This lesson explores how Terraform manages resource attributes and dependencies for dynamic infrastructure provisioning.
In this lesson, we explore how Terraform manages resource attributes and dependencies. When you provision a resource, Terraform stores various details (attributes) related to that resource. These attributes can then be referenced throughout your configuration, enabling you to create dynamic and interconnected infrastructures.
Earlier, we created an AWS key pair resource that required a user-supplied public key. After its creation, Terraform exported several attributes, which you can inspect using the Terraform show command.Below is the configuration used to create the AWS key pair resource:
This output reveals exported attributes such as ARN, fingerprint, ID, key name, public key, and tags. For more detailed explanations of these attributes, refer to the Terraform Documentation for each resource.
Remember: Utilizing exported attributes allows you to build dependencies between resources, enabling dynamic infrastructure provisioning.
Exported resource attributes are often used as inputs for configuring other resources. Consider a scenario where you need to configure an EC2 instance using the AWS key pair resource. You can reference the key pair’s attributes in your EC2 instance configuration as follows:
In this configuration, the EC2 instance’s key_name parameter is set using the reference expression:resourceType.ResourceName.attributeHere, aws_key_pair.alpha.key_name refers to the key_name attribute of the key pair resource named “alpha”. By running terraform apply, both the key pair and the EC2 instance are provisioned in the correct order. Terraform automatically ensures that the key pair is created before the EC2 instance, thanks to the inherent resource dependency.The sample console output from running terraform apply is shown below:
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$ terraform apply...aws_key_pair.alpha: Creating...aws_key_pair.alpha: Creation complete after 1s [id=alpha]aws_instance.cerberus: Creating...aws_instance.cerberus: Still creating... [10s elapsed]aws_instance.cerberus: Creation complete after 10s [id=i-c791dc46a6639d4a7]Apply complete! Resources: 2 added, 0 changed, 0 destroyed
Terraform automatically manages the creation order by analyzing resource dependencies. During deletion, resources are removed in reverse order, ensuring a safe teardown.
Sometimes, two resources might not implicitly depend on each other—for instance, two EC2 instances that do not reference one another. In such cases, you can enforce a creation order using the depends_on meta-argument.Imagine you have two EC2 instances: one for your database server and another for your web server. To ensure that Terraform creates the database instance before the web instance, modify the configuration as follows:
With the depends_on argument, Terraform provisions the database instance first. When removing resources, it deletes the web instance before the database instance, preserving the dependency order.
Be cautious with explicit dependencies. Overusing depends_on can lead to unnecessarily complex dependency graphs, which might complicate the execution plan.
This lesson has reviewed how Terraform manages resource attributes and dependencies. By leveraging both implicit and explicit dependencies, you can efficiently control resource creation and deletion order, ensuring a robust infrastructure lifecycle management.Happy building with Terraform!
