Terraform Wait for Resource: A Comprehensive Guide

Estimated reading time: 8 minutes

Last updated on November 8th, 2024 at 02:38 pm

Terraform provides a powerful way to manage the infrastructure as a code with the ability to manage and orchestrate resource dependencies. In, resource orchestration waiting for resources to be ready before the next provisioning steps is a critical feature.

This guide explores how to effectively use “Terraform Wait for Resource” with practical examples.

Introduction to Terraform Wait for Resource Functionality

Understanding Resource Dependencies in Terraform

While provisioning the infrastructure in Terraform, the resource can depend on other resources for their configuration. Understanding dependencies is crucial for defining an infrastructure.

For example, In AWS, a security group can be dependent on a virtual machine.

First, you create the virtual machine resource, and then you configure the security group. This ensures that the security group settings are applied only after the virtual machine is ready.

Terraform handles these dependencies through the depends_on attribute, but what if you need to wait for a resource to be fully ready? ( eg: Virtual machine creation takes time )

Why Waiting for Resources is Crucial in Terraform

Simply answering “Why Waiting for Resources is Crucial in Terraform?” is to prevent errors related to resources:

1. Resource availability such as VM is ready & running.
2. Attempting to connect to a database before it is fully provisioned.

Waiting for resources ensures that all prerequisites are met and ready before moving forward with the deployment.

How to Implement Terraform Wait for Resource

1. Using the “depends_on” Attribute

The depends_on attribute explicitly defines dependencies between resources. It informs Terraform to create resources in a specific order. For example:

resource "aws_security_group" "example" {
  name        = "example"
  description = "Example security group"
  
  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  tags = {
    Name = "ExampleInstance"
  }

  depends_on = [aws_security_group.example]
}

In this example, aws_instance.example depends on aws_security_group.example, ensuring the instance is created only after the security group is provisioned.

aws_instance.example -> aws_security_group.example

2. Implementing “null_resource” with Local Exec

For granular control, you can use the null_resource with local-exec provisions.

This method allows you to run scripts or commands to check the readiness of a resource.

Here’s how you can implement it:

resource "null_resource" "wait_for_instance" {
  provisioner "local-exec" {
    command = "echo Waiting for instance to be ready..."
  }

  triggers = {
    instance_id = aws_instance.example.id
  }
}

This setup runs a local command to wait for the instance to be ready.

3. Leveraging Terraform Provider-Specific Features

Providers offer built-in functionality to handle resource readiness.

For example, AWS’s aws_instance the resource has attributes like instance_state that you can use to manage dependencies:

resource "aws_instance" "example" {
  ami           = "ami-0c55c158cbfafe1f0"
  instance_type = "t2.micro"

  provisioner "local-exec" {
    command = "aws ec2 wait instance-running --instance-ids ${self.id}"
  }

  depends_on = [aws_security_group.example]
}

Practical Examples of Terraform Wait for Resource

1. Waiting for an EC2 Instance to be Ready

resource "aws_security_group" "example" {
  name        = "example"
  description = "Example security group"
  
  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }
}

resource "aws_instance" "example" {
  ami           = "ami-055c158cbfafe1f0"
  instance_type = "t2.micro"

  tags = {
    Name = "ExampleInstance"
  }

  depends_on = [aws_security_group.example]

  provisioner "local-exec" {
    command = "aws ec2 wait instance-running --instance-ids ${self.id}"
  }
}

Use the local-exec provisioner to wait for an instance to be fully launched.

2. Ensuring Database Availability Before App Deployment

resource "aws_db_instance" "example" {
  allocated_storage    = 20
  storage_type         = "gp2"
  engine               = "mysql"
  engine_version       = "5.7"
  instance_class       = "db.t2.micro"
  db_name              = "mydatabase"
  username             = "admin"
  password             = "password"
  parameter_group_name = "default.mysql5.7"

  provisioner "local-exec" {
    command = "aws rds wait db-instance-available --db-instance-identifier ${self.id}"
  }

  depends_on = [aws_security_group.example]
}

This ensures that the database instance is ready before the application starts deploying.

