Terraform local-exec provisioner to check if web service is ready.

• The Terraform local-exec provisioner can be used to delay when Terraform marks a resource created until after the resource has passed a health check.
• In this example, the health check is a user defined shell script wait-for-endpoint.sh.

Test the example.

Clone this repo and change into the working directory.

git clone https://github.com/tmarfil/local-exec-provisioner.git
cd local-exec-provisioner

Terraform apply

terraform init
terraform plan
terraform apply

Terminal output

aws_instance.web: Creating...
aws_instance.web: Still creating... [10s elapsed]
aws_instance.web: Provisioning with 'local-exec'...
aws_instance.web (local-exec): Executing: ["/bin/sh" "-c" "./wait-for-endpoint.sh http://52.70.89.22 -t 300"]
aws_instance.web: Still creating... [20s elapsed]
aws_instance.web (local-exec): wait-for-endpoint.sh: waiting 300 seconds for http://52.70.89.22
aws_instance.web: Still creating... [30s elapsed]
aws_instance.web (local-exec): wait-for-endpoint.sh: http://52.70.89.22 is alive after 13 seconds
aws_instance.web: Creation complete after 33s [id=i-015ff25ee2df7ba9f]

• Terraform creates aws_instance "web" and computes the assigned public_ip attribute for aws_instance "web".
• local-exec provisioner is a part of the aws_instance "web" resource and runs the ./wait-for-endpoint.sh shell script against the public_ip attribute of aws_instance "web".
• wait-for-endpoint.sh either:
  • receives a successful http 200 response code, then Terraform marks the resource "created" or...
  • times out and Terraform fails

Below is the relevant snippet from main.tf.

resource "aws_instance" "web" {
  ami                    = data.aws_ami.ubuntu.id
  instance_type          = "t3.micro"
  vpc_security_group_ids = [aws_security_group.f5_management.id]
  tags = {
    Name = "marfil-test"
  }

  user_data = <<-EOF
    #!/bin/bash
    mkdir -p /var/www && cd /var/www
    echo "App v${var.release}" >> index.html
    python3 -m http.server 80
  EOF

  lifecycle {
    create_before_destroy = true
  }
  provisioner "local-exec" {
    command = "./wait-for-endpoint.sh http://${self.public_ip} -t 300"
# Simulate a failed health check by commenting out the line above and uncommenting the line below:
#   command = "./wait-for-endpoint.sh http://too.much.fail.biz -t 15"
  }
}

To simulate a failed health check, run the wait-for-endpoint.sh script against a bogus url with a short timeout.

command = "./wait-for-endpoint.sh http://too.much.fail.biz -t 15"

Citations

Based on an example from Terraform in Action.

Uses the wait-for-endpoint.sh shell script.