Target Capacity Reservations from your Auto Scaling group - Amazon EC2 Auto Scaling
Use a Capacity Reservation specification with your Auto Scaling group
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Target Capacity Reservations from your Auto Scaling group

To target Amazon EC2 Capacity Reservations from your Auto Scaling group, you can configure a Capacity Reservation preference and target on the group itself. For step-by-step procedures, see Configure your Auto Scaling group to launch instances with Capacity Reservations.

Use a Capacity Reservation specification with your Auto Scaling group

Use a Capacity Reservation specification (CapacityReservationSpecification) to indicate to your Auto Scaling group how and where to consume Capacity Reservations. A Capacity Reservation specification has two parts: a preference that tells Auto Scaling how strongly to prefer reserved capacity over On-Demand capacity, and an optional target that identifies which reservations to consume.

A Capacity Reservation specification alone handles consuming On-Demand Capacity Reservations from your Auto Scaling group.

Capacity Reservation preference

The Capacity Reservation preference (CapacityReservationPreference) tells Auto Scaling how to consume Capacity Reservations when launching instances. Choose one of the following values.

  • Default (default) – Auto Scaling uses the Capacity Reservation specification from your launch template.

  • Capacity Reservations only (capacity-reservations-only) – Auto Scaling only launches instances into a Capacity Reservation or Capacity Reservation Resource Group. If no reserved capacity is available, instances fail to launch.

  • Capacity Reservations first (capacity-reservations-first) – Auto Scaling launches instances into a Capacity Reservation or Capacity Reservation Resource Group when available. If no reserved capacity is available, instances launch as On-Demand.

  • None (none) – Auto Scaling does not launch instances into a Capacity Reservation. Instances run as On-Demand capacity.

Capacity Reservation target

The Capacity Reservation target (CapacityReservationTarget) tells Auto Scaling which Capacity Reservations it can launch instances into. Choose one of the following options.

  • Specific Capacity Reservation (CapacityReservationIds) – Provide a Capacity Reservation ID. Auto Scaling launches instances into this Capacity Reservation.

  • Capacity Reservation Resource Group (CapacityReservationResourceGroupArns) – Provide a Capacity Reservation Resource Group ARN. Auto Scaling launches instances into any Capacity Reservation in this group with matching attributes.

  • Open – Don't specify a target on your Auto Scaling group or launch template. Auto Scaling launches instances into any open Capacity Reservation in your account with compatible attributes.

  • From launch template – Don't specify a target on your Auto Scaling group. Auto Scaling uses the Capacity Reservation target from your launch template.

Important

Only a single Capacity Reservation ID or Capacity Reservation Resource Group ARN is allowed as a Capacity Reservation target.

Availability Zone balance

By default, Auto Scaling prioritizes Availability Zone balance when consuming Capacity Reservations. The exact behavior depends on whether On-Demand fallback is enabled:

  • With On-Demand fallback – Auto Scaling distributes instances evenly across Availability Zones. If reserved capacity isn't available in an Availability Zone, instances launch as On-Demand to maintain the balance.

  • Without On-Demand fallback – Auto Scaling only launches instances into Capacity Reservations. This can result in uneven distribution across Availability Zones, or the Auto Scaling group might not reach its desired capacity if reserved capacity is insufficient.

Example

If you have 10 Capacity Reservations in AZ-a, 3 in AZ-b, 1 in AZ-c, and a desired capacity of 9 instances:

  • With On-Demand fallback, Auto Scaling launches 3 instances per Availability Zone (maintaining balance), with some instances potentially launching as On-Demand.

  • Without On-Demand fallback, Auto Scaling launches into whichever Capacity Reservations are available, which can result in uneven distribution across Availability Zones. If matching reservations don't cover the full desired capacity, the Auto Scaling group might not launch all 9 instances.

Note

To prioritize Capacity Reservation utilization over Availability Zone balance, use the reservations-then-balanced Availability Zone distribution strategy. For more information, see Auto Scaling group Availability Zone distribution.