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Module Composition

In a simple Terraform configuration with only one root module, we create a flat set of resources and use Terraform's expression syntax to describe the relationships between these resources:

resource "aws_vpc" "example" {
  cidr_block = "10.1.0.0/16"
}

resource "aws_subnet" "example" {
  vpc_id = aws_vpc.example.id

  availability_zone = "us-west-2b"
  cidr_block        = cidrsubnet(aws_vpc.example.cidr_block, 4, 1)
}

When we introduce module blocks, our configuration becomes hierarchical rather than flat: each module contains its own set of resources, and possibly its own child modules, which can potentially create a deep, complex tree of resource configurations.

However, in most cases we strongly recommend keeping the module tree flat, with only one level of child modules, and use a technique similar to the above of using expressions to describe the relationships between the modules:

module "network" {
  source = "./modules/aws-network"

  base_cidr_block = "10.0.0.0/8"
}

module "consul_cluster" {
  source = "./modules/aws-consul-cluster"

  vpc_id     = module.network.vpc_id
  subnet_ids = module.network.subnet_ids
}

We call this flat style of module usage module composition, because it takes multiple composable building-block modules and assembles them together to produce a larger system. Instead of a module embedding its dependencies, creating and managing its own copy, the module receives its dependencies from the root module, which can therefore connect the same modules in different ways to produce different results.

The rest of this page discusses some more specific composition patterns that may be useful when describing larger systems with Terraform.

Dependency Inversion

In the example above, we saw a consul_cluster module that presumably describes a cluster of HashiCorp Consul servers running in an AWS VPC network, and thus it requires as arguments the identifiers of both the VPC itself and of the subnets within that VPC.

An alternative design would be to have the consul_cluster module describe its own network resources, but if we did that then it would be hard for the Consul cluster to coexist with other infrastructure in the same network, and so where possible we prefer to keep modules relatively small and pass in their dependencies.

This dependency inversion approach also improves flexibility for future refactoring, because the consul_cluster module doesn't know or care how those identifiers are obtained by the calling module. A future refactor may separate the network creation into its own configuration, and thus we may pass those values into the module from data sources instead:

data "aws_vpc" "main" {
  tags = {
    Environment = "production"
  }
}

data "aws_subnet_ids" "main" {
  vpc_id = data.aws_vpc.main.id
}

module "consul_cluster" {
  source = "./modules/aws-consul-cluster"

  vpc_id     = data.aws_vpc.main.id
  subnet_ids = data.aws_subnet_ids.main.ids
}

Conditional Creation of Objects

In situations where the same module is used across multiple environments, it's common to see that some necessary object already exists in some environments but needs to be created in other environments.

For example, this can arise in development environment scenarios: for cost reasons, certain infrastructure may be shared across multiple development environments, while in production the infrastructure is unique and managed directly by the production configuration.

Rather than trying to write a module that itself tries to detect whether something exists and create it if not, we recommend applying the dependency inversion approach: making the module accept the object it needs as an argument, via an input variable.

For example, consider a situation where a Terraform module deploys compute instances based on a disk image, and in some environments there is a specialized disk image available while other environments share a common base disk image. Rather than having the module itself handle both of these scenarios, we can instead declare an input variable for an object representing the disk image. Using AWS EC2 as an example, we might declare a common subtype of the aws_ami resource type and data source schemas:

variable "ami" {
  type = object({
    # Declare an object using only the subset of attributes the module
    # needs. Terraform will allow any object that has at least these
    # attributes.
    id           = string
    architecture = string
  })
}

The caller of this module can now itself directly represent whether this is an AMI to be created inline or an AMI to be retrieved from elsewhere:

# In situations where the AMI will be directly managed:

resource "aws_ami_copy" "example" {
  name              = "local-copy-of-ami"
  source_ami_id     = "ami-abc123"
  source_ami_region = "eu-west-1"
}

module "example" {
  source = "./modules/example"

  ami = aws_ami_copy.example
}

# Or, in situations where the AMI already exists:

data "aws_ami" "example" {
  owner = "9999933333"

  tags = {
    application = "example-app"
    environment = "dev"
  }
}

module "example" {
  source = "./modules/example"

  ami = data.aws_ami.example
}

This is consistent with Terraform's declarative style: rather than creating modules with complex conditional branches, we directly describe what should already exist and what we want Terraform to manage itself.

By following this pattern, we can be explicit about in which situations we expect the AMI to already be present and which we don't. A future reader of the configuration can then directly understand what it is intending to do without first needing to inspect the state of the remote system.

In the above example, the object to be created or read is simple enough to be given inline as a single resource, but we can also compose together multiple modules as described elsewhere on this page in situations where the dependencies themselves are complicated enough to benefit from abstractions.

Assumptions and Guarantees

Every module has implicit assumptions and guarantees that define what data it expects and what data it produces for consumers.

  • Assumption: A condition that must be true in order for the configuration of a particular resource to be usable. For example, an aws_instance configuration can have the assumption that the given AMI will always be configured for the x86_64 CPU architecture.
  • Guarantee: A characteristic or behavior of an object that the rest of the configuration should be able to rely on. For example, an aws_instance configuration can have the guarantee that an EC2 instance will be running in a network that assigns it a private DNS record.

