In this tutorial, you will provision a VPC, load balancer, and EC2 instances on
AWS. Then you will refactor your configuration to provision multiple projects
with the for_each argument and a data structure.
The for_each argument will iterate over a data structure to configure
resources or modules with each item in turn. It works best when the duplicate
resources need to be configured differently but share the same lifecycle.
Tip: Terraform 0.13+ supports the for_each argument on both resource and
module blocks. Prior versions only supported it on resource blocks.
»Prerequisites
- The Terraform CLI, version 0.13 or later.
- AWS Credentials configured for use with Terraform.
- The git CLI.
»Apply initial configuration
Clone the example GitHub repository.
Change into the new directory.
Check out the initial configuration.
The configuration in main.tf will provision a VPC with public and private
subnets, a load balancer, and EC2 instances in each private subnet. The
variables located in variables.tf allow you to configure the VPC. For
instance, the private_subnets_per_vpc variable controls the number of private
subnets the configuration will create.
Initialize Terraform in this directory. Terraform will install the AWS provider
and the vpc, app_security_group, lb_security_group, and elb_http
modules.
Once your directory has been initialized, apply the configuration, and remember
to confirm with a yes.
Note: The load balancer's domain name is part of the output. It may take a few minutes after the apply step before you can visit this domain name. Be sure to connect via HTTP, not HTTPS.
Refactor the VPC and related configuration so that Terraform can deploy multiple projects at the same time, each with their own VPC and related resources.
Note: Use separate Terraform projects or
workspaces
instead of for_each to manage resources' lifecycles independently. For
example, if production and development environments share the same Terraform
project running terraform destroy will destroy both.
You can either implement the changes below manually, or check out the foreach-multiple-projects branch for the completed configuration.
»Define a map to configure each project
Define a map for project configuration in variables.tf that for_each will
iterate over to configure each resource.
Note: The for_each argument also supports lists and sets.
Since the project variable includes most of the options that were
configured by individual variables, comment out or remove these variables from
variables.tf.
»Add for_each to the VPC
Now use for_each to iterate over the project map in the VPC module block of
main.tf, which will create one VPC for each key/value pair in the map.
Terraform will provision multiple VPCs, assigning each key/value pair in the
var.project map to each.key and each.value respectively. With a list or
set, each.key will be the index of the item in the collection, and
each.value will be the value of the item.
In this example, the project map includes values for the number of private and
public subnets in each VPC. Update the subnet configuration in the vpc module
block in main.tf to use each.value to refer to these values.
Update the app_security_group module to iterate over the project variable to
get the security group name, VPC ID, and CIDR blocks for each project.
You can differentiate between instances of resources and modules configured with
for_each by using the keys of the map you use. In this example,
using module.vpc[each.key].vpc_id to define the VPC means that the security
group for a given project will be assigned to the corresponding VPC.
»Update the load balancer and its security group
Update the configuration for the load balancer security groups to iterate over
the project variable to get their names and VPC IDs.
Update the elb_http block so that each VPC's load balancer name will also include the name of the project, the
environment, and will use the corresponding security groups and subnets.
»Move EC2 instance to a module
You will also need to update the instance resource block to assign EC2 instances
to each VPC. However, the block already uses count. You cannot use both
count and for_each in the same block.
To solve this, you will move the aws_instance resource into a module,
including the count argument, and then use for_each when referring to the
module in your main.tf file. The example repository includes a module with
this configuration in the modules/aws-instance directory. For a detailed example on how to move a configuration to a local module, try the Create a Terraform Module tutorial.
Remove the resource "aws_instance" "app" and data "aws_ami" "amazon_linux"
blocks from your root module's main.tf file, and replace them with a reference
to the aws-instance module.
Note: You cannot include a provider block in modules that use count or
for_each. They must inherit provider configuration from the root module.
Resources created by the module will all use the same provider configuration.
Next, replace the references to the EC2 instances in the module "elb_http"
block with references to the new module.
Finally, replace the entire contents of outputs.tf in your root module with
the following.
The for expressions used here will map the project names to the corresponding
values in the Terraform output.
Note: for and for_each are different features. for_each provisions
similar resources in module and resource blocks. for creates a list or map
by iterating over a collection, such as another list or map. You can read more
about for expressions in the Terraform
documentation.
»Apply scalable configuration
Initialize the new module.
Now apply the changes. Remember to respond to the confirmation prompt with
yes.
This configuration creates separate VPCs for each project defined in
variables.tf. count and for_each allow you to create more flexible
configurations, and reduce duplicate resource and module blocks.
»Clean up resources
After verifying that the projects deployed successfully, run terraform destroy
to destroy them. Remember to respond to the confirmation prompt with yes.
»Next steps
Now that you have used for_each in your configuration, explore the
following resources.
- Read the Terraform documentation for the for_each meta-argument.
- Learn how to create and use Terraform modules.
