terraform/website/source/intro/getting-started/dependencies.html.md

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intro	Resource Dependencies	gettingstarted-deps

Resource Dependencies

In this page, we're going to introduce resource dependencies, where we'll not only see a configuration with multiple resources for the first time, but also scenarios where resource parameters use information from other resources.

Up to this point, our example has only contained a single resource. Real infrastructure has a diverse set of resources and resource types. Terraform configurations can contain multiple resources, multiple resource types, and these types can even span multiple providers.

On this page, we'll show a basic example of multiple resources and how to reference the attributes of other resources to configure subsequent resources.

Assigning an Elastic IP

We'll improve our configuration by assigning an elastic IP to the EC2 instance we're managing. Modify your example.tf and add the following:

resource "aws_eip" "ip" {
	instance = "${aws_instance.example.id}"
}

This should look familiar from the earlier example of adding an EC2 instance resource, except this time we're building an "aws_eip" resource type. This resource type allocates and associates an elastic IP to an EC2 instance.

The only parameter for aws_eip is "instance" which is the EC2 instance to assign the IP to. For this value, we use an interpolation to use an attribute from the EC2 instance we managed earlier.

The syntax for this interpolation should be straightforward: it requests the "id" attribute from the "aws_instance.example" resource.

Plan and Execute

Run terraform plan to view the execution plan. The output will look something like the following:

$ terraform plan
...

+ aws_eip.ip
    instance:   "" => "${aws_instance.example.id}"
    private_ip: "" => "<computed>"
    public_ip:  "" => "<computed>"

+ aws_instance.example
    ami:               "" => "ami-aa7ab6c2"
    availability_zone: "" => "<computed>"
    instance_type:     "" => "t1.micro"
    key_name:          "" => "<computed>"
    private_dns:       "" => "<computed>"
    private_ip:        "" => "<computed>"
    public_dns:        "" => "<computed>"
    public_ip:         "" => "<computed>"
    security_groups:   "" => "<computed>"
    subnet_id:         "" => "<computed>"

Terraform will create two resources: the instance and the elastic IP. In the "instance" value for the "aws_eip", you can see the raw interpolation is still present. This is because this variable won't be known until the "aws_instance" is created. It will be replaced at apply-time.

Next, run terraform apply. The output will look similar to the following:

aws_instance.example: Creating...
  ami:           "" => "ami-aa7ab6c2"
  instance_type: "" => "t1.micro"
aws_eip.ip: Creating...
  instance: "" => "i-0e737b25"

Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

It is clearer to see from actually running Terraform, but Terraform creates the EC2 instance before the elastic IP address. Due to the interpolation earlier where the elastic IP requires the ID of the EC2 instance, Terraform is able to infer a dependency, and knows to create the instance first.

Implicit and Explicit Dependencies

Most dependencies in Terraform are implicit: Terraform is able to infer dependencies based on usage of attributes of other resources.

Using this information, Terraform builds a graph of resources. This tells Terraform not only what order to create resources, but also what resources can be created in parallel. In our example, since the IP address depended on the EC2 instance, they could not be created in parallel.

Implicit dependencies work well and are usually all you ever need. However, you can also specify explicit dependencies with the depends_on parameter which is available on any resource. For example, we could modify the "aws_eip" resource to the following, which effectively does the same thing and is redundant:

resource "aws_eip" "ip" {
	instance = "${aws_instance.example.id}"
	depends_on = ["aws_instance.example"]
}

If you're ever unsure about the dependency chain that Terraform is creating, you can use the terraform graph command to view the graph. This command outputs a dot-formatted graph which can be viewed with Graphviz.

Non-Dependent Resources

We can now augment the configuration with another EC2 instance. Because this doesn't rely on any other resource, it can be created in parallel to everything else.

resource "aws_instance" "another" {
	ami = "ami-aa7ab6c2"
	instance_type = "t1.micro"
}

You can view the graph with terraform graph to see that nothing depends on this and that it will likely be created in parallel.

Before moving on, remove this resource from your configuration and terraform apply again to destroy it. We won't use the second instance anymore in the getting started guide.

Next

In this page you were introduced to both multiple resources as well as basic resource dependencies and resource attribute interpolation.

Moving on, we'll parameterize our configuration for better shareability and maintainability.