Availability Zones are distinct geographical locations engineered to be insulated from failures in other zones.  It is critically important from a HA perspective to run your application stack in more than one zone. In addition, avoid dependencies across the zones as far as possible. For sites with very high request loads, a 3-zone configuration may be the preferred configuration to handle zone-level failures. In this situation, if one zone goes down, then other two AZs can ensure continuing high availability and uninterrupted customer experience.

In the event of a zone failure in a multi-AZ configuration, there are several challenges. The DNS record will contain multiple IP addresses and DNS round robin can be used to balance traffic between the availability zones. Using multiple AZs can result in traffic imbalances between AZs due to clients caching DNS records. However, ELBs can help reduce the impact of this caching.