Variable Interpolation

This page provides reference information on interpreting node attributes and runtime environment variables in a Nomad workload. Learn the syntax for interpreting variables in a Nomad job specification. Review node attributes for clients, hardware, platform, and OS. Review Review job, network, and Consul environment variables.

Introduction

Nomad supports interpreting two classes of variables: node attributes and runtime environment variables. Node attributes are interpretable in constraints, task environment variables, and certain driver fields. Runtime environment variables are not interpretable in constraints because they are only defined once the scheduler has placed them on a particular node.

Nomad supports interpreting two classes of variables: node attributes and runtime environment variables. Node attributes are interpretable in constraints, task environment variables, and certain task driver fields—for example the labels attribute of the Docker config.

Syntax

The syntax for interpreting variables in the Nomad job specification is ${variable_name}. The template block uses the env function to retrieve these variables from the environment, using {{env "variable_name"}} instead. Examples can be seen below:

task "docs" {
  driver = "docker"

  # Drivers support interpreting node attributes and runtime environment
  # variables
  config {
    image = "my-app"

    # Interpret runtime variables to inject the address to bind to and the
    # location to write logs to.
    args = [
      "--bind", "${NOMAD_ADDR_RPC}",
      "--logs", "${NOMAD_ALLOC_DIR}/logs",
    ]

    port_map {
      RPC = 6379
    }
  }

  # Constraints only support node attributes as runtime environment variables
  # are only defined after the task is placed on a node.
  constraint {
    attribute = "${attr.kernel.name}"
    value     = "linux"
  }

  template {
    destination = "template.txt"
    data = <<EOT
{{- /*
      Environment variables are available to templates via the env function,
      rather than the ${...} syntax.
*/ -}}
Running on {{env "attr.unique.hostname"}}.
EOT
  }

  # Environment variables are interpreted and can contain both runtime and
  # node attributes. These environment variables are passed into the task.
  env {
    DC      = "Running on datacenter ${node.datacenter}"
    VERSION = "Version ${NOMAD_META_VERSION}"
  }

  # Meta keys are also interpretable.
  meta {
    VERSION = "v0.3"
  }
}

Node Attributes

Below is a full listing of node attributes that are interpretable. These attributes are interpreted by both constraints and within the task and driver.

Below is a table documenting common node properties.

The full list of node attributes can be obtained by running nomad node status -verbose [node].

Here are some examples of using node attributes and properties in a job file:

job "docs" {
  # This will constrain this job to only run on 64-bit clients.
  constraint {
    attribute = "${attr.cpu.arch}"
    value     = "amd64"
  }

  # This will restrict the job to only run on clients with 4 or more cores.
  # Note: you may also declare a resource requirement for CPU for a task.
  constraint {
    attribute = "${cpu.numcores}"
    operator  = ">="
    value     = "4"
  }

  # Only run this job on a memory-optimized AWS EC2 instance.
  constraint {
    attribute = "${attr.platform.aws.instance-type}"
    value     = "m4.xlarge"
  }
}

Environment Variables

The following are runtime environment variables that describe the environment the task is running in. These are only defined once the task has been placed on a particular node and as such can not be used in constraints.

Environment variables should be enclosed in brackets ${...} for interpolation or accessed using the env function inside the template block—{{env "..."}}

Dots in Variables

Nomad interprets dots in names as object notation. This causes names that have multiple consecutive dots to be considered invalid. For example, an environment variable named invalid...name cannot be interpolated using the standard "${invalid...name}" syntax. If you do, the parser will return an Extra characters after interpolation expression error. Nomad provides a variable—env—that can access any environment variable, regardless of its name, using index syntax.

job "sample" {
  datacenters = ["dc1"]
  group "g1" {
    task "redis" {

      # Note: to set an environment variable with an invalid name, you must
      # use the HCL2 map assignment syntax for `env`. Otherwise, the job spec
      # parser will throw an `Argument or block definition required` error
      env = {
        "invalid...name" = "value1"
        "valid.name"     = "value2"
      }

      driver = "docker"
      config {
        image  = "redis:7"
        labels {
          label1 = "${env["invalid...name"]}"
          label2 = "${valid.name}"
        }
      }
    }
  }
}

Job-related variables

Network-related Variables

Consul-related Variables