To all of you who enquired about the whereabouts of Application Cache, I regret that I have to inform you that he is no longer with our company. This was not an easy decision to take, but we believe it was the right one.
While it has been no secret for some time that Application Cache was a douchebag, this was not necessarily apparent at first. Application Cache promised so much, and we believed him because he could prove his claims to a large extent. However, his way of working was so much at odds with the way other web components work (especially long-time pillar of web infrastructure HTTP cache) that his core value proposition was harder to exploit than it should have been (with many unfortunate pitfalls, as Jake Archibald documented); and worse, his more advanced promises, while working in basic scenarios, had some ancillary troubles, which unexpectedly turned out to be intractable no matter how hard we tried, and so these promises never came to light.
Because he was useful despite the issues, we tried to work with him on these, with many counseling sessions with HR; however, Application Cache was adamant that this was his fundamental mode of operation and he could not work any other way, and that others would have to adapt to him. This, of course, was not remotely acceptable, but we could not find any way to make him change either, so little progress was made. There was some, as we did manage to make him more transparent; some claimed that made him no longer a douchebag, but in truth he remained one.
Still, we believed that it could still be worth keeping him just for his core value proposition of using web apps while offline. But as time went on, it became clear that even that was not going to be worth the bother, again as a consequence of his fundamentally different way of working. Things came to a head when we tried to solve race conditions resulting from the possibility that a user load the initial HTML page before the web app is updated, and its dependencies (including the manifest) after the web app is updated: the manifest has to be updated at the same URL (it acts as a fixed entry point of sorts for users who already have the web app in Application cache), so we could not rely on the HTML pointing to a new manifest URL so that the update of the entry point would atomically result in the update of the web app. Even with the provision that the manifest be redownloaded after the entry point, and checked against the manifest downloaded before in the case of an app already in Application Cache (so as to try to have the manifest always loaded after the entry point, at least conceptually), we were stuck.
Some solutions were found, though limited to ideal situations; there was no solution available for the case of a serving infrastructure, such as content distribution networks, with only “eventually consistent” or other weak guarantees, and there was no solution either if even minimal use of
FALLBACK: was required. Moreover, even in ideal situations those solutions bring a lot of burden on the web developer, too much burden considering that offline web apps ought to work correctly in the face of these race conditions by default, or at least with minimal care. In the end, Application Cache was let go a few months ago.
If you were relying on the services provided by Application Cache, don’t worry. While there will be no future evolution (in particular, don’t expect bugs to get fixed), a new guy was hired to perform the tasks of Application Cache exactly as the latter did them. This new guy, Service Worker, will also provide a new service allowing web apps to work offline, this time in harmony with the other web components: for instance, out of the box he makes it possible to throttle checks for updated versions simply by setting a cache control header on the service worker (the period being a day at most); something which was exceedingly hard, if not impossible, with Application Cache due to his bad interactions with HTTP cache. He was already available in Chrome, and with the recently released Firefox 44, two independent, non-experimental implementations have now shipped, so you should take the time to make his acquaintance.