Terraform: Applying Security Group changes to aws_instance should update, not destroy/recreate

[bloo]

Terraform Version

0.9.4

Affected Resource(s)

• aws_instance
• aws_security_group

Expected Behavior

Altering aws_security_group definitions should recreate those, then update the aws_instances that were attached to them.

It looks like you can update instances via the AWS API:

http://docs.aws.amazon.com/cli/latest/reference/ec2/modify-instance-attribute.html

Actual Behavior

AWS instances are destroyed and recreated, instead of updated in place.

[grubernaut]

Hey @bloo, thanks for the issue!

Do you have an example configuration that can reproduce the issue? Thanks!

[bloo]

Thanks @grubernaut - below is a simple terraform that contains 2 aws_security_groups and 1 aws_instance. The instance references only 1 of the security_groups to start.

1. terraform apply - creates all 3 resources
2. uncomment line 13: #"access-https"
3. terraform apply - should update aws_instance in place, but instead destroys and recreates it.

provider "aws" {
  access_key = "???"
  secret_key = "???"
  region     = "us-east-1"
}

resource "aws_instance" "dummy" {
  ami               = "ami-6d1c2007"
  instance_type     = "t2.micro"
  availability_zone = "us-east-1a"
  security_groups = [
    "access-http",
    #"access-https"
  ]
}

resource "aws_security_group" "access-https" {
  name = "access-https"
  ingress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

resource "aws_security_group" "access-http" {
  name = "access-http"
  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

After uncommenting line 13 and running terraform plan, this is the output:

$ terraform plan
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.

aws_security_group.access-https: Refreshing state... (ID: sg-474ad539)
aws_security_group.access-http: Refreshing state... (ID: sg-2a4fd054)
aws_instance.dummy: Refreshing state... (ID: i-0debc5cd1bc48deba)
The Terraform execution plan has been generated and is shown below.
Resources are shown in alphabetical order for quick scanning. Green resources
will be created (or destroyed and then created if an existing resource
exists), yellow resources are being changed in-place, and red resources
will be destroyed. Cyan entries are data sources to be read.

Note: You didn't specify an "-out" parameter to save this plan, so when
"apply" is called, Terraform can't guarantee this is what will execute.

-/+ aws_instance.dummy
    ami:                         "ami-6d1c2007" => "ami-6d1c2007"
    associate_public_ip_address: "true" => "<computed>"
    availability_zone:           "us-east-1a" => "us-east-1a"
    ebs_block_device.#:          "0" => "<computed>"
    ephemeral_block_device.#:    "0" => "<computed>"
    instance_state:              "running" => "<computed>"
    instance_type:               "t2.micro" => "t2.micro"
    ipv6_addresses.#:            "0" => "<computed>"
    key_name:                    "" => "<computed>"
    network_interface_id:        "eni-0113fac8" => "<computed>"
    placement_group:             "" => "<computed>"
    private_dns:                 "ip-172-31-75-42.ec2.internal" => "<computed>"
    private_ip:                  "172.31.75.42" => "<computed>"
    public_dns:                  "ec2-54-208-206-103.compute-1.amazonaws.com" => "<computed>"
    public_ip:                   "54.208.206.103" => "<computed>"
    root_block_device.#:         "1" => "<computed>"
    security_groups.#:           "1" => "2" (forces new resource)
    security_groups.2441721151:  "" => "access-https" (forces new resource)
    security_groups.3803437083:  "access-http" => "access-http"
    source_dest_check:           "true" => "true"
    subnet_id:                   "subnet-2f25cf03" => "<computed>"
    tenancy:                     "default" => "<computed>"
    vpc_security_group_ids.#:    "0" => "<computed>"


Plan: 1 to add, 0 to change, 1 to destroy.

[bloo]

Hi @grubernaut - just wondering if there's anything more I can contribute to this?

[grubernaut]

Hey @bloo, extremely sorry, but I haven't had a chance to look at this issue yet. I'll attempt to dive in tomorrow!

[rfletcher]

@bloo Your example defines an EC2 Classic instance. In EC2 Classic, an instance's security groups can only be set at launch time. They can't be modified. That is possible for VPC instances though, and Terraform behaves as you expect there.

The EC2 Classic limitation is indicated (though subtly) by the AWS documentation you've linked in the original report (emphasis mine):

--groups (list)

[EC2-VPC] Changes the security groups of the instance. You must specify at least one security
group, even if it's just the default security group for the VPC. You must specify the security group ID, not the security group name.

I don't think there's a Terraform bug here.

[bloo]

@rfletcher to clarify, I should use the 2nd parameter, not the first one - and it'll allocate a non-classic EC2 instance?

security_groups - The associated security groups.
vpc_security_group_ids - The associated security groups in non-default VPC

[rfletcher]

That's how you'll set any security groups once it is in VPC, but to launch it in VPC you need to specify the subnet. See the subnet_id argument.

[bloo]

@rfletcher thanks for the clarification!

[grubernaut]

Thanks for the response here @rfletcher! :smile:

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