In terraform how to skip creation of resource if the resource already exist?

In a Terraform project there are situations where a Developer feels a need to pre-check if the resource exists or not and many times I also wondered if is it possible to check the existence of the resource to avoid the duplicity of the resource. In this blog, we are gonna see -

1. Why it is not possible to decide dynamically if the resource exists or not?
2. What are alternate ways we have in Terraform to check the existence of resources?
3. Conclusion

1. Why it is not possible to decide dynamically if the resource exists or not?

Why it is not possible? -

"Terraform does not support this kind of pre-check on resources because there is no direct way to decide dynamically in Terraform. Let's assume Terraform does provide support to check the existence of resources dynamically then it would result in the no-deterministic state when you issue the terraform destroy command."

Example - Let's take a hypothetical example to understand the non-deterministic situation of Terraform. In this example we will specially talk about project_B and terraform destroy command-

 1# main.tf
 2
 3# project_A - Does not have any pre-check condition 
 4resource "google_project_service" "project_A" {
 5...
 6...
 7}
 8
 9# project_B - Let's assume there is the pre-exist-check condition
10resource "google_project_service" "project_B" {
11...
12...
13}

Flow of Terraform Commands for above terraform example -

1. terraform apply - Terraform apply command would work without a problem. It will first create project_A and then based on the condition it will decide to create project_B.
2. terraform destroy - Things will get more interesting over here because once you issue terraform destroy it will delete all terraform resources but when terraform destroy command try to evaluate the condition over the project_B it can not decide whether to delete the resource or retain the resource which leads to the non-deterministic situation

2. What alternate ways do we have in Terraform to check the existence of resources?

The other alternate approach would be to use external Data Source. You can read more in-depth in this article but to elaborate a bit Terraform External Data Source can help you to fetch the information about the pre-existing resource.

Let's take a very basic example to understand how to check if the resource exists or not.

Scenario - I have and EC2 Instance which I have manually setup on AWS. Here are the details of the EC2 Instance -

1. AMI - ami-0767046d1677be5a0
2. Instance Type - t2.micro
3. Tag - Terraform EC2

How to Use Terraform Data Source? - Now we know the scenario let's write the Data source to fetch the information about the EC2 resource.

 1
 2data "aws_instance" "myawsinstance" {
 3    filter {
 4        name = "tag:Name"
 5        values = ["Terraform EC2"]
 6    }
 7
 8    depends_on = [
 9      "aws_instance.ec2_example"
10    ]
11}
12
13# the following is_ec2_instance_exist local should return value 1 if resource exists
14locals {
15  is_ec2_instance_exist = "${data.aws_instance.myawsinstance.public_ip)}"
16}
17
18# Here is the output block printing the existence of the resource onto the console
19output "fetched_info_from_aws" {
20  value = data.aws_instance.myawsinstance.public_ip
21}

Here is the key takeaway -

1. The above approach can help you to make a dynamic decision but it is not a silver bullet to all of your problems to determine the existence of the resource.
2. The above might return the in-consistent result or it might fail when the resource does not exist.
3. Never take an approach that is against the working principle of Terraform workflow because Terraform has a very linear workflow where it simply relies on the following facts -

• If the resource is present in main.tf create a resource
• If the resource is not present in the main.tf then remove the resource if it is previously created
• terraform destory will always remove all the resources.
• Terraform always has a fix .tfstate, it can never have non-deterministic state.

3. Conclusion

I hope this article will help you to solve your problem of finding existing resources using Terraform and also to understand why it is not a good practice to find the resource dynamically.