We recommend using custom conditions help capture and test for assumptions and guarantees. This helps future maintainers understand the configuration design and intent. Custom conditions also return useful information about errors earlier and in context, helping consumers more easily diagnose issues in their configurations.

The following examples creates a precondition that checks whether the EC2 instance has an encrypted root volume.

output "api_base_url" {
  value = "https://${aws_instance.example.private_dns}:8433/"

  # The EC2 instance must have an encrypted root volume.
  precondition {
    condition     = data.aws_ebs_volume.example.encrypted
    error_message = "The server's root volume is not encrypted."
  }
}

Multi-cloud Abstractions

Terraform itself intentionally does not attempt to abstract over similar services offered by different vendors, because we want to expose the full functionality in each offering and yet unifying multiple offerings behind a single interface will tend to require a "lowest common denominator" approach.

However, through composition of Terraform modules it is possible to create your own lightweight multi-cloud abstractions by making your own tradeoffs about which platform features are important to you.

Opportunities for such abstractions arise in any situation where multiple vendors implement the same concept, protocol, or open standard. For example, the basic capabilities of the domain name system are common across all vendors, and although some vendors differentiate themselves with unique features such as geolocation and smart load balancing, you may conclude that in your use-case you are willing to eschew those features in return for creating modules that abstract the common DNS concepts across multiple vendors:

module "webserver" {
  source = "./modules/webserver"
}

locals {
  fixed_recordsets = [
    {
      name = "www"
      type = "CNAME"
      ttl  = 3600
      records = [
        "webserver01",
        "webserver02",
        "webserver03",
      ]
    },
  ]
  server_recordsets = [
    for i, addr in module.webserver.public_ip_addrs : {
      name    = format("webserver%02d", i)
      type    = "A"
      records = [addr]
    }
  ]
}

module "dns_records" {
  source = "./modules/route53-dns-records"

  route53_zone_id = var.route53_zone_id
  recordsets      = concat(local.fixed_recordsets, local.server_recordsets)
}

In the above example, we've created a lightweight abstraction in the form of a "recordset" object. This contains the attributes that describe the general idea of a DNS recordset that should be mappable onto any DNS provider.

We then instantiate one specific implementation of that abstraction as a module, in this case deploying our recordsets to Amazon Route53.

If we later wanted to switch to a different DNS provider, we'd need only to replace the dns_records module with a new implementation targeting that provider, and all of the configuration that produces the recordset definitions can remain unchanged.

We can create lightweight abstractions like these by defining Terraform object types representing the concepts involved and then using these object types for module input variables. In this case, all of our "DNS records" implementations would have the following variable declared:

variable "recordsets" {
  type = list(object({
    name    = string
    type    = string
    ttl     = number
    records = list(string)
  }))
}

While DNS serves as a simple example, there are many more opportunities to exploit common elements across vendors. A more complex example is Kubernetes, where there are now many different vendors offering hosted Kubernetes clusters and even more ways to run Kubernetes yourself.

If the common functionality across all of these implementations is sufficient for your needs, you may choose to implement a set of different modules that describe a particular Kubernetes cluster implementation and all have the common trait of exporting the hostname of the cluster as an output value:

output "hostname" {
  value = azurerm_kubernetes_cluster.main.fqdn
}

You can then write other modules that expect only a Kubernetes cluster hostname as input and use them interchangeably with any of your Kubernetes cluster modules:

module "k8s_cluster" {
  source = "modules/azurerm-k8s-cluster"

  # (Azure-specific configuration arguments)
}

module "monitoring_tools" {
  source = "modules/monitoring_tools"

  cluster_hostname = module.k8s_cluster.hostname
}

Data-only Modules

Most modules contain resource blocks and thus describe infrastructure to be created and managed. It may sometimes be useful to write modules that do not describe any new infrastructure at all, but merely retrieve information about existing infrastructure that was created elsewhere using data sources.

As with conventional modules, we suggest using this technique only when the module raises the level of abstraction in some way, in this case by encapsulating exactly how the data is retrieved.

A common use of this technique is when a system has been decomposed into several subsystem configurations but there is certain infrastructure that is shared across all of the subsystems, such as a common IP network. In this situation, we might write a shared module called join-network-aws which can be called by any configuration that needs information about the shared network when deployed in AWS:

module "network" {
  source = "./modules/join-network-aws"

  environment = "production"
}

module "k8s_cluster" {
  source = "./modules/aws-k8s-cluster"

  subnet_ids = module.network.aws_subnet_ids
}

The network module itself could retrieve this data in a number of different ways: it could query the AWS API directly using aws_vpc and aws_subnet_ids data sources, or it could read saved information from a Consul cluster using consul_keys, or it might read the outputs directly from the state of the configuration that manages the network using terraform_remote_state.

The key benefit of this approach is that the source of this information can change over time without updating every configuration that depends on it. Furthermore, if you design your data-only module with a similar set of outputs as a corresponding management module, you can swap between the two relatively easily when refactoring.