[system settings] per app data transfer over 3G should be configurable by the user

Sorry, I don't understand this bug report. What is a "sync", exactly?

The "Updates" screen has a setting for which kinds of connection to use when auto-downloading updates. So whatever syncing is, if it is necessary maybe it could be modelled on that setting.

I think it means calendar and contacts sync.

Calendar and contacts are apps, so that would belong in the apps rather than in System Settings.

I disagree and agree with that, it should be a global setting rather than per app.
It could be refined per app (switch off) but in general if apps can do background sync/updates via 3G as well has to be determined in System Settings.

We should split sync from apps. Because:

1 - Apps does not do the sync, it only can ask for sync (Android does not allow that, the sync is completely invisible by the user)
2- The sync can happen even if the app is not running (push-notifications-syncs, database changes)
3- A database can be accessed by different apps and changing this database could trigger a sync that can happen during the application life time or not
4 - Apps does not run in background so if the sync was attached to the app the sync will be suspended
5 - Syncs should to be a system wide process that runs: periodical , push-notifications events, user request, or several others ways based on device status (baterry, network, disk space).

I have attached two examples of android system settings sync options.

I am not saying that we should use the same approach, these images is just to show how android does.

You can receive phone calls without the Phone app running, you can continue them with the Phone app in the background, and whether you can receive them depends on your network settings. Does that mean incoming calls must be presented, or listed afterwards, outside the Phone app rather than inside? No, because those factors aren't relevant when deciding where to put calls in the overall UI. So they aren't relevant when deciding where to put calendar and contact sync in the overall UI, either.

Factors that are relevant are things like:

(A) Where will people usually be when they want to change the settings? Usually in the apps, wondering why their data is out of date. Possibly also in the data usage settings, once they exist (bug 1287267), if you've been using too much data -- but syncs are probably dwarfed bandwidth-wise by video calls and/or streaming.

(B) How complex are the sync-related settings for each account? If complex, it's a burden for each app to reimplement them, and for users to learn any variations in presentation. If simple, it's no big deal for the settings to be in each app.

(C) How often will people use a calendar or contacts app other than the default one? If very often, then it might be annoying to require people to use the default app just to change sync settings that don't happen to be represented in your preferred app. But as far as I know, there are no rival contacts or calendar apps yet.

(In the course of writing this, I realized that Phone settings probably shouldn't be in System Settings either. I've reported bug 1447601 for that.)

This needs separate design for the Contacts app and for the Calendars app.

Our sync service has a global setting that says if the background sync can happen over 3g. And a sync plugin can sync any kind of data (twitter messages, chat-log, calendar, contacts. etc). In my opinion we should not have individual setting for different service.

I would like to keep address-book-app away from the sync service. Today it has a sync button only because we did not find a way yet to auto-sync if the server side changes.

  Since we can have different address-books apps in the phone. In my opinion the sync service should be configured on system-settings. Where you can see all enabled-syncs, ask a new manual sync, check sync logs, etc..

Renato, that is covered by my point (C) above: it's only a theoretical problem as long as no other address book apps even exist. If we get to a point where >30% or so of Ubuntu users are using a non-default address book or calendar app, then it might become true that most people don't look in the default app to find the sync setting.

There are several issues to consider here:

Using the Phone as an example is a bad idea because it is a special case. The phone app does NOT requite sync but access to different areas on the device. Notifications, contacts, even messages to some extent. Calls, incoming, missed and outgoing are associated with the phone hence a list of recent calls wold be naturally found within the phone app.

Syncing however can be both, specific and generic. As per comment #9 you might be wondering why your data for a specific app is outdated while you are using it (C) but this might have nothing to do with the actual app but with the fact that you are not willing to use 3G/4G for syncing and instead WiFi. If a user has to set that generic setting for every single application which requires sync and is installed on the device, then it starts to become a very cumbersome and very annoying.

I agree with Renato, we need a way to have an overview on all apps which need sync and the best place for that are System Settings. Also, not every developer will invest time into sync settings and might even assume that we are handling at least the high level ones. I also agree with Matthew, some app specific settings (e.g. do not sync images over a certain size) should be app specific.

We do not know how often users will use different apps, apart from the default ones. Some might want to handle private and work related events in different calendars, different email applications for work and family, etc. Meaning, the argument that we do not have alternatives to default apps yet, doesn't mean we won't have it in (near) future (C). In design and development we always deal with theoretical problems and trying to solve them before the user actually complains. ;)

Bottom line:
We need decisions and agreement on what is considered a "generic" setting for sync. Anything which goes outside this definition will have to go into app specific settings.

The issue here is really more about connection management than just "sync". Applications need high level access to connection and data management, hence it should be handled in System Settings.