Resources

Resource bocks represent one more infrastructure object.

One of the advantages of terraform is that it offers resource provisioning that is agnostic of the provider platform. This allows you to avoid learning the underlying provider API.

You can configure a resource by using a resource block followed by the provider platform and type of resource and then a name for the resource being provisioned that can be referred in the rest of the terraform file.

resource "<PROVIDER>_<RESOURCE_TYPE>" "<RESOURCE_NAME>" {
    # resource specific configuration
}

Some resources in terraform are purely associative. For example, you may need to link two separate resources after they are created, for this you would use an associative resource, such as aws_route_table_association to associciate a subnet and route table using the id of each separately created resource.

Dependencies

Some resources must be defined in order of the things they depend on. On example of this is an AWS Elastic IP requiring a Network Gateway to be defined before its declaration so that it can work. If a resource does have a dependency its reccommended to use the depends_on key inside the resource. This should point to the entire dependency as an object in a list.

resource "some_resource" "name" {
    # resource config
}

resource "dependent_resource" "dependent_name" {
    # resource config
    depends_on = [
        some_resource
    ]
}

AWS

You can deploy an Amazon EC2 instance using the "aws_instance" resource type and provide an AMI ID to the image that you want for the instance as well as a type for the instance size.

resource "aws_instance" "myInstance" {
    ami = "<AMI_ID>"
    instance_type = "t3.medium"
}

If you do not specify an availability zone on a resource then a random AZ will be picked. This can be problematic if you have multiple components that depend on one another so its better to specify your availability zone on resources with the availability_zone key.

You can run a bash script on your an AWS instance at start up by using the user_data key with the <<-EOF pipe and the script.

resource "aws_instance" "myInstance" {
    ami = "<AMI_ID>"
    instance_type = "t3.medium"

    user_data = <<-EOF
                #!/bin/bash
                sudo apt update -y
                sudo apt install apache2 -y
}