Configure EC2 Fleet for On-Demand backup - Amazon Elastic Compute Cloud
Prioritize instance types for On-Demand capacityUse Capacity Reservations for On-Demand Instances
Services or capabilities described in Amazon Web Services documentation might vary by Region. To see the differences applicable to the China Regions, see Getting Started with Amazon Web Services in China (PDF).

Configure EC2 Fleet for On-Demand backup

If you have urgent, unpredictable scaling needs, such as a news website that must scale during a major news event or game launch, we recommend that you specify alternative instance types for your On-Demand Instances, in the event that your preferred option does not have sufficient available capacity. For example, you might prefer c5.2xlarge On-Demand Instances, but if there is insufficient available capacity, you'd be willing to use some c4.2xlarge instances during peak load. In this case, EC2 Fleet attempts to fulfill all of your target capacity using c5.2xlarge instances, but if there is insufficient capacity, it automatically launches c4.2xlarge instances to fulfill the target capacity.

Prioritize instance types for On-Demand capacity

When EC2 Fleet attempts to fulfill your On-Demand capacity, it defaults to launching the lowest priced instance type first. If AllocationStrategy is set to prioritized, EC2 Fleet uses priority to determine which instance type to use first in fulfilling On-Demand capacity. The priority is assigned to the launch template override, and the highest priority is launched first.

Example: Prioritize instance types

In this example, you configure three launch template overrides, each with a different instance type.

The On-Demand price for the instance types range in price. The following are the instance types used in this example, listed in order of price, starting with the least expensive instance type:

  • m4.large – least expensive

  • m5.large

  • m5a.large

If you do not use priority to determine the order, the fleet fulfills the On-Demand capacity by starting with the least expensive instance type.

However, say you have unused m5.large Reserved Instances that you want to use first. You can set the launch template override priority so that the instance types are used in the order of priority, as follows:

  • m5.large – priority 1

  • m4.large – priority 2

  • m5a.large – priority 3

Use Capacity Reservations for On-Demand Instances

You can configure a fleet to use On-Demand Capacity Reservations first when launching On-Demand Instances by setting the usage strategy for Capacity Reservations to use-capacity-reservations-first. You can use this setting in conjunction with the allocation strategy for On-Demand Instances (lowest-price or prioritized).

When unused Capacity Reservations are used to fulfil On-Demand capacity:

  • The fleet uses unused Capacity Reservations to fulfill On-Demand capacity up to the target On-Demand capacity.

  • If multiple instance pools have unused Capacity Reservations, the On-Demand allocation strategy (lowest-price or prioritized) is applied.

  • If the number of unused Capacity Reservations is less than the On-Demand target capacity, the remaining On-Demand target capacity is launched according to the On-Demand allocation strategy (lowest-price or prioritized).

You can only use unused On-Demand Capacity Reservations for fleets of type instant.

For examples of how to configure a fleet to use Capacity Reservations to fulfil On-Demand capacity, see EC2 Fleet example configurations. For more information, see On-Demand Capacity Reservations and the On-Demand Capacity Reservation FAQs.