Setup overview

This topic provides an overview and best practices for creating a mixed instances group.

Overview

To create a mixed instances group, you have two options:

Manual selection

The following steps describe how to create a mixed instances group by manually choosing instance types:

1. Choose a launch template that has the parameters to launch an EC2 instance. Parameters in launch templates are optional, but Amazon EC2 Auto Scaling can't launch an instance if the Amazon Machine Image (AMI) ID is missing from the launch template.

2. Choose the option to override the launch template.

3. Manually choose the instance types that suit your workload.

4. Specify the percentages of On-Demand Instances and Spot Instances to launch.

5. Choose allocation strategies that determine how Amazon EC2 Auto Scaling fulfills your On-Demand and Spot capacities from the possible instance types.

6. Choose the Availability Zones and VPC subnets to launch your instances in.

7. Specify the initial size of the group (the desired capacity) and the minimum and maximum size of the group.

Overrides are necessary to override the instance type declared in the launch template and use multiple instances types that are embedded in the Auto Scaling group's own resource definition. For more information about the instance types that are available, see Instance types in the Amazon EC2 User Guide for Linux Instances.

You can also configure the following optional parameters for each instance type:

• LaunchTemplateSpecification – You can assign a different launch template to an instance type as needed. This option is currently not available from the console. For more information, see Use a different launch template for an instance type.

• WeightedCapacity – You decide how much the instance counts toward the desired capacity relative to the rest of the instances in your group. If you specify a WeightedCapacity value for one instance type, you must specify a WeightedCapacity value for all of them. By default, each instance counts as one toward your desired capacity. For more information, see Configure an Auto Scaling group to use instance weights.

Attribute-based selection

To let Amazon EC2 Auto Scaling choose your instance types automatically based on their specific instance attributes, use the following steps to create a mixed instances group by specifying your compute requirements:

1. Choose a launch template that has the parameters to launch an EC2 instance. Parameters in launch templates are optional, but Amazon EC2 Auto Scaling can't launch an instance if the Amazon Machine Image (AMI) ID is missing from the launch template.

2. Choose the option to override the launch template.

3. Specify instance attributes that match your compute requirements, such as vCPUs and memory requirements.

4. Specify the percentages of On-Demand Instances and Spot Instances to launch.

5. Choose allocation strategies that determine how Amazon EC2 Auto Scaling fulfills your On-Demand and Spot capacities from the possible instance types.

6. Choose the Availability Zones and VPC subnets to launch your instances in.

7. Specify the initial size of the group (the desired capacity) and the minimum and maximum size of the group.

Overrides are necessary to override the instance type declared in the launch template and use a set of instance attributes that describe your compute requirements. For supported attributes, see InstanceRequirements in the Amazon EC2 Auto Scaling API Reference. Alternatively, you can use a launch template that already has your instance attributes definition.

You can also configure the LaunchTemplateSpecification parameter within the overrides structure to assign a different launch template to a set of instance requirements as needed. This option is currently not available from the console. For more information, see LaunchTemplateOverrides in the Amazon EC2 Auto Scaling API Reference.

By default, you set the number of instances as the desired capacity of your Auto Scaling group.

Alternatively, you can set the value for desired capacity to the number of vCPUs or the amount of memory. To do so, use the DesiredCapacityType property in the CreateAutoScalingGroup API operation or the Desired capacity type dropdown field in the AWS Management Console. This is a useful alternative to instance weights.

Instance type flexibility

To enhance availability, deploy your application across multiple instance types. It's a best practice to use multiple instance types to satisfy capacity requirements. This way, Amazon EC2 Auto Scaling can launch another instance type if there is insufficient instance capacity in your chosen Availability Zones.

If there is insufficient instance capacity with Spot Instances, Amazon EC2 Auto Scaling keeps trying to launch from other Spot Instance pools. (The pools it uses are determined by your choice of instance types and allocation strategy.) Amazon EC2 Auto Scaling helps you leverage the cost savings of Spot Instances by launching them instead of On-Demand Instances.

We recommend being flexible across at least 10 instance types for each workload. When choosing your instance types, don't limit yourself to the most popular new instance types. Choosing earlier generation instance types tends to result in fewer Spot interruptions because they are less in demand from On-Demand customers.

Availability Zone flexibility

We strongly recommend that you span your Auto Scaling group across multiple Availability Zones. With multiple Availability Zones, you can design applications that automatically fail over between zones for greater resiliency.

