Terraform: resource aws_elasticache_replication_group forces new resource when paramter number_cache_clusters changes

[martin-flaregames]

Not sure if this is really intended by design, but using the AWS console you can remove or add cluster members without the need of recreating the whole cluster + replication group.

Terraform Version

Terraform v0.7.1

Affected Resource(s)

• aws_elasticache_replication_group
  

  Expected Behavior

Just reduce or increase the number of cluster members to the specified one.

This may be a problem just because you need automatic_failover_enabled = true for this to work.

I guess there is also an edge case if you specify number_cache_clusters = 1, because it depends on how the multiple-az mode is setup and automatic_failover_enabled should be then false AFAIK.

Actual Behavior

The redis replication group and cluster were deleted and created anew.

Steps to Reproduce

1. create a replication group with, say, 4 cache cluster members

resource "aws_elasticache_replication_group" "redis_replication_group" {
  replication_group_id = "${var.replication_group_id}"
  replication_group_description = "${var.replication_group_id} redis replication group"
  node_type = "${var.node_type}"
  number_cache_clusters = 4
  port = "${var.port}"
  automatic_failover_enabled = "${data.template_file.automatic_failover_enabled.rendered}"
  engine_version = "${var.engine_version}"
  parameter_group_name = "${var.parameter_group_name}"
  subnet_group_name = "${aws_elasticache_subnet_group.subnet_group.name}"
  security_group_ids = [ "${module.securitygroup.id}" ]
  maintenance_window = "${var.maintenance_window}"
  apply_immediately = "${var.apply_immediately}"
}

1. terraform apply (will create the cluster)
2. modify the number_cache_clusters to another value, like 2.

resource "aws_elasticache_replication_group" "redis_replication_group" {
  replication_group_id = "${var.replication_group_id}"
  replication_group_description = "${var.replication_group_id} redis replication group"
  node_type = "${var.node_type}"
  number_cache_clusters = 2
  port = "${var.port}"
  automatic_failover_enabled = "${data.template_file.automatic_failover_enabled.rendered}"
  engine_version = "${var.engine_version}"
  parameter_group_name = "${var.parameter_group_name}"
  subnet_group_name = "${aws_elasticache_subnet_group.subnet_group.name}"
  security_group_ids = [ "${module.securitygroup.id}" ]
  maintenance_window = "${var.maintenance_window}"
  apply_immediately = "${var.apply_immediately}"
}

1. terraform plan will tell us that the cluster will be destroyed, which actually occurs when executing terraform apply

-/+ module.redis.aws_elasticache_replication_group.redis_replication_group
    apply_immediately:             "true" => "true"
    automatic_failover_enabled:    "true" => "true"
    engine:                        "redis" => "redis"
    engine_version:                "2.8.24" => "2.8.24"
    maintenance_window:            "tue:09:00-tue:10:30" => "tue:09:00-tue:10:30"
    node_type:                     "cache.m3.medium" => "cache.m3.medium"
    number_cache_clusters:         "4" => "2" (forces new resource)
    parameter_group_name:          "default.redis2.8" => "default.redis2.8"
    port:                          "6379" => "6379"
    replication_group_description: "devops-1 replication group" => "devops-1 redis replication group"
    replication_group_id:          "devops-1" => "devops-1"
    security_group_ids.#:          "1" => "1"
    security_group_ids.3159527089: "sg-2b74d751" => "sg-2b74d751"
    security_group_names.#:        "0" => "<computed>"
    snapshot_window:               "07:30-08:30" => "<computed>"
    subnet_group_name:             "zgi-us-vir-devops-1-redeem-sng" => "zgi-us-vir-devops-1-redeem-sng"

Important Factoids

Running in VPC mode.

References

[stack72]

Hi @martin-flaregames

this is what was added for the first set of functionality. I tried to make this explicit in the documentation:

https://www.terraform.io/docs/providers/aws/r/elasticache_replication_group.html#number_cache_clusters

I will continue to make this resource better going forward - we needed it working as a first pass while i evaluate all of the use cases

Paul

[martin-flaregames]

Woops, my fault. Thanks for pointing this out. I really read that documentation but failed to notice the remark. You may close this ticket if you wish.

Edit: And thank you very much for this module. It is a life and time saver!

[stack72]

@martin-flaregames nope - your ticket is a correct request for enhancement :) We should keep it open to make sure the work gets added

[vasiliyb]

Guys, this would be super-useful.

[maxblaze]

Any update on this?

[therapy-lf]

When it would be done? I really wait for this.

[hashibot[bot]]

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