Instances are evenly distributed in different AZs to reduce the impacts of power and network faults on the stability of the system.
A region is a geographic area where resources used by your ECSs are located. Each region contains many AZs where power and networks are physically isolated. AZs in the same region can communicate with each other over an intranet. Each AZ provides cost-effective and low-latency network connections that are unaffected by faults in other AZs.
An AS group can contain ECS instances from one or more AZs in a region. To adjust resources, AS evenly distributes ECS instances in AZs in any of the following cases:
Evenly distributing new instances to balanced AZs
Instances of an AS group should be evenly distributed among the AZs of the AS group. To ensure this, AS adds instances to an AZ with the least number of instances during a scaling action.
For example, four instances are evenly distributed in two AZs used by an AS group. If a scaling action is triggered to add four more instances to the AS group, AS adds two to each AZ.
Re-balancing instances in unbalanced AZs
After you manually add or reduce instances in one of the balanced AZs, these AZs become unbalanced. The next scaling action will preferentially re-balance the instances in the AZs.
For example, three instances are distributed in AZ 1 and AZ 2 used by an AS group, two in AZ 1 and one in AZ 2. If a scaling action is triggered to add five more instances to the AS group, AS adds two to AZ 1 and three to AZ 2.