JSON Configuration Syntax

Most OpenTF configurations are written in the native OpenTF language syntax, which is designed to be relatively easy for humans to read and update.

OpenTF also supports an alternative syntax that is JSON-compatible. This syntax is useful when generating portions of a configuration programmatically, since existing JSON libraries can be used to prepare the generated configuration files.

The JSON syntax is defined in terms of the native syntax. Everything that can be expressed in native syntax can also be expressed in JSON syntax, but some constructs are more complex to represent in JSON due to limitations of the JSON grammar.

OpenTF expects native syntax for files named with a .tf suffix, and JSON syntax for files named with a .tf.json suffix.

The low-level JSON syntax, just as with the native syntax, is defined in terms of a specification called HCL. It is not necessary to know all of the details of HCL syntax or its JSON mapping in order to use OpenTF, and so this page summarizes the most important differences between native and JSON syntax. If you are interested, you can find a full definition of HCL's JSON syntax in its specification.

JSON File Structure​

At the root of any JSON-based OpenTF configuration is a JSON object. The properties of this object correspond to the top-level block types of the OpenTF language. For example:

{
  "variable": {
    "example": {
      "default": "hello"
    }
  }
}

Each top-level object property must match the name of one of the expected top-level block types. Block types that expect labels, such as variable shown above, are represented by one nested object value for each level of label. resource blocks expect two labels, so two levels of nesting are required:

{
  "resource": {
    "aws_instance": {
      "example": {
        "instance_type": "t2.micro",
        "ami": "ami-abc123"
      }
    }
  }
}

After any nested objects representing the labels, finally one more nested object represents the body of the block itself. In the above examples, the default argument for variable "example" and the instance_type and ami arguments for resource "aws_instance" "example" are specified.

Taken together, the above two configuration files are equivalent to the following blocks in the native syntax:

variable "example" {
  default = "hello"
}

resource "aws_instance" "example" {
  instance_type = "t2.micro"
  ami           = "ami-abc123"
}

Within each top-level block type the rules for mapping to JSON are slightly different (see the block-type-specific exceptions below), but the following general rules apply in most cases:

• The JSON object representing the block body contains properties that correspond either to argument names or to nested block type names.

• Where a property corresponds to an argument that accepts arbitrary expressions in the native syntax, the property value is mapped to an expression as described under Expression Mapping below. For arguments that do not accept arbitrary expressions, the interpretation of the property value depends on the argument, as described in the block-type-specific exceptions given later in this page.

• Where a property name corresponds to an expected nested block type name, the value is interpreted as described under Nested Block Mapping below, unless otherwise stated in the block-type-specific exceptions given later in this page.

Expression Mapping​

Since JSON grammar is not able to represent all of the OpenTF language expression syntax, JSON values interpreted as expressions are mapped as follows:

When a JSON string is encountered in a location where arbitrary expressions are expected, its value is first parsed as a string template and then it is evaluated to produce the final result.

If the given template consists only of a single interpolation sequence, the result of its expression is taken directly, without first converting it to a string. This allows non-string expressions to be used within the JSON syntax:

{
  "output": {
    "example": {
      "value": "${aws_instance.example}"
    }
  }
}

The output "example" declared above has the object value representing the given aws_instance resource block as its value, rather than a string value. This special behavior does not apply if any literal or control sequences appear in the template; in these other situations, a string value is always produced.

Nested Block Mapping​

When a JSON object property is named after a nested block type, the value of this property represents one or more blocks of that type. The value of the property must be either a JSON object or a JSON array.

The simplest situation is representing only a single block of the given type when that type expects no labels, as with the lifecycle nested block used within resource blocks:

{
  "resource": {
    "aws_instance": {
      "example": {
        "lifecycle": {
          "create_before_destroy": true
        }
      }
    }
  }
}

The above is equivalent to the following native syntax configuration:

resource "aws_instance" "example" {
  lifecycle {
    create_before_destroy = true
  }
}

When the nested block type requires one or more labels, or when multiple blocks of the same type can be given, the mapping gets a little more complicated. For example, the provisioner nested block type used within resource blocks expects a label giving the provisioner to use, and the ordering of provisioner blocks is significant to decide the order of operations.

The following native syntax example shows a resource block with a number of provisioners of different types:

resource "aws_instance" "example" {
  # (resource configuration omitted for brevity)

  provisioner "local-exec" {
    command = "echo 'Hello World' >example.txt"
  }
  provisioner "file" {
    source      = "example.txt"
    destination = "/tmp/example.txt"
  }
  provisioner "remote-exec" {
    inline = [
      "sudo install-something -f /tmp/example.txt",
    ]
  }
}

In order to preserve the order of these blocks, you must use a JSON array as the direct value of the property representing this block type, as in this JSON equivalent of the above:

{
  "resource": {
    "aws_instance": {
      "example": {
        "provisioner": [
          {
            "local-exec": {
              "command": "echo 'Hello World' >example.txt"
            }
          },
          {
            "file": {
              "source": "example.txt",
              "destination": "/tmp/example.txt"
            }
          },
          {
            "remote-exec": {
              "inline": ["sudo install-something -f /tmp/example.txt"]
            }
          }
        ]
      }
    }
  }
}

Each element of the provisioner array is an object with a single property whose name represents the label for each provisioner block. For block types that expect multiple labels, this pattern of alternating array and object nesting can be used for each additional level.

