Work with Terraform

Lifecycle Rules

By default, Terraform first destroy the resources first and then will create a new resource.

Example

You have local file creating configuration file your directory and the resource was provided already. Now, you want to make a changes in the content of the file like below

resource "local_file" "my-file" {
    filename="./test.txt"
    content="Satheesh Pandian" // earlier it is "Satheesh"
}

When you run terraform apply command. it destroys the existing file first, and then it will create new one.

Create first and Destroy then

If you want to change this behaviour, you can use lifecycle rules.

resource "local_file" "my-file" {
    filename="./test.txt"
    content="My SRE Journey" // earlier it is "Satheesh Pandian Jeganthan"

    lifecycle {
        create_before_destroy = true
    }
}

Do not destroy

If you DO NOT want to destroy any resource, then you need to use the below lifecycle rules. This will be used when you deal with database. Most of the time, we DO NOT want to delete database while changing something. So, you need to update lifecycle rule with prevent_destroy = true.

resource "local_file" "my-file" {
    filename="./test.txt"
    content="My SRE Journey" // earlier it is "Satheesh Pandian Jeganthan"

    lifecycle {
        prevent_destroy = true
    }
}

terraform destroy command still destroys the resource even if you mentioned lifecycle rule with prevent_destroy = true in the configuration file.

Ignore changes

If you DO NOT want to create any resource in case of any particular attribute change. In the below example, I am making changes in content

resource "local_file" "my-file" {
    filename="./test.txt"
    content="Satheesh Pandian Jeganthan" // earlier it is "I am SRE in Bangalore"

    lifecycle {
        ignore_changes = [content] // you can any number of attribute in the list
    }
}

ignore_changes = all means that it won't create any resource even if there is any change in any attribute.

QUICK SUMMARY

It is better NOT to use create_before_destroy = true lifecycle rule for local file because the rule will create the local file first and the same file to be destroyed during the recreate operation.

Data Sources

If you want to read the data from any file which are in your current configuration directory, but not created/maintained by terraform. In that case, you can use that file as a data resource.

>cat data.txt
I am Satheesh from Bangalore and I am working as Sr.SRE in a bank.

The above file is created manually without using terraform commands. Also, terraform does not have any information about this file so far.

resource "local_file" "my-file" {
    filename = "./test.txt"
    content = data.local_file.my-data.content
    lifecycle {
        create_before_destroy = true
    }
}

data "local_file" "my-data" {
    filename="./data.txt"
}

Remember, you CAN NOT do create/update/destroy for data sources. You can ONLY do read the data source

Meta Argument - Count

resource "local_file" "my-file" {
    filename = "./test.txt"
    content = "Satheesh"
    count = 3
}

However, the problem is here that there is ONLY one file created instead of 3. This is because the filename is not unique. Hence, terraform is recreated the same file again and again.

Hence, we need to create three different file. Here, I used variable configuration file for creating three files and modified configuration file to read the filename from variable configuration.

variable filename {
    default = ["./test.txt","./sample.txt","./exam.txt"]
}

resource "local_file" "my-file" {
    filename = var.filename
    content = "Satheesh"
    count = 3

In the above example, even if you have 100 arguments in variable configuration file, this will ALWAYS create 3 files as we hardcoded count = 3. To create the number of files defined in variable configuration file, we need to update the main configuration file to pick the length of the list variable.

resource "local_file" "my-file" {
    filename = var.filename
    content = "Satheesh"
    count = length(var.filename)