stale?

Sets the etag and/or last_modified on the response and checks it against the client request. If the request doesn’t match the options provided, the request is considered stale and should be generated from scratch. Otherwise, it’s fresh and we don’t need to generate anything and a reply of 304 Not Modified is sent.

Parameters:

• :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 set If-None-Match header may return a 304 Not Modified response if it matches the ETag 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 set If-None-Match header may return a 304 Not Modified response if it matches the ETag exactly. A strong ETag implies exact equality: the response must match byte for byte. This is necessary for doing 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 set If-Modified-Since may return a 304 Not Modified response if last_modified <= If-Modified-Since.

• :public By default the Cache-Control header is private, set this to true if you want your application to be cacheable by other devices (proxy caches).

• :cache_control When given will overwrite an existing Cache-Control header. See www.w3.org/Protocols/rfc2616/rfc2616-sec14.html for more possibilities.

• :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.

Example:

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. In this case last_modified will be set by calling updated_at and etag by passing the object itself.

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

You can also pass an object that responds to maximum, such as a collection of active records. In this case last_modified will be set by calling maximum(:updated_at) on the collection (the timestamp of the most recently updated record) and the etag by passing the object itself.

def index
  @articles = Article.all

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

When passing a record or a collection, you can still set the public header:

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

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

When overwriting Cache-Control header:

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

This will set in the response Cache-Control = public, no-cache.

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

def show
  super if stale? @article, template: 'widgets/show'
end