guaranteed delivery for uploads after network reconnect, even if my app is not running

Question

I'm spec-ing an iOS app (which will be built outside of our company) which will upload a user's data entry to a server. If the device is not connected to the Internet, we'd like to save data on the device and upload it when the network is re-connected. (The app will primarily run on iPod Touch devices that will be disconnected most of the time).

If the user unlocks the device and re-opens our app after the network is reconnected, then uploading to the server should be easy because the app is running.

But what if the app is not running, where "not running" can mean one or more of:

• device was power cycled
• user has locked the device and it's sitting in his pocket
• app crashed
• user exited the app
• user started using other apps so our app isn't running in the foreground anymore
• are there other cases?

In the cases above, is there a way (ideally a battery-efficient way) to ensure that local data is uploaded soon after Internet connectivity is restored? Is the answer different depending on which of the cases above caused the app not to be running?

And is there a minimum iOS version the device will need in order to enable some (or all) of the above not-running cases to still upload when the app is not running?

My apologies if these are obvious newbie questions-- I'm not an iOS expert.

Answer 1

There is an interesting technique that is used by among others Instapaper and News.me(the pioneers of this technique) where you use region monitoring to initiate background downloads or uploads. Marco (Instapaper) blogged and talked (in episode 80 of the Build and Analyze podcast) about his communication with Apple so it should be a allowed in the App Store.

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In brief the technique is that you set up certain regions (geofences) like "home" or "work" and respond to the locationManager:didEnterRegion: (and similar) callback(s). Your app will wake up from the background once you enter the pre-specified region and you can check to see if there is any data to upload.

This technique won't guarantee that the data is uploaded when the network reconnects but it will allow your app to automatically upload the information when the iPod Touch users gets home to their WiFi network.

That should most likely be at least once a day which may or may not be frequent enough for you. You could add a timestamp to when the initial upload was attempted and send that along the upload once it succeeds to get the correct order of events (data entries) on your server.

Answer 2

There is no way to ensure this. If your application is "not running" (by the definition described in your question), it will not be capable of responding to a change in the device's network status. It should be setup to resume upload operations the next time the application runs again.

EDIT: Some of the cases you've described may indeed provide different opportunities for your application. Specifically, if the user "exits" the app by pressing the home button or launches another app in the foreground, your application may continue to run the in the background and could potentially respond to a change in network reachability.

The nature of what may be done in the background and for how-long is well documented, and supported by any version of iOS that supports multi-tasking. I recommend you review the documentation pertaining to App States and Background Services.