etag

Created by Loqi.me on September 22

• Tue, September 22 loqi.me prompted by tantek and dfn added by KevinMarks
• Tue, September 22 kevinmarks.com discussion of how to do this dynamically
• Wed, September 23 bear.im create howto section and add flask
• Wed, September 23 tantek.com split why, how to, linky linky, indie web examples none
• Wed, September 23 bear.im PHP
• Wed, September 23 bear.im nodejs
• Wed, September 23 bear.im rework howto text and move examples to bottom of section

This article is a stub. You can help the IndieWebCamp wiki by expanding it.

etag is a header sent in an HTTP response to allow future fetches of the same URL to be skipped if nothing has changed.

Why

When serving a URL that may be fetched multiple times (a feed or hovercard for example) it is good practice to send ETag and Last-Modified headers along with it in order to enable fetching code to optimize for reducing bandwidth from your site, and to theirs.

See https://pythonhosted.org/feedparser/http-etag.html for a discussion on how this can save you bandwidth.

How to

This section is a stub - please expand into a step-by-step How to inline!

If you're using static serving of files, sending ETag and Last-Modified headers is often done automatically by the web server using information from the host file system.

A client that supports this protocol will send back the ETag and If-Modified-Since headers when it next fetches your URL. Your web server will then check to see if there is a new version for them or not. If there is no change, it will return an HTTP 304 result and an empty body.

If you are dynamically generating your site, you have to do more work to create these headers yourself. Hashing is a common way to create an ETag; Last-Modified is a date in a standard format. You'll also need to check for them when serving.

The following are examples of how to do this for different languages

• Google AppEngine Python
• Python Flask
• PHP
• NodeJS -- every NodeJS framework supports etags in a different way:
  • HapiJS
  • ExpressJS
  • Restangular AngularJS doesn't natively support etag's so you have to use a helper component

IndieWeb Examples

none currently.

is anyone programmatically setting ETag / Last-Modified headers?

See Also

• HTTP