| =========================== |
| 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.http.ConditionalGetMiddleware` |
| middleware to set the ``ETag`` header. |
|
|
| When the client next requests the same resource, it might send along a header |
| such as either :rfc:`If-Modified-Since <9110#section-13.1.3>` or |
| :rfc:`If-Unmodified-Since <9110#section-13.1.4>`, containing the date of the |
| last modification time it was sent, or either :rfc:`If-Match |
| <9110#section-13.1.1>` or :rfc:`If-None-Match <9110#section-13.1.2>`, |
| 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. |
|
|
| 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 :rfc:`ETag <9110#section-8.8.3>` 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. |
|
|
| 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 :rfc:`ETag <9110#section-8.8.3>` for the resource, or ``None`` |
| if it doesn't exist. |
|
|
| The decorator sets the ``ETag`` and ``Last-Modified`` headers on the response |
| if they are not already set by the view and if the request's method is safe |
| (``GET`` or ``HEAD``). |
|
|
| Using this feature usefully is probably best explained with an example. |
| Suppose you have this pair of models, representing a small blog system:: |
|
|
| import datetime |
| from django.db import models |
|
|
|
|
| class Blog(models.Model): |
| ... |
|
|
|
|
| class Entry(models.Model): |
| blog = models.ForeignKey(Blog, on_delete=models.CASCADE) |
| 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 9110 <9110#section-15.4.5>` 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: |
|
|
| #. Client requests ``/foo/``. |
| #. Server responds with some content with an ETag of ``"abcd1234"``. |
| #. 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. |
| #. 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". |
| #. 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. |
|
|
| .. admonition:: Validator headers with non-safe request methods |
|
|
| The ``condition`` decorator only sets validator headers (``ETag`` and |
| ``Last-Modified``) for safe HTTP methods, i.e. ``GET`` and ``HEAD``. If you |
| wish to return them in other cases, set them in your view. See |
| :rfc:`9110#section-9.3.4` to learn about the distinction between setting a |
| validator header in response to requests made with ``PUT`` versus ``POST``. |
|
|
| Comparison with middleware conditional processing |
| ================================================= |
|
|
| Django provides conditional ``GET`` handling via |
| :class:`django.middleware.http.ConditionalGetMiddleware`. While being suitable |
| for many situations, the middleware has limitations for advanced usage: |
|
|
| * It's applied globally to all views in your project. |
| * It doesn't save you from generating the response, which may be expensive. |
| * It's 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. |
|
|
