Understanding Once-Per-Session Cache Validation

Last year, I wrote about the IE9 improvements in heuristic expiration, which apply when a server fails to specify how long a cached resource should be treated as fresh. Heuristic Expiration works by calculating an implicit freshness lifetime from the Last-Modified timestamp on the cached resource and the timestamp at which the resource was downloaded from the server.

However, the Heuristic Expiration calculation only applies when the user’s Check for newer versions of stored pages option (a.k.a. "SyncMode") inside Tools > Internet Options > Browsing History Settings is set to Automatically.

As mentioned, the heuristic expiration feature relies upon the server providing a Last-Modified timestamp on the response. If the server fails to provide such a timestamp, then Internet Explorer will fall back to its Once-Per-Session syncmode for that resource. A user may elect to use Once-Per-Session syncmode (for all resources) in lieu of Heuristic Expiration by selecting the Every time I start Internet Explorer radio button.

WinINET keeps track of when the current process has started in a variable called SessionStartTime. Every entry in the WinINET cache has a LastSyncTime. If the cache syncmode is Once-Per-Session, when WinINET is deciding whether or not to reuse a resource of unknown freshness, it compares the resource’s LastSyncTime to the process’ SessionStartTime. If the LastSyncTime is earlier than the SessionStartTime, WinINET will not immediately reuse the cached version and will instead make a network request to the server to check for freshness, potentially downloading an updated version of the resource in the process.

Once-Per-Session syncmode has a few non-obvious subtleties.

1. To establish an upper-bound on reuse, WinINET will also perform revalidation if the current time is more than 12 hours after the LastSyncTime.
2. In any version of IE, if a page’s JavaScript executes the ClearAuthenticationCache command, that invokes the INTERNET_OPTION_END_BROWSER_SESSION operation in WinINET. Internally, the EndBrowserSession option (among many other operations) invokes the INTERNET_OPTION_RESET_URLCACHE_SESSION operation, which sets the current process’ SessionStartTime to the current time. So, if any page invokes this API, any “Once per session” resources will be revalidated before being reused by the current process.
3. If INTERNET_FLAG_HYPERLINK is present on the request, and a cached response doesn’t have a Last-Modified value set, then WinINET will always revalidate the cached resource before reuse, ignoring the Once-Per-Session rule. IE9 uses FLAG_HYPERLINK (via BINDF_HYPERLINK) when performing most document (e.g. top-level or frame) downloads.
4. In Internet Explorer 8, simply opening a new tab (which creates a LCIE Tab process) would reset the SessionStartTime for all other tabs in the current Browser Session. This behavior hurt performance, and was corrected in IE9. In IE9, every new tab process in a Browser Session will inherit the original SessionStartTime for the browser session, eliminating this performance penalty.

So, what does this gobbledygook all mean to you, the web developer? Avoid the confusion and unpredictability of cache heuristics and specify an explicit expiration time!

Thanks,

Eric

Comments

• Anonymous
  July 12, 2011
  The comment has been removed

• Anonymous
  July 13, 2011
  @dunfield: #1: Video URLs are converted from http:// to mms:// by the Windows Media Player ActiveX control. IE itself doesn't participate in or control that behavior. I'm afraid I have no idea why these won't play from the cache. #2: As far as I know, there's no supported way to opt IE out of the "go online automatically" behavior which hits if you try to navigate to a resource which is not already cached locally. (I'll verify and update this remark if I turn up anything different). If you ensure that all resources are cached properly (e.g. nothing is no-cache, Vary: *, etc) then you shouldn't hit the "Go online" behavior at all. More generally, you'll probably want to migrate your application away from running IE in Kiosk mode and instead host a Web Browser Control instance which then hosts your content. This will give you far more control over the behavior and create a more stable platform which is less likely to change between versions.

• Anonymous
  July 14, 2011
  Thanks for your quick reply! #1:  Is there a blog / forum hereabouts where I can ask the Media Player team about that?  Not being able to play wmv's from the cache really cuts down on the usefulness of our application. #2:  It definitely behaves more reliably if I do the Web Browser Control hosting route I'll continue working in that direction from now on.   As I'll show in a minute, I do have no-cache, etc set. Updated jpg's are not being detected reliably unless I set some headers on the server.  The images can be updated as often as once a minute (they may have the time imprinted on them) so it's important that the current one is being displayed.   I've tried all the different 'Check for newer versions of stored pages' options and the best combination I've found is to set the 'Check for newer...' to 'Automatic'  and to add this to my Apache configuration: <FilesMatch ".(jpg|cgi|swf)$"> Header set Cache-Control "max-age=0, must-revalidate" Header set last-modified "Sun Jul 18 00:32:50 GMT 2004" ExpiresActive on ExpiresDefault "access plus 0 minutes" </FilesMatch> With this combination the jpgs are being detected when they change and still play from cache when put offline.   Prior to IE8 I could set 'Every time I visit the webpage' and it would detect the changed images with no changes on the server. Is there some other way I could configure things to make IE detect changes reliably?

• Anonymous
  July 14, 2011
  @dunfield: You should read blogs.msdn.com/.../caching-improvements-in-internet-explorer-9.aspx. "Cache-Control: max-age=0, mustrevalidate" literally means "You may not re-use this element without validating it with the server, and you must not re-use the cached copy if you cannot reach the server, say, because you're offline."

• Anonymous
  July 21, 2011
  @EricLaw : I have read your previous article.  I've removed the conditions from my Apache install and now again have intermittent issues with the images not being updated in a timely fashion.  I can't yet narrow down the conditions. Where can I contact the Media Player team to ask about playing files from the cache when offline.  This is a critical issue and am stuck since I don't see a way to downgrade WMP to a previous version in Win7. Thanks.

• Anonymous
  November 17, 2011
  The comment has been removed

• Anonymous
  November 17, 2011
  @gkiras: Email me using the contact link. The link you posted to your site is broken, and your question is about cookies, not caching.