Apple's iCloud service has had its share of outages and issues, but the latest problem, while apparently limited to a subset of accounts, just might be the most frustrating yet for those affected.
Starting on or around September 18, some iCloud customers found that apps that use iCloud to sync data between devices could no longer do so—they either would no longer sync or appeared to have lost their data completely. The apps otherwise functioned normally, but simply didn't show any saved information or could not update or sync existing info.
The issue doesn't just affect third-party iOS apps—it also affects Apple's own software, including some OS X apps. For example, some users are reporting that OS X apps that store documents in the cloud, such as TextEdit, Pages, and Preview, currently show no documents available when you choose File > Open and switch to iCloud view; similarly, iCloud-saved documents no longer appear in the File > Open Recent menu.
Apple's online iWork for iCloud apps have also fallen victim to this issue. If your account is affected, you can log in to your account on iCloud.com, but clicking the Pages, Numbers, or Keynote icon to use one of the Web-app versions immediately logs you out of your account and returns you to the iCloud.com login screen. (Developers who have been bitten by the bug get a similar behavior when trying to view data on Apple's developer iCloud site, developer.icloud.com: Clicking the Documents icon—which should let you test and troubleshoot iCloud storage—dumps you back to the login screen.)
Apple's System Status webpage has yet to indicate any iCloud issues.
What's made the issue so frustrating for affected users is that other iCloud-hosted services, such as email, contacts, and calendars, continue to function normally, and even allow third-party apps to sync with them. (Behind the scenes, that's because these services, though also part of iCloud, use a different system than Documents & Data syncing.) In fact, if you don't use more than one or two iOS apps that sync data using iCloud, it's easy enough to assume that the app is at fault. For example, I first noticed the issue when I discovered that my movie list in TodoMovies was blank. As the problem started soon after I updated my iPhone to iOS 7, I assumed it was a bug with the app and iOS 7, so I contacted the developer, who couldn't reproduce the problem (and apparently hadn't yet received enough reports of problems to realize that we were seeing an iCloud issue).
Users discover details
While the issue clearly isn't affecting all iCloud users—or even most, it appears—it's generated a number of lengthy discussions on Apple's Support Communities forums. User Grant Grueninger has provided perhaps the best summary, including some more details about what's going on behind the scenes.
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