RebalanceInstances - Auto Scaling - Alibaba Cloud Documentation Center

Operation description

The rebalancing operation compensates for imbalanced zones by creating new ECS instances to replace existing ones. New ECS instances are started before existing ones are terminated, so the rebalancing operation does not affect your application performance or availability.

This operation is supported only for multi-zone scaling groups whose MultiAZPolicy is set to BALANCE, and is used to balance the distribution of ECS instances across zones.
You can perform a rebalancing operation only when the instance distribution in the scaling group is severely imbalanced. A single rebalancing activity can replace up to 20 ECS instances.
During a rebalancing activity, if the number of instances in the scaling group approaches or reaches the specified maximum number of ECS instances (MaxSize) but further balancing is required, Auto Scaling allows the number of ECS instances to temporarily exceed MaxSize by 10%. This excess state persists until the instance distribution is balanced, typically for 1 to 6 minutes.

Note
If 10% of the maximum number of instances is not an integer, the decimal part is rounded up to allow one additional instance. For example, if the maximum number of instances is 15, 10% is 1.5, so a temporary excess of 2 instances is allowed.

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Test

RAM authorization

The table below describes the authorization required to call this API. You can define it in a Resource Access Management (RAM) policy. The table's columns are detailed below:

Action: The actions can be used in the Action element of RAM permission policy statements to grant permissions to perform the operation.
API: The API that you can call to perform the action.
Access level: The predefined level of access granted for each API. Valid values: create, list, get, update, and delete.
Resource type: The type of the resource that supports authorization to perform the action. It indicates if the action supports resource-level permission. The specified resource must be compatible with the action. Otherwise, the policy will be ineffective.
- For APIs with resource-level permissions, required resource types are marked with an asterisk (*). Specify the corresponding Alibaba Cloud Resource Name (ARN) in the Resource element of the policy.
- For APIs without resource-level permissions, it is shown as All Resources. Use an asterisk (*) in the Resource element of the policy.
Condition key: The condition keys defined by the service. The key allows for granular control, applying to either actions alone or actions associated with specific resources. In addition to service-specific condition keys, Alibaba Cloud provides a set of common condition keys applicable across all RAM-supported services.
Dependent action: The dependent actions required to run the action. To complete the action, the RAM user or the RAM role must have the permissions to perform all dependent actions.

Action

Access level

Resource type

Condition key

Dependent action

ess:RebalanceInstances

update

*ScalingGroup

acs:ess:{#regionId}:{#accountId}:scalinggroup/{#ScalingGroupId}

None

Request parameters

Parameter	Type	Required	Description	Example
ScalingGroupId	string	Yes	The ID of the scaling group.	asg-bp18p2yfxow2dloq****
RegionId	string	No	The region ID of the scaling group.	cn-hangzhou

Response elements

Element	Type	Description	Example
	object
ScalingActivityId	string	The ID of the scaling activity.	asa-kjgffgdfadah****
RequestId	string	The request ID.	473469C7-AA6F-4DC5-B3DB-A3DC0DE3****

Examples

Success response

JSON format

{
  "ScalingActivityId": "asa-kjgffgdfadah****",
  "RequestId": "473469C7-AA6F-4DC5-B3DB-A3DC0DE3****"
}

Error codes

See Error Codes for a complete list.

Release notes

See Release Notes for a complete list.