You can use ETags to allow clients to use cached responses, if your application would send the same contents as before.

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Besides what "actually" defines your response's contents, your application probably also considers "global" conditions, like which user is signed in:

class ApplicationController < ActionController::Base

  etag { current_user&.id }
  etag { current_user&.updated_at }

end

Under the hood, Rails generates an ETag header value like W/"f14ce3710a2a3187802cadc7e0c8ea99". In doing so, all objects from that etaggers list are serialized and Time objects lose their millisecond resolution.

You likely won't notice for a while (ask me how I know) but occasionally encounter tests which are "randomly failing".

Example

When users change their name and visit a page they visited less than a second ago (yes, that is not an issue with humans), your application will respond with an HTTP 304 and the browser will re-use the response from before which still contained the old username.

To avoid that, convert any Time objects into a string or number before an ETag is generated from them.

class ApplicationController < ActionController::Base

  etag { current_user&.id }
  etag { current_user&.updated_at&.to_f }

end

You might even want to patch Rails' etagging logic so you don't have to remember doing that. Here you go:

class ApplicationController < ActionController::Base
  
  etag { current_user&.id }
  etag { current_user&.updated_at }
  
  private
  
  def combine_etags(...)
    # When using Time objects for etagging, their milliseconds are ignored because of how
    # ActiveSupport::Cache#expand_cache_key works (which is called to build ETag contents).
    # We convert them to floats to avoid that.
    super.map! { |object| object.is_a?(Time) ? object.to_f : object }
  end

end