ETags

Conditional-gets are a facility of the HTTP spec that provide a way for web servers to tell browsers that the response to a GET request hasn’t changed since the last request and can be safely pulled from the browser cache.

 

They work by using the HTTP_IF_NONE_MATCH and HTTP_IF_MODIFIED_SINCE headers to pass back and forth both a unique content identifier and the timestamp of when the content was last changed. If the browser makes a request where the content identifier (etag) or last modified since timestamp matches the server’s version then the server only needs to send back an empty response with a not modified status.

 

It is the server’s (i.e. our) responsibility to look for a last modified timestamp and the if-none-match header and determine whether or not to send back the full response.

 

class ArticlesController < ApplicationController

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

    # Set the response headers to accurately reflect the state of the
    # requested object(s)
    response.last_modified = @article.published_at.utc
    response.etag = @article

    # If the request's state is the same as the server's state then we know
    # we don't have to send back the whole body
    if request.fresh?(response)
      head :not_modified
    else
      respond_to do |wants|
        # normal response processing
      end
    end
end

 

The etag value is calculated for you with the etag= setter method. All you have to do is provide a single object or array of objects that uniquely identify this request. In this example the article itself contains all the information that uniquely identifies the state of this request. However, you may need to use more than one key in your app. For instance, if the request is user specific:

 

response.etag = [@article, current_user]

 

The request.fresh?(response) method is what will then tell you if the incoming request matches either the last-modified-since or if-none-match values of the outgoing response. If it does you can avoid passing the full body of the response back and save some bandwidth.

 

It’s also possible that you can avoid hitting the database all together if your application deals with completely static resources (though this is rare):

 

class ArticlesController < ApplicationController

  def show

    # If articles don't change, the etag can be based solely
    # on items we have in the request
    response.etag = [:article, params[:id]]

    # If the request's state is the same as the server's state then we can
    # avoid the db call all together
    if request.fresh?(response)
      head :not_modified
    else
      @article = Article.find(params[:id])
      respond_to do |wants|
        ...
      end
    end
end