Auto Scaling:RebalanceInstances

Usage notes

Auto Scaling creates new ECS instances to replace the existing ECS instances to fulfill the rebalancing purpose. Auto Scaling starts the new ECS instances before stopping the existing ECS instances. The rebalancing operation does not affect the performance or service availability of your application.

• This operation is supported by only multi-zone scaling groups whose MultiAZPolicy is set to BALANCE.

• A rebalancing operation is required only when the distribution of the instances of a multi-zone scaling group is significantly unbalanced. In a rebalancing activity, Auto Scaling replaces up to 20 ECS instances to rectify the unbalanced distribution.

• During the execution of a rebalancing operation, if the number of instances in the scaling group approaches or hits the value of MaxSize but the rebalancing operation needs to continue, Auto Scaling allows the total number of ECS instances to momentarily exceed the value of MaxSize by 10%. This temporary surplus condition persists for a duration until equilibrium in the distribution of ECS instances is achieved. Typically, it takes 1 to 6 minutes.

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  Note If the 10% increment of the maximum number of instances in a scaling group yield a non-integer value, the decimal portion is always rounded up to ensure an additional instance is accounted for. For example, you have a scaling group that holds a maximum of 15 ECS instances. During a rebalancing operation, Auto Scaling would permit the total number of instances to momentarily surpass this limit by 2, instead of the calculated 10% (which is 1.5).