XWiki 15.10.2, 15.10.3 and 15.10.4 are containing a version of a migration (named R151002000XWIKI21448) that leads to the permanent deletion of all notification filters related to subwiki pages. If you performed an upgrade to one of this version and you used a subwiki then you're impacted by this regression and should follow this procedure. If you upgraded to a later version (15.10.5 or after) or if you don't have any subwiki, then you're not impacted by this regression.
This recovery procedure is only possible if you have a backup of your database (and more specifically of the main wiki database), prior to the upgrade to the incriminated version. Without a backup of the database, there is sadly nothing to be done, the notification filters have been lost: you can only ask your users to check their filters in the notification settings and to manually set them by watching the pages again.
If you do have a backup, then you can perform the following steps:
- Create a new temporary database that will be used to hold the data coming from the backup
- Restore in that location the main wiki database, and specifically the table named notification_filters_pref: you only need that table to be created back with all data in it
- Import in your wiki by going in the Administration > Content > Import
- Go to the page Main/NotificationFiltersRecovery (this page is hidden so you won't see it in the navigation by default)
- Type the name of your temporary database in the dedicated field and click on the "Start recovery" button
- Wait for the action to be finished: you should see some logs in your console logs and you will have the same output on the page when the process is finished
- Delete the page Main/NotificationFiltersRecovery
- Restart the wiki
