Help Center/ Auto Scaling/ FAQs/ Instance/ If the AS Configuration of an AS Group Uses Multiple Flavors, In What Order Are the Flavors Selected in During Scale-out Actions?
Updated on 2025-08-25 GMT+08:00
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If the AS Configuration of an AS Group Uses Multiple Flavors, In What Order Are the Flavors Selected in During Scale-out Actions?

The order depends on the AZs and multiple-AZ scaling policy used by the AS group. This section describes how flavors are selected during scale-out events in these the following two different scenarios:

Single AZ

If the AS group uses only one AZ, new instances will be created in the AZ by following the configured flavor selection policy which determines the flavor priority order.

  • Sequenced: In this policy, the flavors are used in the order they were selected. For example, if you selected flavors 2, 3, and 1, and in that sequence, the system attempts to use flavor 2 to create instances at first. If flavor 2 is not available in the current AZ or an instance fails to be created due to other reasons, the system follows up with flavor 3 and again with flavor 1.
  • Cost-centered: In this policy, the least expensive flavor is used first. For example, if you select flavors 1, 2, and 3 in sequence, where flavor 1 is the most expensive, followed by flavor 3, and flavor 2 is the least expensive, the system preferentially scales out the AS group by starting with flavor 2, then flavor 3, and then flavor 1.

Multiple AZs

If the AS group spans two or more AZs, you need to configure the multi-AZ scaling policy. The system will scale out the AS group by following the scaling policy you configured using the flavor with the highest priority.

  • Balanced: When scaling out an AS group, the system preferentially distributes ECS instances evenly across AZs used by the AS group. If it fails in the target AZ, it automatically selects another AZ based on the sequenced policy. For example, if:
    • You have selected AZ 1, AZ 2, and AZ 3 in sequence.
    • You have selected flavors 1, 2, and 3, where the flavor priority order is 2, 3, and 1.
    • AZ 1, AZ 2, and AZ 3 have, respectively, 3, 2, and 3 instances.

    AZ 2 has fewer instances. The system will try to create instances in AZ 2 first using flavor 2 (the one with the highest priority). If this attempt succeeds, the scaling action is complete. If this attempt fails, the system will try to create instances in AZ 2 using flavor 3, and then flavor 1. If all attempts fail in AZ 2, the system will try to fulfill the required capacity by starting with flavor 2, then flavor 3, and then flavor 1 in the AZ 1 based on the Sequenced policy. If all attempts fail in AZ 1, the system will try in AZ 3.

  • Sequenced: When scaling out an AS group, the system distributes ECS instances to the AZs based on the order in which AZs are specified. For example, if:
    • You have selected AZ 1, AZ 2, and AZ 3 in sequence.
    • You have selected flavors 1, 2, and 3, where the flavor priority order is 2, 3, and 1.

    Regardless of how instances are distributed in three AZs, the system will try to use flavor 2 to create instances in AZ 1. If this attempt fails, the system will try flavor 3 and then flavor 1 in AZ 1. If all attempts fail in AZ 1, the system will do that again in AZ 2, and then AZ 3.

The flavor priority order is determined by the flavor selection policy in the AS configuration. For details, see Single AZ.

Parent topic: Instance