Terraform: Can't change subnet on ELB

[FransUrbo]

I'm converting a ELB to be an external (internal = false) one. That was done in a previous run, but I also realised I need to put it on the/a public network.

So changing the subnet doesn't seem to work.

Terraform Version

0.9.1

Affected Resource(s)

• aws_elb

Terraform Configuration Files

From

  subnets                       = [ # One per AZ
    "${aws_subnet.vpc_subnet_0.id}"
    "${aws_subnet.vpc_subnet_1.id}"
    "${aws_subnet.vpc_subnet_2.id}"
  ]

to

  subnets                       = [
    "${aws_subnet.vpc_subnet_public.id}"
  ]

Gives me:

module.mymodule.aws_elb.myelb: Modifying... (ID: myelb)
  subnets.#:          "3" => "1"
  subnets.12345678902: "subnet-abcdefghi" => ""
  subnets.23456789012: "subnet-bcdefghij" => ""
  subnets.34567890123:  "subnet-cdefghijk" => ""
  subnets.45678901234:  "" => "subnet-defghijkl"
Error applying plan:

1 error(s) occurred:

* module.mymodule.aws_elb.myelb: 1 error(s) occurred:

* aws_elb.myelb: Failure removing ELB subnets: InvalidConfigurationRequest: Requested configuration change for LoadBalancer "myelb" is invalid because you attempted to detach all the subnets for this LoadBalancer and a LoadBalancer cannot be attached to zero subnets in VPC.
        status code: 409, request id: adeeeb86-2c0e-11e7-93ae-92361f002671

Expected Behavior

ELB subnet should be created without failing. ALTERNATIVELY (?) destroyed and created again with the new settings.

Actual Behavior

TF error out.

Steps to Reproduce

Please list the steps required to reproduce the issue, for example:

1. Setup ELB with subnet (more than one?)
2. Apply (create) ELB
3. Change subnet (remove all, add another?)
4. Apply (modify) ELB

[catsby]

Hey @FransUrbo – Terraform is trying to remove all subnets and then add new ones. The closest test we have for this doesn't remove all of them, so I don't suppose we've hit this error before :( We'll add a regression test for that.

A workaround would be to do this in 2 steps:

• add new subnet, apply
• remove old subnets, apply

Or any combination of steps that doesn't result in the ELB having zero subnets.

Unfortunately it's not as easy as swapping the code to add subnets new first and then remove the old ones; ELB won't allow users to add more than one subnet in an AZ. I imagine we can work this out with a call to DescribeSubnets and logic to determine if we'd be violating either rule and deciding how to proceed then, but that would be a bit of work and I can't promise it to come this week or next :/

[FransUrbo]

I'm still in a build phase, so it was just simpler to delete the ELB in the Console and then re-run TF.

But the workaround looks decent enough. Update the docs until a correct solution can be found?

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