dynamic Blocks

Within top-level block constructs like resources, expressions can usually be used only when assigning a value to an argument using the name = expression form. This covers many uses, but some resource types include repeatable nested blocks in their arguments, which typically represent separate objects that are related to (or embedded within) the containing object:

resource "aws_elastic_beanstalk_environment" "tfenvtest" {
  name = "tf-test-name" # can use expressions here

  setting {
    # but the "setting" block is always a literal block
  }
}

You can dynamically construct repeatable nested blocks like setting using a special dynamic block type, which is supported inside resource, data, provider, and provisioner blocks:

resource "aws_elastic_beanstalk_environment" "tfenvtest" {
  name                = "tf-test-name"
  application         = "${aws_elastic_beanstalk_application.tftest.name}"
  solution_stack_name = "64bit Amazon Linux 2018.03 v2.11.4 running Go 1.12.6"

  dynamic "setting" {
    for_each = var.settings
    content {
      namespace = setting.value["namespace"]
      name = setting.value["name"]
      value = setting.value["value"]
    }
  }
}

A dynamic block acts much like a for expression, but produces nested blocks instead of a complex typed value. It iterates over a given complex value, and generates a nested block for each element of that complex value.

• The label of the dynamic block ("setting" in the example above) specifies what kind of nested block to generate.
• The for_each argument provides the complex value to iterate over.
• The iterator argument (optional) sets the name of a temporary variable that represents the current element of the complex value. If omitted, the name of the variable defaults to the label of the dynamic block ("setting" in the example above).
• The labels argument (optional) is a list of strings that specifies the block labels, in order, to use for each generated block. You can use the temporary iterator variable in this value.
• The nested content block defines the body of each generated block. You can use the temporary iterator variable inside this block.

Since the for_each argument accepts any collection or structural value, you can use a for expression or splat expression to transform an existing collection.

The iterator object (setting in the example above) has two attributes:

• key is the map key or list element index for the current element. If the for_each expression produces a set value then key is identical to value and should not be used.
• value is the value of the current element.

A dynamic block can only generate arguments that belong to the resource type, data source, provider or provisioner being configured. It is not possible to generate meta-argument blocks such as lifecycle and provisioner blocks, since OpenTF must process these before it is safe to evaluate expressions.

The for_each value must be a collection with one element per desired nested block. If you need to declare resource instances based on a nested data structure or combinations of elements from multiple data structures you can use OpenTF expressions and functions to derive a suitable value. For some common examples of such situations, see the flatten and setproduct functions.

Multi-level Nested Block Structures​

Some providers define resource types that include multiple levels of blocks nested inside one another. You can generate these nested structures dynamically when necessary by nesting dynamic blocks in the content portion of other dynamic blocks.

For example, a module might accept a complex data structure like the following:

variable "load_balancer_origin_groups" {
  type = map(object({
    origins = set(object({
      hostname = string
    }))
  }))
}

If you were defining a resource whose type expects a block for each origin group and then nested blocks for each origin within a group, you could ask OpenTF to generate that dynamically using the following nested dynamic blocks:

  dynamic "origin_group" {
    for_each = var.load_balancer_origin_groups
    content {
      name = origin_group.key

      dynamic "origin" {
        for_each = origin_group.value.origins
        content {
          hostname = origin.value.hostname
        }
      }
    }
  }

When using nested dynamic blocks it's particularly important to pay attention to the iterator symbol for each block. In the above example, origin_group.value refers to the current element of the outer block, while origin.value refers to the current element of the inner block.

If a particular resource type defines nested blocks that have the same type name as one of their parents, you can use the iterator argument in each of dynamic blocks to choose a different iterator symbol that makes the two easier to distinguish.

Best Practices for dynamic Blocks​

Overuse of dynamic blocks can make configuration hard to read and maintain, so we recommend using them only when you need to hide details in order to build a clean user interface for a re-usable module. Always write nested blocks out literally where possible.

If you find yourself defining most or all of a resource block's arguments and nested blocks using directly-corresponding attributes from an input variable then that might suggest that your module is not creating a useful abstraction. It may be better for the calling module to define the resource itself then pass information about it into your module. For more information on this design tradeoff, see When to Write a Module and Module Composition.