1. CS193H: High Performance Web Sites Lecture 17: Rule 14 – Make Ajax Cacheable Steve Souders Google souders@cs.stanford.edu

2. announcements 11/17 – guest lecturer: Robert Johnson (Facebook), "Fast Data at Massive Scale - lessons learned at Facebook"

3. Web 2.0, DHTML, Ajax communities rich UI mashups Asynchronous JS and XML layer between data and UI JS, CSS, & DOM used to modify the page

4. Async == Instantaneous (?) try Google Maps on dialup passive Ajax requests in anticipation of user's needs e.g., download address book for webmail active Ajax requests in response to user action e.g., email search request user is waiting; not instantaneous optimize these first

5. Ajax optimizations add an Expires header gzip components minify JavaScript avoid redirects configure ETags most important

6. cache Ajax? Really?! but... it's dynamic yes, but for this [user | time | browser] it's [always the same | doesn't change for awhile] it's data sometimes data doesn't change it's private :no-store doesn't always work should you use https?

7. caching XHR vs. HTML XHR and HTML often contain dynamic, personalized data why can I cache XHR, but not HTML? the URL for HTML is often frozen bookmarks cross-references prettier memorized developers control what's in XHR URLs the XHR URL is contained inside the HTML the developer can modify the XHR URL with data to avoid a cache hit

8. ex: Google Calendar http://www.google.com/calendar/contacts HTTP/1.x 200 OK Cache-Control: no-cache, no-store, max-age=0, must- revalidate Pragma: no-cache Expires: Fri, 01 Jan 1990 00:00:00 GMT Content-Type: text/javascript; charset=UTF-8 Content-Encoding: gzip Date: Mon, 10 Nov 2008 22:00:57 GMT Content-Length: 1562 solution: add modification timestamp to URL http://www.google.com/calendar/contacts_1226354800

9. Homework 11/12 3:15pm – Web 100 Double Check look at your rows in Web 100 spreadsheet double-check your entries for any rows in red update incorrect entries enter "y" in "Double Checked" column

10. Questions Which performance rules so far apply to Ajax? What are passive Ajax requests? active? Why may developers view the cacheability of Ajax responses differently than other content? If I can cache XHRs, why can't I cache HTML that also contains dynamic data?