JSON Configuration Syntax

Waypoint configuration is usually written in HCL syntax which is designed to be relatively easy for humans to read and update. Waypoint also supports JSON. This syntax is useful when generating configurations programmatically, since existing JSON libraries can be used to prepare the generated configuration files.

Waypoint expects HCL syntax to be in waypoint.hcl and JSON syntax to be in waypoint.hcl.json. If both files exist, waypoint.hcl will be be loaded instead of the JSON version.

JSON File Structure

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

{
  "app": {
    "web": {
      "build": {
        "use": {
          "docker": {
            "dockerfile" = "${path.app}/Dockerfile"
          }
        }
      }
    }
  }
}

Each top-level object property must match the name of one of the expected top-level stanza types. Stanza types that expect labels, such as variable shown above, are represented by one nested object value for each level of label.

After any nested objects representing the labels, finally one more nested object represents the body of the stanza itself. In the above examples, the dockerfile parameter is specified.

Taken together, the above configuration file is equivalent to the following in HCL:

app "web" {
  build {
    use "docker" {
      dockerfile = "${path.app}/Dockerfile"
    }
  }
}

Expression Mapping

Since JSON grammar is not able to represent all of the HCL 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 stanza 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.

Comments

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 stanza bodies using a special property name:

{
  "app": {
    "web": {
      "//": "This app is our main frontend.",

      "build": {}
    }
  }
}

In any object that represents a stanza body, properties named "//" are ignored by Waypoint 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-waypoint.py. DO NOT HAND-EDIT!",

  "app": {
    "web": {
      "build": {}
    }
  }
}

Nested Stanza Mapping

When a JSON object property is named after a nested stanza type, the value of this property represents one or more stanzas 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 stanza of the given type when that type expects no labels, as with the build nested stanza used within app stanzas:

{
  "app": {
    "web": {
      "build": {}
    }
  }
}

The above is equivalent to the following native syntax configuration:

app "web" {
  build {}
}

When the nested stanza type requires one or more labels, or when multiple stanzas of the same type can be given, the mapping gets a little more complicated. For example, the hook nested stanza type used within the build stanza can be repeated and the ordering of hook stanzas is significant to decide the order of operations.

The following native syntax example shows a hook stanza with a number of invocations:

app "web" {
  build {
    hook {
      when    = "before"
      command = ["./validate-creds.sh"]
    }

    hook {
      when    = "before"
      command = ["./prepare-files.sh"]
    }
  }
}

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

{
  "app": {
    "web": {
      "build": {
        "hook": [
          {
            "when": "before",
            "command": ["./validate-creds.sh"]
          },
          {
            "when": "before",
            "command": ["./prepare-files.sh"]
          }
        ]
      }
    }
  }
}