Task status query

[John Lilley]

How does a YARN application master typically query ongoing status (like 
percentage completion) of its tasks?
I would like to be able to ultimately relay information to the user like:
100 tasks are scheduled
10 tasks are complete
4 tasks are running and they are (4%, 10%, 50%, 70%) complete
But, given that YARN tasks are simply executables, how can the AM even get at 
this information?  Can the AM get access to stdout/stderr?
Thanks
John