Cloudwatch alarms for Redis Nodes


I want to provision cloud watch alarms for Redis. I have the following code.

resource "aws_elasticache_replication_group" "this" {
  replication_group_id = var.replication_group_id
  description          = var.description

  port                 = var.port
  node_type            = var.node_type
  parameter_group_name = var.parameter_group_name

  security_group_ids = var.security_group_ids
  subnet_group_name  = var.subnet_group_name

  at_rest_encryption_enabled = true
  automatic_failover_enabled = true
  multi_az_enabled           = true

  num_node_groups         = var.num_node_groups
  replicas_per_node_group = var.replicas_per_node_group

resource "aws_cloudwatch_metric_alarm" "cache_memory" {

    for_each = toset(tolist(flatten(aws_elasticache_replication_group.this.member_clusters)))

  alarm_name          = "${each.key}-freeable-memory"
  alarm_description   = "Elasticache ${each.key} average freeable memory is less than ${var.alarm_memory_threshold_bytes} bytes"
  comparison_operator = "LessThanThreshold"
  evaluation_periods  = 1
  metric_name         = "FreeableMemory"
  namespace           = "AWS/ElastiCache"
  period              = 600
  statistic           = "Average"
  threshold           = var.alarm_memory_threshold_bytes
  alarm_actions       = var.alarm_actions

  dimensions = {
    CacheClusterId = each.key

I am getting the following error. I tried adding depends on but it does not fix it,.

│ Error: Invalid for_each argument
│   on modules/redis/ line 3, in resource "aws_cloudwatch_metric_alarm" "cache_memory":
│    3:   for_each = toset(tolist(flatten(aws_elasticache_replication_group.this.member_clusters)))
│     ├────────────────
│     │ aws_elasticache_replication_group.this.member_clusters is a set of string, known only after apply
│ The "for_each" set includes values derived from resource attributes that
│ cannot be determined until apply, and so Terraform cannot determine the
│ full set of keys that will identify the instances of this resource.
│ When working with unknown values in for_each, it's better to use a map
│ value where the keys are defined statically in your configuration and where
│ only the values contain apply-time results.
│ Alternatively, you could use the -target planning option to first apply
│ only the resources that the for_each value depends on, and then apply a
│ second time to fully converge.


Hi @rohit.jha,

The API and AWS provider don’t seem to offer anything suitable for use in for_each to identify these objects, since the only data available is something that will be decided during the apply step.

Therefore in this particular case you will need to do what the error message describes in its last paragraph:

  • terraform apply -target=module.redis.aws_elasticache_replication_group.this first, to get the replication group created.
  • Then terraform apply without extra arguments, to finish creating everything else including the cloudwatch alarms.

Once the replication group exists you won’t need to use -target again unless the set of clusters associated with this replication group changes, because the set of clusters will be known.