As an added benefit, you can access a deeper Amazon EC2 capacity pool when compared to groups in a single Availability Zone. Because capacity fluctuates independently for each instance type in each Availability Zone, you can often get more compute capacity with flexibility for both the instance type and the Availability Zone.

For more information about using multiple Availability Zones, see Example: Distribute instances across Availability Zones.

Spot max price

When you create your Auto Scaling group using the AWS CLI or an SDK, you can specify the SpotMaxPrice parameter. The SpotMaxPrice parameter determines the maximum price that you're willing to pay for a Spot Instance hour.

When you specify the WeightedCapacity parameter in your overrides (or "DesiredCapacityType": "vcpu" or "DesiredCapacityType": "memory-mib" at the group level), the maximum price represents the maximum unit price, not the maximum price for a whole instance.

We strongly recommend that you do not specify a maximum price. Your application might not run if you do not receive any Spot Instances, such as when your maximum price is too low. If you don't specify a maximum price, the default maximum price is the On-Demand price. You pay only the Spot price for the Spot Instances that you launch. You still receive the steep discounts provided by Spot Instances. These discounts are possible because of the stable Spot pricing that's available with the Spot pricing model. For more information, see Pricing and savings in the Amazon EC2 User Guide for Linux Instances.

Proactive capacity rebalancing

If your use case allows, we recommend Capacity Rebalancing. Capacity Rebalancing helps you maintain workload availability by proactively augmenting your fleet with a new Spot Instance before a running Spot Instance receives the two-minute Spot Instance interruption notice.

When Capacity Rebalancing is enabled, Amazon EC2 Auto Scaling attempts to proactively replace Spot Instances that have received a rebalance recommendation. This provides an opportunity to rebalance your workload to new Spot Instances that are not at elevated risk of interruption.

For more information, see Use Capacity Rebalancing to handle Amazon EC2 Spot interruptions.

Scaling behavior

When you create a mixed instances group, it uses On-Demand Instances by default. To use Spot Instances, you must modify the percentage of the group to be launched as On-Demand Instances. You can specify any number from 0 to 100 for the On-Demand percentage.

Optionally, you can also designate a base number of On-Demand Instances to start with. If you do so, Amazon EC2 Auto Scaling waits to launch Spot Instances until after it launches the base capacity of On-Demand Instances when the group scales out. Anything beyond the base capacity uses the On-Demand percentage to determine how many On-Demand Instances and Spot Instances to launch.

Amazon EC2 Auto Scaling converts the percentage to the equivalent number of instances. If the result creates a fractional number, it rounds up to the next integer in favor of On-Demand Instances.

The following table demonstrates the behavior of the Auto Scaling group as it increases and decreases in size.

When the size of the group increases, Amazon EC2 Auto Scaling attempts to balance your capacity evenly across your specified Availability Zones. Then, it launches instance types according to the specified allocation strategy.

When the size of the group decreases, Amazon EC2 Auto Scaling first identifies which of the two types (Spot or On-Demand) should be terminated. Then, it tries to terminate instances in a balanced way across your specified Availability Zones. It also favors terminating instances in a way that aligns closer to your allocation strategies. For information about termination policies, see Configure termination policies for Amazon EC2 Auto Scaling.

Regional availability of instance types

The availability of EC2 instance types varies depending on your AWS Region. For example, the newest generation instance types might not yet be available in a given Region. Due to the variances in instance availability across Regions, you might encounter issues when making programmatic requests if multiple instance types in your overrides are not available in your Region. Using multiple instance types that are not available in your Region might cause the request to fail entirely. To solve the issue, retry the request with different instance types, making sure that each instance type is available in the Region. To search for instance types offered by location, use the describe-instance-type-offerings command. For more information, see Finding an Amazon EC2 instance type in the Amazon EC2 User Guide for Linux Instances.

Related resources

For more best practices for Spot Instances, see Best practices for EC2 Spot in the Amazon EC2 User Guide for Linux Instances.

Limitations

After you add overrides to an Auto Scaling group using a mixed instances policy, you can update the overrides with the UpdateAutoScalingGroup API call but not delete them. To completely remove the overrides, you must first switch the Auto Scaling group to use a launch template or launch configuration instead of a mixed instances policy. Then, you can add a mixed instances policy again without any overrides.