Request Spot Instances for fault-tolerant and flexible applications - Amazon EC2 Auto Scaling
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Request Spot Instances for fault-tolerant and flexible applications

In your launch template, you can optionally request Spot Instances with no end date or duration. Amazon EC2 Spot Instances are spare capacity available at steep discounts compared to the EC2 On-Demand price. Spot Instances are a cost-effective choice if you can be flexible about when your applications run and if your applications can be interrupted. For more information about creating a launch template that requests Spot Instances, see Configure advanced settings for your launch template.

Important

Spot Instances are typically used to supplement On-Demand Instances. For this scenario, you can specify the same settings that are used to launch Spot Instances as part of the settings of your Auto Scaling group. When you specify the settings as part of the Auto Scaling group, you can request to launch Spot Instances only after launching a certain number of On-Demand Instances and then continue to launch some combination of On-Demand Instances and Spot Instances as the group scales. For more information, see Auto Scaling groups with multiple instance types and purchase options.

This topic describes how to launch only Spot Instances in your Auto Scaling group by specifying settings in a launch template, rather than in the Auto Scaling group itself. The information in this topic also applies to Auto Scaling groups that request Spot Instances with a launch configuration. The difference is that a launch configuration requires a maximum price, but for launch templates, the maximum price is optional.

When you create a launch template to launch only Spot Instances, keep the following considerations in mind:

  • Spot price. You pay only the current Spot price for the Spot Instances that you launch. This pricing changes slowly over time based on long-term trends in supply and demand. For more information, see Spot Instances and Pricing and savings in the Amazon EC2 User Guide for Linux Instances.

  • Setting your maximum price. You can optionally include a maximum price per hour for Spot Instances in your launch template. If your maximum price exceeds the current Spot price, the Amazon EC2 Spot service fulfills your request immediately if capacity is available. If the price for Spot Instances rises above your maximum price for a running instance in your Auto Scaling group, it terminates your instance.

    Warning

    Your application might not run if you do not receive any Spot Instances, such as when your maximum price is too low. To take advantage of the Spot Instances available for as long as possible, set your maximum price close to the On-Demand price.

  • Balancing across Availability Zones. If you specify multiple Availability Zones, Amazon EC2 Auto Scaling distributes the Spot requests across the specified zones. If your maximum price is too low in one Availability Zone for any requests to be fulfilled, Amazon EC2 Auto Scaling checks whether requests were fulfilled in the other zones. If so, Amazon EC2 Auto Scaling cancels the requests that failed and redistributes them across the Availability Zones that have requests fulfilled. If the price in an Availability Zone with no fulfilled requests drops enough that future requests succeed, Amazon EC2 Auto Scaling rebalances across all of the Availability Zones.

  • Spot Instance termination. Spot Instances can be terminated at any time. The Amazon EC2 Spot service can terminate Spot Instances in your Auto Scaling group as the availability of, or price for, Spot Instances changes. When scaling or performing health checks, Amazon EC2 Auto Scaling can also terminate Spot Instances in the same way that it can terminate On-Demand Instances. When an instance is terminated, any storage is deleted.

  • Maintaining your desired capacity. When a Spot Instance is terminated, Amazon EC2 Auto Scaling attempts to launch another Spot Instance to maintain the desired capacity for the group. If the current Spot price is less than your maximum price, it launches a Spot Instance. If the request for a Spot Instance is unsuccessful, it keeps trying.

  • Changing your maximum price. To change your maximum price, create a new launch template or update an existing launch template with the new maximum price, and then associate it with your Auto Scaling group. The existing Spot Instances continue to run as long as the maximum price specified in the launch template used for those instances is higher than the current Spot price. If you did not set a maximum price, the default maximum price is the On-Demand price.