This article explains how Elastigroup uses your account’s reserved instances when the Utilize reserved instances setting is turned on. Understanding this behavior helps you predict how Elastigroup launches and replaces instances during scaling.

How Elastigroup uses reserved instances

Elastigroup monitors the reserved instances in your account. When it needs to launch an instance, it checks whether a matching reservation is available based on the specific instance type and Availability Zone. If a reservation is available, Elastigroup uses it and applies the reserved instance billing method.

When the group scales up and launches an on‑demand instance, Elastigroup uses a reservation if one is available for that instance type in that zone.

What happens when Utilize reserved instances is enabled

When you enable this setting, Elastigroup continuously checks whether any group instance can be replaced with a reserved instance. If a reserved instance is available, Elastigroup begins replacing existing instances (including spot instances) to use the available reservations.

This behavior applies to the entire group and occurs automatically.

Launch priority

When Elastigroup needs to launch an instance, it follows this order:

1. Launch a reserved instance if a matching reservation is available.

2. If no reservation is available, launch a spot instance.

3. If a spot instance is unavailable, launch an on‑demand instance based on the fallback configuration.

This ensures your group uses reservations whenever possible while still maintaining capacity.