Caveats When Building ElastiCache Redis 6.x with Terraform

ToC

• Overview
• Problem when specifying redis engine_version = 6.x

Overview

There was a certain ritual to go through when using the ElastiCache Redis 6.x line with Terraform, so here is a memo for future reference.

Problem when specifying redis engine_version = 6.x

To build ElastiCache > Redis with the 6.x line, you need to specify engine_version = "6.x" as shown below.

resource "aws_elasticache_replication_group" "cache_store" {
    engine         = "redis"
    engine_version = "6.x"
  ...

Then, after building it, running terraform plan produces the following change.

~ resource "aws_elasticache_replication_group" "cache_store" {
    engine         = "redis"
  ~ engine_version = "6.0.5" -> "6.x"

An issue had been filed about this behavior.

https://github.com/hashicorp/terraform-provider-aws/issues/15625

As a workaround, I want to avoid adding ignore_changes = [engine_version] as much as possible.
https://github.com/hashicorp/terraform-provider-aws/issues/15625#issuecomment-727759811

For now, I handle it by specifying engine_version = "6.0.5" after creation, as shown below.

resource "aws_elasticache_replication_group" "cache_store" {
    engine         = "redis"
    engine_version = "6.0.5"

You might think, “Why not just specify engine_version = "6.0.5" at build time!” but that results in an error.

So leaving a comment at build time is kind to reviewers.

resource "aws_elasticache_replication_group" "cache_store" {
    engine         = "redis"
    # TODO: After building, change this to the version that was actually adopted.
    #       This is because a diff in engine_version appears in terraform plan after the build.
    # see: https://github.com/hashicorp/terraform-provider-aws/issues/15625
    engine_version = "6.x"

That’s all.
I hope this is helpful.