!terraform.output

The !terraform.output YAML function allows reading the outputs (remote state) of components directly in Atmos stack manifests by internally executing a terraform output or tofu output command.

Usage

The !terraform.output function can be called with either two or three parameters:

  # Get the `output` of the `component` in the current stack
  !terraform.output <component> <output>

  # Get the `output` of the `component` in the provided `stack`
  !terraform.output <component> <stack> <output>

  # Get the output of the `component` by evaluating the YQ expression
  !terraform.output <component> <yq-expression>

 # Get the output of the `component` in the provided `stack` by evaluating the YQ expression
  !terraform.output <component> <stack> <yq-expression>

Arguments

component

Atmos component name

stack

(Optional) Atmos stack name

output or yq-expression

Terraform output or YQ expression to evaluate the output

tip

You can use Atmos Stack Manifest Templating in the !terraform.output YAML function expressions. Atmos processes the templates first, and then executes the !terraform.output function, allowing you to provide the parameters to the function dynamically.

!terraform.output Function Execution Flow

When processing the !terraform.output YAML function for a component in a stack, Atmos executes the following steps:

• Stack and Component Context Resolution Atmos resolves the full context for the specified component within the given stack, including all inherited and merged configuration layers (globals, environment and component-level config).

• Terraform/OpenTofu Variables Based on the resolved context, Atmos generates a varfile with all the variables for the component in the stack.

• Terraform/OpenTofu Backend Atmos generates a backend configuration file for the component in the stack based on the resolved context.

• Terraform/OpenTofu Provider Overrides Atmos generates a provider override file for the component in the stack (if provider overrides are configured in the stack manifests).

• Terraform/OpenTofu Workspace Based on the resolved context, Atmos selects (or creates new) Terraform/OpenTofu workspace by executing terraform/tofu workspace commands.

• Terraform/OpenTofu Init Atmos executes a terraform/tofu init command to initialize the component and download all Terraform/OpenTofu modules and providers (if they are not present in the cache).

• Terraform/OpenTofu Output Atmos executes a terraform/tofu output command to read the outputs of the component in the stack.

• Output Parsing and Interpolation The relevant output variable is extracted from the state file (using a YQ parser). Atmos parses and interpolates the value into the final configuration structure, replacing the !terraform.output directive in the YAML stack manifest with the final value.

tip

Since Atmos executes the terraform/tofu init and terraform/tofu output commands when processing the !terraform.output YAML functions, it can significantly impact performance because it requires initializing the components, which initializes all Terraform/OpenTofu modules and downloads all Terraform/OpenTofu providers (if they are not present in the cache).

To improve performance, consider using the !store or !terraform.state YAML function.

To understand the performance implications of the !terraform.output and !terraform.state functions, compare the !terraform.output Execution Flow with the !terraform.state Execution Flow.

Using YQ Expressions to retrieve items from complex output types

To retrieve items from complex output types such as maps and lists, or do any kind of filtering or querying, you can utilize YQ expressions.

For example:

• Retrieve the first item from a list

subnet_id1: !terraform.output vpc .private_subnet_ids[0]

• Read a key from a map

username: !terraform.output config .config_map.username

For more details, review the following docs:

• YQ Guide
• YQ Recipes

Using YQ Expressions to provide a default value

If the component for which you are reading the output has not been provisioned yet, the !terraform.output function will return the string <no value> unless you specify a default value in the YQ expression, in which case the function will return the default value.

This will allow you to mock outputs when executing atmos terraform plan where there are dependencies between components, and the dependent components are not provisioned yet.

note

To provide a default value, you use the // YQ operator. The whole YQ expression contains spaces, and to make it a single parameter, you need to double-quote it.

YQ requires the strings in the default values to be double-quoted as well. This means that you have to escape the double-quotes in the default values by using two double-quotes.

