feat: allow mixture of on-demand and spot instances (#140)

# Description

Added the possibility of choosing the preferred balance to mix on-demand
and Spot instances on the `on_spot` aws_autoscaling_group resource. As
spot instances might be stopped without further notice, it might be
useful to deploy at least one on-demand instance.

Use the new variable `instances_distribution` to customize the minimum
number of on_demand instances and the percentage of on_demand instances
(excluding the on_demand base capacity).

```hcl
module "bastion" {
  ...
  instances_distribution = {
    on_demand_base_capacity                  = 1
    on_demand_percentage_above_base_capacity = 20
    spot_allocation_strategy                 = "lowest-price"
  }
}
```

# Verification

- [x] simple example applied, still running
- [x] mixture of 3:1 (spot:on-demand) instances (4 in total). Both
instances are created and I am able to connect.

# Checklist

- [x] My code follows the style guidelines of the project
- [x] I have performed a self-review of my own code
- [x] I have made corresponding changes to the documentation

---------

Co-authored-by: kayma <kayma@hlag.com>
Co-authored-by: Matthias Kay <github-public@matthiaskay.de>

autoscaling.tf

@@ -50,10 +50,13 @@ resource "aws_autoscaling_group" "on_spot" {
 
   termination_policies = ["OldestInstance"]
 
+  capacity_rebalance = true # https://docs.aws.amazon.com/autoscaling/ec2/userguide/ec2-auto-scaling-capacity-rebalancing.html
+
   mixed_instances_policy {
     instances_distribution {
-      on_demand_percentage_above_base_capacity = 0
-      on_demand_base_capacity                  = 0
+      on_demand_percentage_above_base_capacity = var.instances_distribution.on_demand_percentage_above_base_capacity
+      on_demand_base_capacity                  = var.instances_distribution.on_demand_base_capacity
+      spot_allocation_strategy                 = var.instances_distribution.spot_allocation_strategy
     }
 
     launch_template {

examples/spot/main.tf

@@ -14,6 +14,12 @@ module "bastion_host" {
     enable_spot = true
   }
 
+  instances_distribution = {
+    on_demand_base_capacity                  = 1
+    on_demand_percentage_above_base_capacity = 0
+    spot_allocation_strategy                 = "lowest-price"
+  }
+
   vpc_id     = module.vpc.vpc_id
   subnet_ids = module.vpc.private_subnets
 

variables.tf

@@ -90,6 +90,22 @@ variable "instance" {
   }
 }
 
+variable "instances_distribution" {
+  type = object({
+    on_demand_base_capacity                  = number # absolute minimum amount of on_demand instances
+    on_demand_percentage_above_base_capacity = number # percentage split between on-demand and Spot instances
+    spot_allocation_strategy                 = string
+  })
+
+  description = "Defines the parameters for mixed instances policy auto scaling"
+
+  default = {
+    on_demand_base_capacity                  = 0
+    on_demand_percentage_above_base_capacity = 0
+    spot_allocation_strategy                 = "lowest-price"
+  }
+}
+
 variable "tags" {
   type        = map(string)
   description = "A list of tags to add to all resources."
@@ -100,4 +116,4 @@ variable "tags" {
 variable "ami_name_filter" {
   description = "The search filter string for the bastion AMI."
   default     = "amzn2-ami-hvm-*-x86_64-ebs"
-}
+}