TTL feature for field , auto deleting (or update) a field after x seconde , use case :

Manage easely online mebers in your collection.
A members is connected , we put a flag online , at the application level each X seconde we reput the flag to maintain the online statut .
When a search for members online comes, profiles no more online is easely identified because " TTL for fields feature deleted (or update) the online flag.

Actually to doing this, i need a cron task to delete (or update) the online flag with the help of others 'flagtime' etc , creating some overhead..