[whatwg] Fwd: fallback section taking over for 4xx and 5xx responses while online

Ian Hickson ian at hixie.ch
Mon Dec 10 11:37:25 PST 2012

On Fri, 24 Aug 2012, Josh Sharpe wrote:
> I have a manifest that looks something like this:
> # e4a75fb378cb627a0d51a80c1cc5684c2d918d93e267f5854a511aa3c8db5b1a
> /a/application.js
> /a/application.css
> *
> / /offline
> Notably, it has a "/ /offline" fallback section which is, obviously, a 
> prefix for every page on my site.  This is good, because the goal is to 
> have my users redirected to what's at /offline when they navigate to 
> www.mydomain.com while offline.

Note that the Offline Application Cache feature is for Offline 
Applications, not Offline Sites. What you're trying to do here is not what 
appcache was designed to do.

Also, it's not clear what you mean by "offline". For the record, in the 
spec, "offline" includes "I'm on wifi but there's a captive portal" and 
"I'm online but the server is broken".

> As the fallback section is a prefix for everything, it's a prefix for any
> url/path that results in an error condition such as a 404 or 500 response.

These are cases where it's assumed that the server is broken, i.e. 
offline, and the cache is therefore used.

> It seems that the application cache, when it encounters an error state 
> such as a 404 or 500, doesn't check to see if the browser is still in 
> the 'online' state, and immediately falls over to the fallback section.

It does check -- by definition, if it receives a 4xx or 5xx, it's assumed 
that the server is offline (broken).

> While online, I would expect my 4xx and 5xx page to be rendered 
> normally.

Offline Application Cache doesn't have a "while online" mode, it just 
always works as if you were offline and tries to get the data from the 
server while the server is able to respond.

This is an important facet of how appcache works: it doesn't "work online" 
or "work offline". It always acts in "offline" mode (or rather, always 
works in "internet connection is flaky" mode).

> Finally, the fallback section in my example is very typical of most 
> examples I find in various docs, including the whatwg spec.  I don't 
> think I'm doing anything abnormal here.

What's abnormal is having the user visit pages that return 4xx or 5xx 
error codes when there's no problem. :-)

> Should I design this differently or is there something missing from the 
> spec?

It's hard to know exactly without understanding more about your use case. 
Can you elaborate on what you're trying to do?

We could just exclude 4xx (and maybe 5xx? Though that seems less 
reasonable) error codes from being considered "offline" for fallback- 
supported resources, if people are commonly linking people to missing 
pages intentionally (and don't want the problem hidden from users by 
having it fall back to locally-generated pages). But that seems like a 
weird thing to do...

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'

More information about the whatwg mailing list