Packer is a tool that automates the creation of machine images. When you’re building images, especially if you’re doing it frequently or building large, complex images, the cost of the compute instances you use for building can add up. Spot Instances are a way to dramatically cut those costs, often by 70-90%, by leveraging Amazon’s spare EC2 capacity. The catch? These instances can be terminated by AWS with a two-minute warning.
Let’s see how this plays out in practice. Imagine you’re building a new AMI for your web application. This involves launching an EC2 instance, installing dependencies, configuring software, and then capturing the state of that instance as an AMI.
Here’s a snippet of a Packer amazon-ebs.json template that configures it to use Spot Instances:
{
"builders": [
{
"type": "amazon-ebs",
"region": "us-east-1",
"instance_type": "t3.medium",
"ami_name": "my-web-app-{{timestamp}}",
"spot_price": "0.04",
"tags": {
"Name": "Packer Builder",
"Environment": "Development"
}
}
],
"provisioners": [
{
"type": "shell",
"inline": [
"sudo apt-get update",
"sudo apt-get install -y nginx",
"sudo systemctl enable nginx"
]
}
]
}
The key here is the spot_price parameter. This tells Packer to request a Spot Instance with a maximum bid price of $0.04 per hour. If the current Spot price for a t3.medium in us-east-1 is below $0.04, your instance will launch. If it’s above, Packer will wait or retry based on its configuration.
Packer itself handles the complexities of requesting Spot Instances. When you run packer build your-template.json, Packer initiates the process. It first checks your spot_price against the current Spot market price for the specified instance_type and region. If your bid is higher than the current market price, AWS launches a Spot Instance for you. If your bid is lower, Packer will wait. You can configure how long Packer waits using the spot_instance_interruption_behavior and spot_instance_max_search_duration options, though the default behavior is usually sufficient for many use cases.
Once the instance is running, Packer proceeds with its provisioners. This is where you install software, copy files, run scripts, and configure the operating system to your desired state. For our web app example, we’re updating package lists and installing Nginx.
The real magic happens when the build is complete and Packer is about to capture the AMI. If AWS needs the capacity back, it will send a two-minute warning to your Spot Instance. Packer is designed to gracefully handle this. It will attempt to finalize the AMI capture and terminate the instance cleanly. If the interruption occurs before the AMI capture is complete, the build will fail, and you’ll need to rerun it. This is why it’s crucial to have idempotent provisioners – scripts that can be run multiple times without changing the result beyond the initial application.
The instance_type is critical. You’re not just picking any instance; you’re picking one that has spare capacity in the region you’re targeting. If a specific instance type is in high demand, its Spot price will rise, and you might have trouble getting capacity or your bid might be too low. This is why it’s often a good idea to have a fallback instance_type in mind or to configure Packer to try multiple types.
The ami_name is important for organization. Using a timestamp like {{timestamp}} ensures you get a unique AMI name for each build, preventing conflicts and making it easy to track versions.
The tags are not just for organizational purposes; they can also be used to identify your Packer builder instances for cost allocation or management.
The core problem this solves is the significant cost associated with transient compute resources used for building immutable infrastructure. Instead of paying on-demand prices for instances that are only active for a few hours at a time, you leverage AWS’s unused capacity at a fraction of the cost. This is particularly impactful for organizations with CI/CD pipelines that frequently build and update AMIs.
Here’s a more nuanced point: the spot_price isn’t a fixed price you pay forever. It’s your maximum bid. The actual price you pay is the current Spot market price. If the market price drops below your bid, you pay the lower price. If it rises above your bid, your instance is at risk of interruption. This dynamic pricing is what allows AWS to utilize its spare capacity so effectively and offer such deep discounts.
The next logical step after mastering Spot Instance builds is to explore how to make your Packer builds more resilient to interruptions, perhaps by using multi-instance strategies or by integrating with services that can automatically resume interrupted builds.