Autoscaling with min 0 read replicas not possible

[thomasfinstad]

Description

Due to the instance count being taken directly from var.replica_scale_min the writer node also gets deleted when minimum read replicas is set to zero

Versions

• Terraform: 0.14.7

• Provider(s):
  • provider registry.terraform.io/datadog/datadog v2.26.1
  • provider registry.terraform.io/hashicorp/archive v2.2.0
  • provider registry.terraform.io/hashicorp/aws v3.56.0
  • provider registry.terraform.io/hashicorp/external v2.1.0
  • provider registry.terraform.io/hashicorp/local v2.1.0
  • provider registry.terraform.io/hashicorp/null v3.1.0
  • provider registry.terraform.io/hashicorp/random v3.1.0

• Module: 5.2.0

Reproduction

Steps to reproduce the behavior:
setup a cluster with var.replica_scale_enabled turned on, with var.replica_scale_min set to one you will always have a replica even when not needed (a scale event might need to be triggered once for the autoscale policy to take effect). Setting var.replica_scale_min to zero will remove the writer node as well as the read replica

Code Snippet to Reproduce

Expected behavior

When var.create_cluster is set to true the instance count should use the var.replica_scale_min as it does now, unless that is zero/less than 1, at which point the value should be set to 1 as a fallback

Actual behavior

Directly drawing from var.replica_scale_min, forcing a minimum of 1 read replica, which means 2 nodes is the minimum if auto scaling is desired

Additional context

This would be a nice money saving feature for the module.

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