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

With On-Demand Capacity Reservations, you can reserve compute capacity for your On-Demand Instances in a specified Availability Zone for any duration. You can configure an EC2 Fleet to use the Capacity Reservations first when launching On-Demand Instances.

Capacity Reservations are configured as either open or targeted. EC2 Fleet can launch On-Demand Instances into either open or targeted Capacity Reservations, as follows:

• If a Capacity Reservation is open, On-Demand Instances that have matching attributes automatically run in the reserved capacity.

• If a Capacity Reservation is targeted, On-Demand Instances must specifically target it to run in the reserved capacity. This is useful for using up specific Capacity Reservations or for controlling when to use specific Capacity Reservations.

If you use targeted Capacity Reservations in your EC2 Fleet, there must be enough Capacity Reservations to fulfil the target On-Demand capacity, otherwise the launch fails. To avoid a launch fail, rather add the targeted Capacity Reservations to a resource group, and then target the resource group. The resource group doesn't need to have enough Capacity Reservations; if it runs out of Capacity Reservations before the target On-Demand capacity is fulfilled, the fleet can launch the remaining target capacity into regular On-Demand capacity.

To use Capacity Reservations with EC2 Fleet

1. Configure the fleet as type instant. You can't use Capacity Reservations for fleets of other types.

2. Configure the usage strategy for Capacity Reservations as use-capacity-reservations-first.

3. In the launch template, for Capacity reservation, choose either Open or Target by group. If you choose Target by group, specify the Capacity Reservations resource group ID.

When the fleet attempts to fulfil the On-Demand capacity, if it finds that multiple instance pools have unused matching Capacity Reservations, it determines the pools in which to launch the On-Demand Instances based on the On-Demand allocation strategy (lowest-price or prioritized).

For examples of how to configure a fleet to use Capacity Reservations to fulfil On-Demand capacity, see EC2 Fleet example configurations, specifically Examples 5 through 7.

For information about configuring Capacity Reservations, see On-Demand Capacity Reservations and the On-Demand Capacity Reservation FAQs.