The Zone Imbalance Problem
A load balancer often has a node in each availability zone. Without cross zone load balancing, each zone node only sends traffic to targets in its own zone, which can spread load unevenly when zones hold different numbers of targets.
How The Skew Happens
Imagine two zones, one with eight targets and one with two.
- Each zone node receives half of incoming traffic if clients resolve evenly.
- The zone with two targets then gives each target a larger share than the zone with eight.
- Enabling cross zone balancing lets any node send to all targets, smoothing the load.
The Trade Off
Cross zone balancing evens out per target load but can add cross zone data transfer, which may carry a charge. Some balancer types enable it by default while others make it optional, so it pays to know the behavior of the type you are using before assuming even distribution.
Key idea
Cross zone load balancing lets every balancer node reach targets in all zones, which evens out per target load when zones have different target counts, at the possible cost of cross zone transfer.