github.com/nathanielks/terraform@v0.6.1-0.20170509030759-13e1a62319dc/website/source/intro/getting-started/dependencies.html.md (about) 1 --- 2 layout: "intro" 3 page_title: "Resource Dependencies" 4 sidebar_current: "gettingstarted-deps" 5 description: |- 6 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. 7 --- 8 9 # Resource Dependencies 10 11 In this page, we're going to introduce resource dependencies, 12 where we'll not only see a configuration with multiple resources 13 for the first time, but also scenarios where resource parameters 14 use information from other resources. 15 16 Up to this point, our example has only contained a single resource. 17 Real infrastructure has a diverse set of resources and resource 18 types. Terraform configurations can contain multiple resources, 19 multiple resource types, and these types can even span multiple 20 providers. 21 22 On this page, we'll show a basic example of multiple resources 23 and how to reference the attributes of other resources to configure 24 subsequent resources. 25 26 ## Assigning an Elastic IP 27 28 We'll improve our configuration by assigning an elastic IP to 29 the EC2 instance we're managing. Modify your `example.tf` and 30 add the following: 31 32 ```hcl 33 resource "aws_eip" "ip" { 34 instance = "${aws_instance.example.id}" 35 } 36 ``` 37 38 This should look familiar from the earlier example of adding 39 an EC2 instance resource, except this time we're building 40 an "aws\_eip" resource type. This resource type allocates 41 and associates an 42 [elastic IP](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html) 43 to an EC2 instance. 44 45 The only parameter for 46 [aws\_eip](/docs/providers/aws/r/eip.html) is "instance" which 47 is the EC2 instance to assign the IP to. For this value, we 48 use an interpolation to use an attribute from the EC2 instance 49 we managed earlier. 50 51 The syntax for this interpolation should be straightforward: 52 it requests the "id" attribute from the "aws\_instance.example" 53 resource. 54 55 ## Plan and Execute 56 57 Run `terraform plan` to view the execution plan. The output 58 will look something like the following: 59 60 ``` 61 $ terraform plan 62 63 + aws_eip.ip 64 allocation_id: "<computed>" 65 association_id: "<computed>" 66 domain: "<computed>" 67 instance: "${aws_instance.example.id}" 68 network_interface: "<computed>" 69 private_ip: "<computed>" 70 public_ip: "<computed>" 71 72 + aws_instance.example 73 ami: "ami-b374d5a5" 74 availability_zone: "<computed>" 75 ebs_block_device.#: "<computed>" 76 ephemeral_block_device.#: "<computed>" 77 instance_state: "<computed>" 78 instance_type: "t2.micro" 79 key_name: "<computed>" 80 placement_group: "<computed>" 81 private_dns: "<computed>" 82 private_ip: "<computed>" 83 public_dns: "<computed>" 84 public_ip: "<computed>" 85 root_block_device.#: "<computed>" 86 security_groups.#: "<computed>" 87 source_dest_check: "true" 88 subnet_id: "<computed>" 89 tenancy: "<computed>" 90 vpc_security_group_ids.#: "<computed>" 91 ``` 92 93 Terraform will create two resources: the instance and the elastic 94 IP. In the "instance" value for the "aws\_eip", you can see the 95 raw interpolation is still present. This is because this variable 96 won't be known until the "aws\_instance" is created. It will be 97 replaced at apply-time. 98 99 Next, run `terraform apply`. The output will look similar to the 100 following: 101 102 ``` 103 $ terraform apply 104 aws_instance.example: Creating... 105 ami: "" => "ami-b374d5a5" 106 instance_type: "" => "t2.micro" 107 [..] 108 aws_instance.example: Still creating... (10s elapsed) 109 aws_instance.example: Creation complete 110 aws_eip.ip: Creating... 111 allocation_id: "" => "<computed>" 112 association_id: "" => "<computed>" 113 domain: "" => "<computed>" 114 instance: "" => "i-f3d77d69" 115 network_interface: "" => "<computed>" 116 private_ip: "" => "<computed>" 117 public_ip: "" => "<computed>" 118 aws_eip.ip: Creation complete 119 120 Apply complete! Resources: 2 added, 0 changed, 0 destroyed. 121 ``` 122 123 It is clearer to see from actually running Terraform, but 124 Terraform creates the EC2 instance before the elastic IP 125 address. Due to the interpolation earlier where the elastic 126 IP requires the ID of the EC2 instance, Terraform is able 127 to infer a dependency, and knows to create the instance 128 first. 129 130 ## Implicit and Explicit Dependencies 131 132 Most dependencies in Terraform are implicit: Terraform is able 133 to infer dependencies based on usage of attributes of other 134 resources. 135 136 Using this information, Terraform builds a graph of resources. 137 This tells Terraform not only in what order to create resources, 138 but also what resources can be created in parallel. In our example, 139 since the IP address depended on the EC2 instance, they could 140 not be created in parallel. 141 142 Implicit dependencies work well and are usually all you ever need. 143 However, you can also specify explicit dependencies with the 144 `depends_on` parameter which is available on any resource. For example, 145 we could modify the "aws\_eip" resource to the following, which 146 effectively does the same thing and is redundant: 147 148 ```hcl 149 resource "aws_eip" "ip" { 150 instance = "${aws_instance.example.id}" 151 depends_on = ["aws_instance.example"] 152 } 153 ``` 154 155 If you're ever unsure about the dependency chain that Terraform 156 is creating, you can use the [`terraform graph` command](/docs/commands/graph.html) to view 157 the graph. This command outputs a dot-formatted graph which can be 158 viewed with 159 [Graphviz](http://www.graphviz.org/). 160 161 ## Non-Dependent Resources 162 163 We can now augment the configuration with another EC2 instance. 164 Because this doesn't rely on any other resource, it can be 165 created in parallel to everything else. 166 167 ```hcl 168 resource "aws_instance" "another" { 169 ami = "ami-b374d5a5" 170 instance_type = "t2.micro" 171 } 172 ``` 173 174 You can view the graph with `terraform graph` to see that 175 nothing depends on this and that it will likely be created 176 in parallel. 177 178 Before moving on, remove this resource from your configuration 179 and `terraform apply` again to destroy it. We won't use the 180 second instance anymore in the getting started guide. 181 182 ## Next 183 184 In this page you were introduced to both multiple resources 185 as well as basic resource dependencies and resource attribute 186 interpolation. 187 188 Moving on, [we'll use provisioners](/intro/getting-started/provision.html) 189 to do some basic bootstrapping of our launched instance.