How to set ETAGS on Google App Engine for Python?

Question

I am serving some JSON content from a Google App Engine server. I need to serve an ETAG for the content to know if its changed since i last loaded the data. Then my app will remove its old data and use the new JSON data to populate its views.

    self.response.headers['Content-Type'] = "application/json; charset=utf-8"
    self.response.out.write(json.dumps(to_dict(objects,"content")))

Whats the best practice to set ETAGs for the response? Do i have to calculate the ETAG myself? Or is it a way to get the HTTP protocol to do this?

Answer 1

You'll have to calculate the e-tag value yourself. E-tags are opaque strings that only have meaning to the application.

Best practice is to just concatenate all the input variables (converted to string) that determine the JSON content; anything that, if changed, would result in a change in the JSON output, should be part of this. If there is anything sensitive in those strings you wouldn't want to be exposed, use the MD5 hash of the values instead.

For example, in a CMS application that I administer, the front page has the following e-tag:

|531337735|en-us;en;q=0.5|0|Eli Visual Theme|1|943ed3c25e6d44497deb3fe274f98a96||

The input variables that we care about have been concatenated with the | symbol into an opaque value, but it does represent several distinct input values, such as a last-modified timestamp (the number), the browser accepted language header, the current visual theme, and a internal UID that is retrieved from a browser cookie (and which determines what context the content on the front page is taken from). If any of those variables would change, the page is likely to be different and the cached copy would be stale.

Note that an e-tag is useless without a means to verify it quickly. A client will include it in a If-None-Match request header, and the server should be able to quickly re-calculate the e-tag header from the current variables and see if the tag is still current. If that re-calculation would take the same amount of time as regenerating the content body, you only save a little bandwidth sending the 304 Not Modified response instead of a full JSON body in a 200 OK response.

Answer 2

If you're using webapp2, it can add an md5 ETag based on the response body automatically.

self.response.md5_etag()

http://webapp-improved.appspot.com/guide/response.html