The company i work for have been using the described app for the...

The company i work for have been using the described app for the last two years
late cancel the T/L simply ticks a couple
boxes on a screen on computer it then works out and messages out to available pool of non rostered people with required skillset, first in best dressed for the shift, no obligation to have notifications on, your either available or not.
I might add casuals are sent rosters on a Wednesday for following week depending
on company requirements and whether
you have made yourself unavailable or not.