HCL Language - 41 - Resource Dependencies and Ordering

Understanding Resource Dependencies and Ordering in HCL

Resource dependencies and ordering are crucial aspects of managing infrastructure as code (IaC) using the HashiCorp Configuration Language (HCL). These features allow you to define and control the relationships between resources in your infrastructure, ensuring that they are created, updated, and destroyed in the desired sequence. In this discussion, we’ll explore the concept of resource dependencies and ordering, their significance, and how to use them effectively in HCL.

The Significance of Resource Dependencies

Resource dependencies are vital for several reasons:

Sequential Execution: They ensure that resources are created, updated, or destroyed in the desired order, preventing potential issues and conflicts.
Inter-Resource Communication: Dependencies enable one resource to communicate with and reference another resource, allowing for configuration and integration.
Resource Lifecycle Management: Dependencies help you manage the lifecycle of resources, ensuring that they are properly initialized and terminated when needed.
State Management: They facilitate the management of resource states and their interdependencies, making it easier to maintain your infrastructure code.

Defining Resource Dependencies in HCL

In HCL, you can define resource dependencies explicitly using the “depends_on” argument within a resource block. This argument specifies the resources that a given resource depends on. Here’s the basic syntax:

Resource Dependency Syntax


resource "resource_type" "resource_name" {
  # Resource attributes

  depends_on = [resource_type.resource_name, ...]
}

In this syntax, “resource_type” represents the type of resource, “resource_name” is a name for the resource block, and “depends_on” is an array of resource dependencies. You can specify one or more dependencies to ensure the correct order of resource execution.

Resource Dependency Example

Let’s take a practical example of using resource dependencies in HCL to ensure that an AWS EC2 instance is created only after an associated security group is established. In this scenario, we specify the dependency using the “depends_on” argument.

Example of Resource Dependencies


resource "aws_security_group" "example" {
  name_prefix = "example-"
  # Security group attributes
}

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

  depends_on = [aws_security_group.example]
}

In this example, we ensure that the “aws_instance” resource depends on the “aws_security_group” resource, guaranteeing that the security group is created before the EC2 instance.

Resource Ordering in HCL

Resource ordering is the mechanism by which Terraform defines the sequence in which resources are created, updated, or destroyed. Terraform automatically determines this sequence based on resource dependencies and references. Resources with dependencies are created after the resources they depend on. However, you can override the default order using the “count” argument.

Overriding Resource Ordering

You can override resource ordering by using the “count” argument and conditionally creating or not creating a resource. This allows you to control when a resource is created or updated based on specific conditions.

Overriding Resource Ordering Example


resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  count         = var.create_instance ? 1 : 0
}

In this example, the “aws_instance” resource will be created only if the “create_instance” variable is set to true. This conditional resource creation allows you to control the resource ordering based on dynamic conditions.

Benefits of Resource Dependencies and Ordering

Resource dependencies and ordering offer several advantages for infrastructure management:

Sequential Execution: They ensure resources are created, updated, or destroyed in the correct order, preventing conflicts and issues.
Inter-Resource Communication: Dependencies enable resources to reference each other, facilitating configuration and integration.
State Management: They help manage the state of resources and their interdependencies, simplifying infrastructure maintenance.
Control and Flexibility: Overriding resource ordering provides control and flexibility in determining when resources are created based on specific conditions.

Conclusion

Resource dependencies and ordering are essential elements in HCL for managing infrastructure in a controlled and organized manner. By defining dependencies and using the “depends_on” argument, you can ensure that resources are created in the desired sequence. Additionally, overriding resource ordering allows you to conditionally create or update resources, providing flexibility in your infrastructure as code projects.

HCL Language – 41 – Resource Dependencies and Ordering