When a user sharing group has a large number of files, it may be prohibitive to try to download all of those at once when a new user joins (or when a recovery operation takes place). Note that this is different than the issue of background downloading because while some number of files could be downloaded in the background, presumably there are limits on this. If a sharing group had thousands of files, presumably it wouldn't be suitable to attempt to download all of those in the background.

Some other options:

1. Allow an app to be selective about which files get downloaded. For example, for SharedImages it might be sufficient to have both icon image files and full sized image files. Downloading icons might be sufficient. Date range restrictions might also be useful.

2. A strategy such as the above could require some method(s) on the SyncServer client interface to explicitly request files to be downloaded. For example, if only an icon was downloaded, the client could explicitly request the large image be downloaded when the user taps on the icon.

3. After applying a filtering or selection to which files should be downloaded, the remaining files could be downloaded, automatically, and gradually. Say, a collection of 10 or less files could be queued up for background downloading each time the app was in the foreground and there were not already files downloading.