fresh_when

Sets the etag, last_modified, or both on the response, and renders a 304 Not Modified response if the request is already fresh.

Options

:etag

Sets a “weak” ETag validator on the response. See the :weak_etag option.

:weak_etag

Sets a “weak” ETag validator on the response. Requests that specify an If-None-Match header may receive a 304 Not Modified response if the ETag matches exactly.

A weak ETag indicates semantic equivalence, not byte-for-byte equality, so they’re good for caching HTML pages in browser caches. They can’t be used for responses that must be byte-identical, like serving Range requests within a PDF file.

:strong_etag

Sets a “strong” ETag validator on the response. Requests that specify an If-None-Match header may receive a 304 Not Modified response if the ETag matches exactly.

A strong ETag implies exact equality – the response must match byte for byte. This is necessary for serving Range requests within a large video or PDF file, for example, or for compatibility with some CDNs that don’t support weak ETags.

:last_modified

Sets a “weak” last-update validator on the response. Subsequent requests that specify an If-Modified-Since header may receive a 304 Not Modified response if last_modified <= If-Modified-Since.

:public

By default the Cache-Control header is private. Set this option to true if you want your application to be cacheable by other devices, such as proxy caches.

:cache_control

When given, will overwrite an existing Cache-Control header. For a list of Cache-Control directives, see the article on MDN.

:template

By default, the template digest for the current controller/action is included in ETags. If the action renders a different template, you can include its digest instead. If the action doesn’t render a template at all, you can pass template: false to skip any attempt to check for a template digest.

Examples

def show
  @article = Article.find(params[:id])
  fresh_when(etag: @article, last_modified: @article.updated_at, public: true)
end

This will send a 304 Not Modified response if the request specifies a matching ETag and If-Modified-Since header. Otherwise, it will render the show template.

You can also just pass a record:

def show
  @article = Article.find(params[:id])
  fresh_when(@article)
end

etag will be set to the record, and last_modified will be set to the record’s updated_at.

You can also pass an object that responds to maximum, such as a collection of records:

def index
  @articles = Article.all
  fresh_when(@articles)
end

In this case, etag will be set to the collection, and last_modified will be set to maximum(:updated_at) (the timestamp of the most recently updated record).

When passing a record or a collection, you can still specify other options, such as :public and :cache_control:

def show
  @article = Article.find(params[:id])
  fresh_when(@article, public: true, cache_control: { no_cache: true })
end

The above will set Cache-Control: public, no-cache in the response.

When rendering a different template than the controller/action’s default template, you can indicate which digest to include in the ETag:

before_action { fresh_when @article, template: "widgets/show" }