Define Auto Scaling Configuration Variables

Auto Scaling Configuration Variables

Auto scaling settings are critical for both performance and cost optimization. Well-defined variables with validation ensure scaling policies make sense before deployment.

Variable Definitions

variable "min_capacity" {
  type        = number
  description = "Minimum number of instances in the auto scaling group"
  default     = 1

  validation {
    condition     = var.min_capacity >= 0
    error_message = "Minimum capacity must be non-negative."
  }
}

variable "max_capacity" {
  type        = number
  description = "Maximum number of instances in the auto scaling group"
  default     = 4

  validation {
    condition     = var.max_capacity >= 1
    error_message = "Maximum capacity must be at least 1."
  }
}

variable "desired_capacity" {
  type        = number
  description = "Desired number of instances in the auto scaling group"
  default     = 2

  validation {
    condition     = var.desired_capacity >= 1
    error_message = "Desired capacity must be at least 1."
  }
}

Cross-Field Validation with Locals

Terraform variable validation cannot reference other variables. Use a locals block with a check block (Terraform 1.5+) for cross-field validation:

check "scaling_bounds" {
  assert {
    condition     = var.min_capacity <= var.desired_capacity
    error_message = "min_capacity must be <= desired_capacity"
  }

  assert {
    condition     = var.desired_capacity <= var.max_capacity
    error_message = "desired_capacity must be <= max_capacity"
  }
}

Environment-Specific Scaling

# dev.tfvars
min_capacity     = 1
max_capacity     = 2
desired_capacity = 1

# staging.tfvars
min_capacity     = 2
max_capacity     = 4
desired_capacity = 2

# prod.tfvars
min_capacity     = 3
max_capacity     = 20
desired_capacity = 5

Combining with Instance Type

Pairing scaling variables with the instance type variable lets you right-size for each environment: dev uses small instances with minimal scaling, while prod uses larger instances with wider scaling ranges.