Cannot use memoryReservation in Terraform ECS provider - amazon-web-services

Question

I am running tests where it is not desirable for my containers to have hard memory limits as I programatically swapping the vms for bigger sized ones and need the containers to be able to leverage the increment in CPU and memory automatically.
I want to explore memoryReservation as it is softlimit and will allows the containers scale up, if the memory of the VM is not low.
Unfortunately, this parameter does not seem to work in the task definition. Any ideas?
Task definition:
resource "aws_ecs_task_definition" "quorum" {
family = "quorum-${var.consensus_mechanism}-${var.tx_privacy_engine}-${var.network_name}"
container_definitions = "${replace(element(compact(local.container_definitions), 0), "/\"(true|false|[0-9]+)\"/", "$1")}"
requires_compatibilities = ["${var.ecs_mode}"]
# cpu = "4096"
# memory = "81920"
memoryReservation = "8192"
network_mode = "${var.ecs_network_mode}"
task_role_arn = "${aws_iam_role.ecs_task.arn}"
execution_role_arn = "${aws_iam_role.ecs_task.arn}"
volume {
name = "${local.shared_volume_name}"
}
volume {
name = "docker_socket"
host_path = "/var/run/docker.sock"
}
}
Error:
[FINAL] Summary execution:
Wrote summarry output to: .mjolnir//output.log
2 errors occurred:
* aws_ecs_service.quorum: 5 errors occurred:
* aws_ecs_service.quorum[3]: Resource 'aws_ecs_task_definition.quorum' not found for variable 'aws_ecs_task_definition.quorum.revision'
* aws_ecs_service.quorum[0]: Resource 'aws_ecs_task_definition.quorum' not found for variable 'aws_ecs_task_definition.quorum.revision'
* aws_ecs_service.quorum[4]: Resource 'aws_ecs_task_definition.quorum' not found for variable 'aws_ecs_task_definition.quorum.revision'
* aws_ecs_service.quorum[2]: Resource 'aws_ecs_task_definition.quorum' not found for variable 'aws_ecs_task_definition.quorum.revision'
* aws_ecs_service.quorum[1]: Resource 'aws_ecs_task_definition.quorum' not found for variable 'aws_ecs_task_definition.quorum.revision'
* output._status: Resource 'aws_ecs_task_definition.quorum' not found for variable 'aws_ecs_task_definition.quorum.revision'
Restoring env variables.
Error occured: 4
I will be deeply appreciative of pointers

Answer 1

ECS task definitions are made up of a multiple of container definitions with some extra parameters that can set hard limits for the whole task and also set things like placement constraints and networking configuration.
To set the memory soft limit that a task is allowed to use in ECS rather than the hard limit you need to use memoryReservation from the container definition rather than the task definition.
The code in your question doesn't show how you are defining the container definitions in your local but a basic example of setting soft memory limits in an ECS task would look something like this:
resource "aws_ecs_task_definition" "service" {
family = "service"
container_definitions = <<EOF
[
{
"name": "first",
"image": "service-first",
"cpu": 10,
"memoryReservation": 512,
"essential": true,
"portMappings": [
{
"containerPort": 80,
"hostPort": 80
}
]
},
{
"name": "second",
"image": "service-second",
"cpu": 10,
"memoryReservation": 256,
"essential": true,
"portMappings": [
{
"containerPort": 443,
"hostPort": 443
}
]
}
]
EOF
}