However, the same headers can be
used for all HTTP methods (``POST``, ``PUT``, ``DELETE``, etc.).

For each page (response) that Django sends back from a view, it might provide
two HTTP headers: the ``ETag`` header and the ``Last-Modified`` header. These
headers are optional on HTTP responses. They can be set by your view function,
or you can rely on the :class:`~django.middleware.common.CommonMiddleware`
middleware to set the ``ETag`` header.

When the client next requests the same resource, it might send along a header
such as either `If-modified-since`_ or `If-unmodified-since`_, containing the
date of the last modification time it was sent, or either `If-match`_ or
`If-none-match`_, containing the last ``ETag`` it was sent.
If the current version of the page matches the ``ETag`` sent by the client, or
if the resource has not been modified, a 304 status code can be sent back,
instead of a full response, telling the client that nothing has changed.
Depending on the header, if the page has been modified or does not match the
``ETag`` sent by the client, a 412 status code (Precondition Failed) may be
returned.

.. _If-match: https://tools.ietf.org/html/rfc7232#section-3.1
.. _If-none-match: https://tools.ietf.org/html/rfc7232#section-3.2
.. _If-modified-since: https://tools.ietf.org/html/rfc7232#section-3.3
.. _If-unmodified-since: https://tools.ietf.org/html/rfc7232#section-3.4

When you need more fine-grained control you may use per-view conditional
processing functions.

.. _conditional-decorators:

The ``condition`` decorator
===========================

Sometimes (in fact, quite often) you can create functions to rapidly compute the ETag_
value or the last-modified time for a resource, **without** needing to do all
the computations needed to construct the full view. Django can then use these
functions to provide an "early bailout" option for the view processing.
Telling the client that the content has not been modified since the last
request, perhaps.

.. _ETag: https://tools.ietf.org/html/rfc7232#section-2.3

These two functions are passed as parameters to the
``django.views.decorators.http.condition`` decorator. This decorator uses
the two functions (you only need to supply one, if you can't compute both
quantities easily and quickly) to work out if the headers in the HTTP request
match those on the resource. If they don't match, a new copy of the resource
must be computed and your normal view is called.

The ``condition`` decorator's signature looks like this::

    condition(etag_func=None, last_modified_func=None)

The two functions, to compute the ETag and the last modified time, will be
passed the incoming ``request`` object and the same parameters, in the same
order, as the view function they are helping to wrap. The function passed
``last_modified_func`` should return a standard datetime value specifying the
last time the resource was modified, or ``None`` if the resource doesn't
exist. The function passed to the ``etag`` decorator should return a string
representing the `ETag`_ for the resource, or ``None`` if it doesn't exist.

Using this feature usefully is probably best explained with an example.
Suppose you have this pair of models, representing a simple blog system::

    import datetime
    from django.db import models

    class Blog(models.Model):
        ...

    class Entry(models.Model):
        blog = models.ForeignKey(Blog)
        published = models.DateTimeField(default=datetime.datetime.now)
        ...

If the front page, displaying the latest blog entries, only changes when you
add a new blog entry, you can compute the last modified time very quickly. You
need the latest ``published`` date for every entry associated with that blog.
One way to do this would be::

    def latest_entry(request, blog_id):
        return Entry.objects.filter(blog=blog_id).latest("published").published

You can then use this function to provide early detection of an unchanged page
for your front page view::

    from django.views.decorators.http import condition

    @condition(last_modified_func=latest_entry)
    def front_page(request, blog_id):
        ...

.. admonition:: Be careful with the order of decorators

    When ``condition()`` returns a conditional response, any decorators below
    it will be skipped and won't apply to the response. Therefore, any
    decorators that need to apply to both the regular view response and a
    conditional response must be above ``condition()``. In particular,
    :func:`~django.views.decorators.vary.vary_on_cookie`,
    :func:`~django.views.decorators.vary.vary_on_headers`, and
    :func:`~django.views.decorators.cache.cache_control` should come first
    because :rfc:`RFC 7232 <7232#section-4.1>` requires that the headers they
    set be present on 304 responses.

Shortcuts for only computing one value
======================================

As a general rule, if you can provide functions to compute *both* the ETag and
the last modified time, you should do so. You don't know which headers any
given HTTP client will send you, so be prepared to handle both. However,
sometimes only one value is easy to compute and Django provides decorators
that handle only ETag or only last-modified computations.

The ``django.views.decorators.http.etag`` and
``django.views.decorators.http.last_modified`` decorators are passed the same
type of functions as the ``condition`` decorator. Their signatures are::

    etag(etag_func)
    last_modified(last_modified_func)

We could write the earlier example, which only uses a last-modified function,
using one of these decorators::

    @last_modified(latest_entry)
    def front_page(request, blog_id):
        ...

...or::

    def front_page(request, blog_id):
        ...
    front_page = last_modified(latest_entry)(front_page)

Use ``condition`` when testing both conditions
------------------------------------------------

It might look nicer to some people to try and chain the ``etag`` and
``last_modified`` decorators if you want to test both preconditions. However,
this would lead to incorrect behavior.

::

    # Bad code. Don't do this!
    @etag(etag_func)
    @last_modified(last_modified_func)
    def my_view(request):
        # ...

    # End of bad code.

The first decorator doesn't know anything about the second and might
answer that the response is not modified even if the second decorators would
determine otherwise. The ``condition`` decorator uses both callback functions
simultaneously to work out the right action to take.

Using the decorators with other HTTP methods
============================================

The ``condition`` decorator is useful for more than only ``GET`` and
``HEAD`` requests (``HEAD`` requests are the same as ``GET`` in this
situation). It can also be used to provide checking for ``POST``,
``PUT`` and ``DELETE`` requests. In these situations, the idea isn't to return
a "not modified" response, but to tell the client that the resource they are
trying to change has been altered in the meantime.

For example, consider the following exchange between the client and server:

1. Client requests ``/foo/``.
2. Server responds with some content with an ETag of ``"abcd1234"``.
3. Client sends an HTTP ``PUT`` request to ``/foo/`` to update the
   resource. It also sends an ``If-Match: "abcd1234"`` header to specify
   the version it is trying to update.
4. Server checks to see if the resource has changed, by computing the ETag
   the same way it does for a ``GET`` request (using the same function).
   If the resource *has* changed, it will return a 412 status code,
   meaning "precondition failed".
5. Client sends a ``GET`` request to ``/foo/``, after receiving a 412
   response, to retrieve an updated version of the content before updating
   it.

The important thing this example shows is that the same functions can be used
to compute the ETag and last modification values in all situations. In fact,
you **should** use the same functions, so that the same values are returned
every time.

Comparison with middleware conditional processing
=================================================

You may notice that Django already provides simple and straightforward
conditional ``GET`` handling via the
:class:`django.middleware.http.ConditionalGetMiddleware` and
:class:`~django.middleware.common.CommonMiddleware`. While certainly being
easy to use and suitable for many situations, those pieces of middleware
functionality have limitations for advanced usage:

* They are applied globally to all views in your project
* They don't save you from generating the response itself, which may be
  expensive
* They are only appropriate for HTTP ``GET`` requests.

You should choose the most appropriate tool for your particular problem here.
If you have a way to compute ETags and modification times quickly and if some
view takes a while to generate the content, you should consider using the
``condition`` decorator described in this document. If everything already runs
fairly quickly, stick to using the middleware and the amount of network
traffic sent back to the clients will still be reduced if the view hasn't
changed.
