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Evenly Distributing Instances by AZ

An AS group can contain instances from multiple AZs in a region. When adjusting resources, AS evenly distributes the instances between the AZs in either of the following cases:

Evenly distributing new instances to balanced AZs

AS evenly adds instances to the AZs where an equal number of instances are running.

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.

Figure 1 Evenly distributing instances

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.

Figure 2 Re-balancing instances
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