This is an example of how we replaced NetApp's Spotinst. There are two main pieces to this solution:
This repository is not production-ready, please don't try to download it and terraform apply your way out of Spotinst. We intend this to be more of a guideline. We also do not intend to update this repository with any regularity.
The on-demand increaser Lambda Function handles incoming failed instance launch events by increasing the on-demand base capacity of the autoscaling group with launch failures.
This function can also be used to reduce the on-demand base for a set of autoscaling groups. As it's written, the function will reduce the number of on-demand instances by half. This is not configurable - if you want to only decrease the on-demand base by, say, 20%, you'd have to modify the code itself.
A word of warning: we build and deploy this function via CI. The lambda_od_increaser.tf configuration won't quite work out of the box.
This function handles draining ASG instances; ECS Capacity Providers will not do this for you. This Lambda function is a slightly modified version of the code in this article from AWS.