website: resource dependencies
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Mitchell Hashimoto
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layout: "intro"
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page_title: "Resource Dependencies"
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sidebar_current: "gettingstarted-deps"
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---
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# Resource Dependencies
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In this page, we're going to introduce resource dependencies,
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where we'll not only see a configuration with multiple resources
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for the first time, but also scenarios where resource parameters
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use information from other resources.
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Up to this point, our example has only contained a single resource.
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Real infrastructure has a diverse set of resources and resource
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types. Terraform configurations can contain multiple resources,
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multiple resource types, and these types can even span multiple
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providers.
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On this page, we'll show a basic example of multiple resources
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and how to reference the attributes of other resources to configure
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subsequent resources.
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## Assigning an Elastic IP
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We'll improve our configuration by assigning an elastic IP to
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the EC2 instance we're managing. Modify your `example.tf` and
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add the following:
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```
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resource "aws_eip" "ip" {
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instance = "${aws_instance.example.id}"
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}
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```
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This should look familiar from the earlier example of adding
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an EC2 instance resource, except this time we're building
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an "aws\_eip" resource type. This resource type allocates
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and associates an
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[elastic IP](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html)
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to an EC2 instance.
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The only parameter for
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[aws\_eip](/docs/providers/aws/r/eip.html) is "instance" which
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is the EC2 instance to assign the IP to. For this value, we
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use an interpolation to use an attribute from the EC2 instance
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we managed earlier.
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The syntax for this interpolation should be straightforward:
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it requests the "id" attribute from the "aws\_instance.example"
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resource.
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## Plan and Execute
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Run `terraform plan` to view the execution plan. The output
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will look something like the following:
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```
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$ terraform plan
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...
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+ aws_eip.ip
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instance: "" => "${aws_instance.example.id}"
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private_ip: "" => "<computed>"
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public_ip: "" => "<computed>"
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+ aws_instance.example
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ami: "" => "ami-aa7ab6c2"
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availability_zone: "" => "<computed>"
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instance_type: "" => "t1.micro"
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key_name: "" => "<computed>"
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private_dns: "" => "<computed>"
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private_ip: "" => "<computed>"
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public_dns: "" => "<computed>"
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public_ip: "" => "<computed>"
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security_groups: "" => "<computed>"
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subnet_id: "" => "<computed>"
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```
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Terraform will create two resources: the instance and the elastic
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IP. In the "instance" value for the "aws\_eip", you can see the
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raw interpolation is still present. This is because this variable
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won't be known until the "aws\_instance" is created. It will be
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replaced at apply-time.
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Next, run `terraform apply`. The output will look similar to the
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following:
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```
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aws_instance.example: Creating...
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ami: "" => "ami-aa7ab6c2"
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instance_type: "" => "t1.micro"
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aws_eip.ip: Creating...
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instance: "" => "i-0e737b25"
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Apply complete! Resources: 2 added, 0 changed, 0 destroyed.
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```
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It is clearer to see from actually running Terraform, but
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Terraform creates the EC2 instance before the elastic IP
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address. Due to the interpolation earlier where the elastic
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IP requires the ID of the EC2 instance, Terraform is able
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to infer a dependency, and knows to create the instance
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first.
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## Implicit and Explicit Dependencies
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Most dependencies in Terraform are implicit: Terraform is able
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to infer dependencies based on usage of attributes of other
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resources.
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Using this information, Terraform builds a graph of resources.
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This tells Terraform not only what order to create resources,
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but also what resources can be created in parallel. In our example,
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since the IP address depended on the EC2 instance, they could
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not be created in parallel.
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Implicit dependencies work well and are usually all you ever need.
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However, you can also specify explicit dependencies with the
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`depends_on` parameter which is available on any resource. For example,
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we could modify the "aws\_eip" resource to the following, which
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effectively does the same thing and is redundant:
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```
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resource "aws_eip" "ip" {
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instance = "${aws_instance.example.id}"
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depends_on = ["aws_instance.example"]
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}
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```
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If you're ever unsure about the dependency chain that Terraform
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is creating, you can use the `terraform graph` command to view
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the graph. This command outputs a dot-formatted graph which can be
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viewed with
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[Graphviz](http://www.graphviz.org/).
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## Non-Dependent Resources
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We can now augment the configuration with another EC2 instance.
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Because this doesn't rely on any other resource, it can be
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created in parallel to everything else.
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```
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resource "aws_instance" "another" {
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ami = "ami-aa7ab6c2"
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instance_type = "t1.micro"
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}
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```
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You can view the graph with `terraform graph` to see that
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nothing depends on this and that it will likely be created
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in parallel.
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Before moving on, remove this resource from your configuration
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and `terraform apply` again to destroy it. We won't use the
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second instance anymore in the getting started guide.
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## Next
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In this page you were introduced to both multiple resources
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as well as basic resource dependencies and resource attribute
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interpolation.
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Moving on, we'll parameterize our configuration for better
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shareability and maintainability.
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@ -42,10 +42,6 @@
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<a href="/intro/getting-started/destroy.html">Destroy Infrastructure</a>
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</li>
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<li<%= sidebar_current("gettingstarted-outputs") %>>
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<a href="/intro/getting-started/outputs.html">Output Variables</a>
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</li>
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<li<%= sidebar_current("gettingstarted-deps") %>>
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<a href="/intro/getting-started/dependencies.html">Resource Dependencies</a>
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</li>
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@ -54,6 +50,10 @@
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<a href="/intro/getting-started/variables.html">Input Variables</a>
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</li>
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<li<%= sidebar_current("gettingstarted-outputs") %>>
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<a href="/intro/getting-started/outputs.html">Output Variables</a>
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</li>
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<li<%= sidebar_current("gettingstarted-variables") %>>
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<a href="/intro/getting-started/provisioners.html">Provision</a>
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</li>
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