In itemupdating event

The App Uninstalling event is synchronous and you can use it to cancel the uninstallation, which would leave the add-in in the second stage recycle bin.

To handle list and list item events, you create remote event receivers (RERs), which are web services that run externally to the Share Point farm or Share Point Online.

If Share Point encounters an error when processing any of the three add-in events, it cancels the event and rolls back any changes it has made in connection with the event.

Your add-in event handlers have to integrate with this system because if the part of the event that you are implementing fails, you want the whole event to roll back, rather than continue and leave things in a possibly corrupt state.

Each time it calls the handler, your code starts again from the beginning.

But you generally don't want your handler to redo things it has already done, such as create a list on the host web, and you can't know if your rollback logic was completed, or even triggered, before the handler timed out.

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