If a nested block type requires labels but the order does not matter, you may omit the array and provide just a single object whose property names correspond to unique block labels. This is allowed as a shorthand for the above for simple cases, but the alternating array and object approach is the most general. We recommend using the most general form if systematically converting from native syntax to JSON, to ensure that the meaning of the configuration is preserved exactly.

Comment Properties​

Although we do not recommend hand-editing of JSON syntax configuration files -- this format is primarily intended for programmatic generation and consumption -- a limited form of comments are allowed inside JSON objects that represent block bodies using a special property name:

{
  "resource": {
    "aws_instance": {
      "example": {
        "//": "This instance runs the scheduled tasks for backup",

        "instance_type": "t2.micro",
        "ami": "ami-abc123"
      }
    }
  }
}

In any object that represents a block body, properties named "//" are ignored by OpenTF entirely. This exception does not apply to objects that are being interpreted as expressions, where this would be interpreted as an object type attribute named "//".

This special property name can also be used at the root of a JSON-based configuration file. This can be useful to note which program created the file.

{
  "//": "This file is generated by generate-outputs.py. DO NOT HAND-EDIT!",

  "output": {
    "example": {
      "value": "${aws_instance.example}"
    }
  }
}

Block-type-specific Exceptions​

Certain arguments within specific block types are processed in a special way by OpenTF, and so their mapping to the JSON syntax does not follow the general rules described above. The following sub-sections describe the special mapping rules that apply to each top-level block type.

resource and data blocks​

Some meta-arguments for the resource and data block types take direct references to objects, or literal keywords. When represented in JSON, the reference or keyword is given as a JSON string with no additional surrounding spaces or symbols.

For example, the provider meta-argument takes a <PROVIDER>.<ALIAS> reference to a provider configuration, which appears unquoted in the native syntax but must be presented as a string in the JSON syntax:

{
  "resource": {
    "aws_instance": {
      "example": {
        "provider": "aws.foo"
      }
    }
  }
}

This special processing applies to the following meta-arguments:

• provider: a single string, as shown above
• depends_on: an array of strings containing references to named entities, like ["aws_instance.example"].
• ignore_changes within the lifecycle block: if set to all, a single string "all" must be given. Otherwise, an array of JSON strings containing property references must be used, like ["ami"].

Special processing also applies to the type argument of any connection blocks, whether directly inside the resource block or nested inside provisioner blocks: the given string is interpreted literally, and not parsed and evaluated as a string template.

variable blocks​

All arguments inside variable blocks have non-standard mappings to JSON:

• type: a string containing a type expression, like "string" or "list(string)".
• default: a literal JSON value that can be converted to the given type. Strings within this value are taken literally and not interpreted as string templates.
• description: a literal JSON string, not interpreted as a template.

{
  "variable": {
    "example": {
      "type": "string",
      "default": "hello"
    }
  }
}

output blocks​

The description and sensitive arguments are interpreted as literal JSON values. The description string is not interpreted as a string template.

The value argument is interpreted as an expression.

{
  "output": {
    "example": {
      "value": "${aws_instance.example}"
    }
  }
}

locals blocks​

The value of the JSON object property representing the locals block type must be a JSON object whose property names are the local value names to declare:

{
  "locals": {
    "greeting": "Hello, ${var.name}"
  }
}

The value of each of these nested properties is interpreted as an expression.

module blocks​

The source and version meta-arguments must be given as literal strings. The values are not interpreted as string templates.

The providers meta-argument must be given as a JSON object whose properties are the compact provider addresses to expose into the child module and whose values are the provider addresses to use from the current module, both given as literal strings:

{
  "module": {
    "example": {
      "source": "hashicorp/consul/azurerm",
      "version": "= 1.0.0",
      "providers": {
        "aws": "aws.usw1"
      }
    }
  }
}

provider blocks​

The alias and version meta-arguments must be given as literal strings. The values are not interpreted as string templates.

{
  "provider": {
    "aws": [
      {
        "region": "us-east-1"
      },
      {
        "alias": "usw1",
        "region": "us-west-1"
      }
    ]
  }
}

terraform blocks​

Since no settings within terraform blocks accept named object references or function calls, all setting values are taken literally. String values are not interpreted as string templates.

Since only one backend block is allowed per terraform block, the compact block mapping can be used to represent it, with a nested object containing a single property whose name represents the backend type.

{
  "terraform": {
    "required_version": ">= 0.12.0",
    "backend": {
      "s3": {
        "region": "us-west-2",
        "bucket": "acme-opentf-states"
      }
    }
  }
}