3. Coordinating Multi-Resource Deployments

When dealing with multiple resources, you can chain dependencies to ensure the correct order of creation:

resource "aws_instance" "web" {
  ami           = "ami-055c158cbfafe1f0"
  instance_type = "t2.micro"
  
  depends_on = [aws_security_group.example]
}

resource "aws_instance" "app" {
  ami           = "ami-055c158cbfafe1f0"
  instance_type = "t2.micro"

  depends_on = [aws_instance.web]
}

resource "aws_db_instance" "db" {
  allocated_storage    = 20
  storage_type         = "gp2"
  engine               = "mysql"
  engine_version       = "5.7"
  instance_class       = "db.t2.micro"
  db_name              = "mydatabase"
  username             = "admin"
  password             = "password"

  depends_on = [aws_instance.app]
}

This example ensures that the web server is up before the application and the application is ready before the database.

Terraform Wait for Resource: Common Issues and Troubleshooting

Debugging “depends_on” Failures

Sometimes you might face an error. The most common causes are dependencies might not be sufficient, or resources might be ready but not in the expected state.

You can check Terraform logs and increase verbosity with TF_LOG=DEBUG to troubleshoot:

TF_LOG=DEBUG terraform apply

Handling Timeouts and Delays

When waiting for resources, consider implementing timeouts and retries. You never know when the resources are ready.

Use local-exec with loops to handle delays:

resource "null_resource" "wait_for_instance" {
  provisioner "local-exec" {
    command = <<EOT
    for i in $(seq 1 30); do
      if [ $(aws ec2 describe-instances --instance-ids ${aws_instance.example.id} --query "Reservations[].Instances[].State.Name" --output text) == "running" ]; then
        echo "Instance is running!"
        break
      else
        echo "Waiting for instance to start..."
        sleep 10
      fi
    done
    EOT
  }

  triggers = {
    instance_id = aws_instance.example.id
  }
}

This custom script checks the instance state every 10 seconds for up to 5 minutes.

Best Practices for Efficient Resource Waiting

Minimize Dependencies: Keep the number of dependencies to a minimum to avoid complex interdependencies.

Use count and for_each: For scalable resources, utilize count and for_each to simplify resource management.

Must Read 💡

• How to Enable Terraform Zsh Completion: Easy Guide
• Master Terraform Null Resource

Terraform Wait for Resource: Advanced Techniques

Custom Scripts for Terraform Wait for Resource Checks

Create custom scripts or use Terraform null_resource to check the state of resources before proceeding.

For instance, a script to check an S3 bucket’s availability:

resource "null_resource" "check_s3_bucket" {
  provisioner "local-exec" {
    command = "aws s3 ls s3://mybucket --region us-west-2"
  }

  triggers = {
    bucket_name = aws_s3_bucket.example.bucket
  }
}

Integrating with External Monitoring Tools

Integrate Terraform with tools like Prometheus or Grafana to monitor resource states and automate waiting logic based on real-time data:

resource "null_resource" "monitor_instance" {
  provisioner "local-exec" {
    command = "curl -s http://,y-monitoring-service/api/v1/instance-status?instance_id=${aws_instance.example.id}"
  }
  
  triggers = {
    instance_id = aws_instance.example.id
  }
}

Automating Complex Dependency Chains

Use Terraform’s for_each and count with conditional logic to automate complex dependency chains:

resource "aws_instance" "app_server" {
  count = var.instance_count
  
  ami           = "ami-055c158cbfafe1f0"
  instance_type = "t2.micro"

  lifecycle {
    create_before_destroy = true
  }

  depends_on = [aws_security_group.example]
}

This setup allows dynamic scaling and automated dependency management.

Terraform Wait for Resource: Conclusion

• Terraform’s depends_on and null_resource with local-exec are powerful tools for managing resource dependencies.
• Monitoring and custom scripts enhance resource readiness checks and automate deployment processes.
• Follow Best practices for “Terraform Wait for Resource” such as minimizing dependencies and leveraging external monitoring to ensure efficient and reliable infrastructure management.