while working on MDL-26587, i've come across a backup file that i couldn't replicate.
I've had to basically put in place a check for broken data to allow the restore process to continue.

This effectively introduces data lacking full integrity to the system. quiz short answer for the case of MDL-26587.

While it is being controlled at the GUI level currently, there are times when a moodle stable version is released that didn't enforce such data integrity. i think it is also difficult to ensure this at the database level (foreign keys) with the various databases we support and the simple database relationships within the database.

I think we need a system to help maintain our system's data integrity, just like our UI(forms) is doing. We need to do this with backup files being imported (from older 2.x versions etc) and possibly with other input files. I don't think having the UI parse the file and asking the admin to resolve all the data integrity issues at that point would be practical. I think its better that the data issues get stored to be looked at later, possibly suspending that entire dataset (say an entire question which is missing an answer it should have) from actively existing for access by non-admins.

This would allow more backward compatibility for many backup files at the very least.