Module: ActionController::ConditionalGet

Sets the Cache-Control header, overwriting existing directives. This method will also ensure an HTTP ::Date header for client compatibility.

Defaults to issuing the private directive, so that intermediate caches must not cache the response.

#### Options

:public : If true, replaces the default private directive with the public

directive.

:must_revalidate : If true, adds the must-revalidate directive.

:stale_while_revalidate : Sets the value of the stale-while-revalidate directive.

:stale_if_error : Sets the value of the stale-if-error directive.

:immutable : If true, adds the immutable directive.

Any additional key-value pairs are concatenated as directives. For a list of supported Cache-Control directives, see the [article on MDN](developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control).

#### Examples

expires_in 10.minutes
# => Cache-Control: max-age=600, private

expires_in 10.minutes, public: true
# => Cache-Control: max-age=600, public

expires_in 10.minutes, public: true, must_revalidate: true
# => Cache-Control: max-age=600, public, must-revalidate

expires_in 1.hour, stale_while_revalidate: 60.seconds
# => Cache-Control: max-age=3600, private, stale-while-revalidate=60

expires_in 1.hour, stale_if_error: 5.minutes
# => Cache-Control: max-age=3600, private, stale-if-error=300

expires_in 1.hour, public: true, "s-maxage": 3.hours, "no-transform": true
# => Cache-Control: max-age=3600, public, s-maxage=10800, no-transform=true

Sets an HTTP 1.1 Cache-Control header of no-cache. This means the resource will be marked as stale, so clients must always revalidate. Intermediate/browser caches may still store the asset.

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](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control).

: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" }

Cache or yield the block. The cache is supposed to never expire.

You can use this method when you have an HTTP response that never changes, and the browser and proxies should cache it indefinitely.

• public: By default, HTTP responses are private, cached only on the user’s web browser. To allow proxies to cache the response, set true to indicate that they can serve the cached response to all users.

Sets an HTTP 1.1 Cache-Control header of no-store. This means the resource may not be stored in any cache.

Sets the etag and/or last_modified on the response and checks them against the request. If the request doesn’t match the provided options, it is considered stale, and the response should be rendered from scratch. Otherwise, it is fresh, and a 304 Not Modified is sent.

#### Options

See #fresh_when for supported options.

#### Examples

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

  if stale?(etag: @article, last_modified: @article.updated_at)
    @statistics = @article.really_expensive_call
    respond_to do |format|
      # all the supported formats
    end
  end
end

You can also just pass a record:

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

  if stale?(@article)
    @statistics = @article.really_expensive_call
    respond_to do |format|
      # all the supported formats
    end
  end
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

  if stale?(@articles)
    @statistics = @articles.really_expensive_call
    respond_to do |format|
      # all the supported formats
    end
  end
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])

  if stale?(@article, public: true, cache_control: { no_cache: true })
    @statistics = @articles.really_expensive_call
    respond_to do |format|
      # all the supported formats
    end
  end
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:

def show
  super if stale?(@article, template: "widgets/show")
end