Need information

Hi All, I need information about availability zones in nova. I tried googling but cant find enough information. My questions are , 1. Why is it that we have two concepts of aggregates and AZs ? Is one not enough ? Like exposing aggregates and creating flavors with extra specs to match ? Why do we need AZs also ? 2. Why is it that one node should only be a part of one AZ but not two ? whereas in the case of aggregates, it can overlap ? 3. Also why cant we expose only aggregates like AZs but block the compute member list to the users ? Doing this way will serve the purpose of AZ as well ? Why we dont want to expose aggregates as AZs ? Thanks Kumar