For example:

• Specify a string default value. Read the username output from the config component in the current stack. If the config component has not been provisioned yet, return the default value default-user

username: !terraform.output config ".username // ""default-user"""

• Specify a list default value. Read the private_subnet_ids output from the vpc component in the current stack. If the vpc component has not been provisioned yet, return the default value ["mock-subnet1", "mock-subnet2"]

subnet_ids: !terraform.output vpc ".private_subnet_ids // [""mock-subnet1"", ""mock-subnet2""]"

• Specify a map default value. Read the config_map output from the config component in the current stack. If the config component has not been provisioned yet, return the default value {"api_endpoint": "localhost:3000", "user": "test"}

config_map: !terraform.output 'config ".config_map // {""api_endpoint"": ""localhost:3000"", ""user"": ""test""}"'

For more details, review the following docs:

• YQ Alternative (Default value)

Using YQ Expressions to modify values returned from the remote state

Since the output parameter of the !terraform.output function is a YQ expression, you can use YQ pipes and YQ operators (including YQ string concatenation functions and YQ arithmetic functions) to modify the values returned from the remote state.

For example, suppose you have an aurora-postgres Atmos component which has the output master_hostname.

To read the output without modification, you can use the following expressions:

postgres_url: !terraform.output aurora-postgres master_hostname

postgres_url: !terraform.output aurora-postgres .master_hostname

To prepend and append strings to the output, you can use YQ pipes and the add function (+ operator):

postgres_url: !terraform.output aurora-postgres ".master_hostname | ""jdbc:postgresql://"" + . + "":5432/events"""

note

The double quotes are required around the whole YQ expression, and around the strings inside the YQ expression, hence the double-double quotes "" to escape it.

After the !terraform.output function is executed, the postgres_url variable will have the final value similar to:

postgres_url: "jdbc:postgresql://aurora-postgres-cluster-writer.prod.plat.mydomain.net:5432/events"

For more details, review the following docs:

• YQ pipe
• YQ operators
• YQ string concatenation
• YQ add function

Examples

stack.yaml

components:
  terraform:
    my_lambda_component:
      vars:
        vpc_config:
          # Output of type string
          security_group_id: !terraform.output security-group/lambda id
          security_group_id2: !terraform.output security-group/lambda2 {{ .stack }} id
          security_group_id3: !terraform.output security-group/lambda3 {{ .atmos_stack }} id
          # Output of type list
          subnet_ids: !terraform.output vpc private_subnet_ids
          # Use a YQ expression to get an item from the list
          subnet_id1: !terraform.output vpc .private_subnet_ids[0]
          # Output of type map
          config_map: !terraform.output config {{ .stack }} config_map
          # Use a YQ expression to get a value from the map
          username: !terraform.output config .config_map.username

Specifying Atmos stack

If you call the !terraform.output function with three parameters, you need to specify the stack as the second argument.

There are multiple ways you can specify the Atmos stack parameter in the !terraform.output function.

Hardcoded Stack Name

Use it if you want to get an output from a component from a different (well-known and static) stack. For example, you have a tgw component in a stack plat-ue2-dev that requires the vpc_id output from the vpc component from the stack plat-ue2-prod:

plat-ue2-dev

  components:
    terraform:
      tgw:
        vars:
          vpc_id: !terraform.output vpc plat-ue2-prod vpc_id

Reference the Current Stack Name

Use the .stack (or .atmos_stack) template identifier to specify the same stack as the current component is in (for which the !terraform.output function is executed):

  !terraform.output <component> {{ .stack }} <output>
  !terraform.output <component> {{ .atmos_stack }} <output>

For example, you have a tgw component that requires the vpc_id output from the vpc component in the same stack:

  components:
    terraform:
      tgw:
        vars:
          vpc_id: !terraform.output vpc {{ .stack }} vpc_id

note

Using the .stack or .atmos_stack template identifiers to specify the stack is the same as calling the !terraform.output function with two parameters without specifying the current stack, but without using Go templates. If you need to get an output of a component in the current stack, using the !terraform.output function with two parameters is preferred because it has a simpler syntax and executes faster.

