HCL2

Nomad uses the Hashicorp Configuration Language - HCL - designed to allow concise descriptions of the required steps to get to a job file. Nomad 1.0 adopts HCL2, the second generation of HashiCorp Configuration Language. HCL2 extends the HCL language by adding expressions and input variables support to improve job spec reusability and readability. Also, the new HCL2 parser improves the error messages for invalid jobs.

HCL Parsing Context

The Nomad API uses JSON, not HCL, to represent Nomad jobs. When running commands like nomad job run and nomad job plan, the Nomad CLI parses HCL and ultimately converts it to JSON. Because this parsing happens locally (i.e., where the operator is running the CLI) before job submission, there are some limits to the capabilities that can be accessed by HCL job specifications. For example, scheduling information is not yet available, including information about the client. Similarly, HCL features that depend on external context will take that context from the local environment of the CLI (e.g., files, environment variables).

JSON Jobs

Since HCL is a superset of JSON, nomad job run example.json will attempt to parse a JSON job using the HCL parser. However, the JSON format accepted by the HCL parser is not the same as the API's JSON format. The HCL parser's JSON format is unspecified, so the API format is preferred. You can use the API format with the -json command line flag:

$ # Generate an HCL formatted job
$ nomad init

$ # Convert HCL to API JSON format
$ nomad job run -output example.nomad.hcl > example.json

$ # Submit with the -json flag
$ nomad job run -json example.json

Arguments, Blocks, and Expressions

The syntax of the HCL language consists of only a few basic elements:

task "example" {
  driver = "docker"
}

<BLOCK TYPE> "<BLOCK LABEL>" {
  # Block body
  <IDENTIFIER> = <EXPRESSION> # Argument
}

Blocks are containers for other content and usually represent the configuration of some kind of object, like a task. Blocks have a block type, can have zero or more labels, and have a body that contains any number of arguments and nested blocks. Block labels must be string literals.
Arguments assign a value to a name. They appear within blocks.
Expressions represent a value, either literally or by referencing and combining other values. They appear as values for arguments, or within other expressions.

For full details about Nomad's syntax, see:

Blocks

Block syntax is as follows, using unquoted attributes and quoted values:

meta {
  team         = "..."
  organization = "..."
}

Additionally, block attributes must be HCL2 valid identifiers. Generally, identifiers may only contain letters, numbers, underscore _, or a dash -, and start with a letter. Notable, meta, and env keys may not contain other symbols (e.g. ., #).

Task driver config fields may require extra attention if they contain invalid identifiers. For example, docker sysctl must use the map assignment syntax if the keys aren't valid:

sysctl = {
  "net.core.somaxconn/docs/drivers/docker#sysctl" = "16384"
}

Additionally, task driver config fields may not nest block syntax within an assignment syntax. The following mounts syntax is no longer valid:

# INVALID in Nomad 1.0
mounts = [
  {
    type = "tmpfs"
    tmpfs_options {  # <- block syntax is not valid here
      size = 10000
    }
  }
]

Here, the tmpfs_options block declaration is invalid HCL2 syntax, and must be an assignment instead:

# VALID in Nomad 1.0
mounts = [
  {
    type = "tmpfs"
    tmpfs_options = {
      size = 10000
    }
  }
]

Or better yet, the new mount syntax, introduced in Nomad 1.0.1, is more appropriate here:

mount {
  type = "tmpfs"
  tmpfs_options {
    size = 10000
  }
}

Multiline "here doc" string

Nomad supports multi-line string literals in the so-called "heredoc" style, inspired by Unix shell languages:

template {
  data = <<EOF
hello
  world
  EOF
}

HCL2 trims the whitespace preceding the delimiter in the last line. So in the above example, data is read as "hello\n world\n " in HCL1, but "hello\n world\n" (note lack of trailing whitespace) in HCL2.

Decimals

HCL2 requires a leading zero for decimal values lower than 1 (e.g. 0.3, 0.59, 0.9).