Design a mobile application where user can edit image (add some effects like Instragram).
Follow up user should be able to continue working on another device. Not a collaborative editing, just continue from the same point.

Solution
I proposed to store image (S3) and actions (nosql) separatly. Each editing is a sorted coollection of actions: {id, actionType=Crop/Blur, actionData=...}
For follow up: when user opens application the request with the last action id is sent to the server. There is a redis with all active sessions. If there is already one -> send back a collection of new ations and apply at the new device, in the same time the old session should be blocked.

I was completly confused by this question and how to tacke this, have never seen something like continue of work. Probably there is some name of this pattern. Any ideas what I've missed?
This round was the worst performance and the reason of a reject.