Use a Format Function

Use the printf template function to construct stack names using static strings and dynamic identifiers. This is convenient when you want to override some identifiers in the stack name:

  !terraform.output <component> {{ printf "%s-%s-%s" .vars.tenant .vars.environment .vars.stage }} <output>

  !terraform.output <component> {{ printf "plat-%s-prod" .vars.environment }} <output>

  !terraform.output <component> {{ printf "%s-%s-%s" .settings.context.tenant .settings.context.region .settings.context.account }} <output>

<component>

Placeholder for an actual component name (e.g. vpc)

<output>

Placeholder for an actual Terraform output (e.g. subnet_ids)

For example, you have a tgw component deployed in the stack plat-ue2-dev. The tgw component requires the vpc_id output from the vpc component from the same environment (ue2) and same stage (dev), but from a different tenant net (instead of plat):

plat-ue2-dev

  components:
    terraform:
      tgw:
        vars:
          vpc_id: !terraform.output vpc {{ printf "net-%s-%s" .vars.environment .vars.stage }} vpc_id

Important

By using the printf "%s-%s-%s" function, you are constructing stack names using the stack context variables/identifiers.

For more information on Atmos stack names and how to define them, refer to stacks.name_pattern and stacks.name_template sections in atmos.yaml CLI config file

Caching the result of !terraform.output function

Atmos caches (in memory) the results of !terraform.output function.

The cache is per Atmos CLI command execution, e.g., each new execution of a command like atmos terraform plan, atmos terraform apply or atmos describe component will create and use a new memory cache, which involves re-invoking terraform outputs after reinitialization.

If you define the function in stack manifests for the same component in a stack more than once, the first call will produce the result and cache it, and all the consecutive calls will just use the cached data. This is useful when you use the !terraform.output function for the same component in a stack in multiple places in Atmos stack manifests. It will speed up the function execution and stack processing.

For example:

components:
  terraform:
    test2:
      vars:
        tags:
          test: !terraform.output test id
          test2: !terraform.output test id
          test3: !terraform.output test {{ .stack }} id

In the example, the test2 Atmos component uses the outputs (remote state) of the test Atmos component from the same stack. The YAML function !terraform.output is executed three times (once for each tag).

After the first execution, Atmos caches the result in memory, and reuses it in the next two calls to the function. The caching makes the stack processing much faster. In a production environment where many components are used, the speedup can be significant.

Using !terraform.output with static remote state backend

Atmos supports brownfield configuration by using the remote state of type static.

For example:

stack.yaml

components:
  terraform:
    # Component `static-backend` is configured with the remote state backend of type `static`
    static-backend:
      remote_state_backend_type: static
      remote_state_backend:
        static:
          region: "us-west-2"
          cluster_name: "production-cluster"
          vpc_cidr: "10.0.0.0/16"
          database:
            type: "postgresql"
            version: "12.7"
            storage_gb: 100
          allowed_ips:
            - "192.168.1.0/24"
            - "10.1.0.0/16"
          tags:
            Environment: "production"
            Owner: "infra-team"

    eks-cluster:
      vars:
        region: !terraform.output static-backend region
        cluster_name: !terraform.output static-backend cluster_name
        vpc_cidr: !terraform.output static-backend vpc_cidr
        db_type: !terraform.output static-backend database.type
        db_storage: !terraform.output static-backend database.storage_gb
        allowed_ips: !terraform.output static-backend allowed_ips
        tags: !terraform.output static-backend tags

When the functions are executed, Atmos detects that the static-backend component has the static remote state configured, and instead of executing terraform output, it just returns the static values from the remote_state_backend.static section.

Executing the command atmos describe component eks-cluster -s <stack> produces the following result:

atmos describe component eks-cluster -s <stack>

vars:
  region: us-west-2
  cluster_name: production-cluster
  vpc_cidr: 10.0.0.0/16
  db_type: postgresql
  db_storage: 100
  allowed_ips:
    - 192.168.1.0/24
    - 10.1.0.0/16
  tags:
    Environment: production
    Owner: infra-team

Considerations

• Using !terraform.output with secrets can expose sensitive data to standard output (stdout) in any commands that describe stacks or components.

• When using !terraform.output with atmos describe affected, Atmos requires access to all referenced remote states. If you operate with limited permissions (e.g., scoped to dev) and reference production stacks, the command will fail.

• Overusing the function within stacks to reference multiple components can significantly impact performance since Atmos internally executes a terraform output or tofu output command, which requires initializing the component, which initializes all Terraform/OpenTofu modules and downloads all Terraform/OpenTofu providers.

• Be mindful of disaster recovery (DR) implications when using it across regions.

• Consider cold-start scenarios: if the dependent component has not yet been provisioned, terraform output will fail.