open-nomad/website/source/docs/runtime/interpolation.html.md

layout	page_title	sidebar_current	description
docs	Interpolation - Runtime	docs-runtime-interpolation	Learn about the Nomad's interpolation and interpreted variables.

Interpolation

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.

The syntax for interpreting variables is ${variable}. An example and a comprehensive list of interpretable fields 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"
  }

  # Environment variables are interpreted and can contain both runtime and
  # node attributes. There 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 Variables

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

Variable	Description	Example Value
${node.unique.id}	36 character unique client identifier	9afa5da1-8f39-25a2-48dc-ba31fd7c0023
${node.datacenter}	Client's datacenter	dc1
${node.unique.name}	Client's name	nomad-client-10-1-2-4
${node.class}	Client's class	linux-64bit
${attr.<property>}	Property given by property on the client	${attr.cpu.arch} => amd64
${meta.<key>}	Metadata value given by key on the client	${meta.foo} => bar

Below is a table documenting common node properties:

Property	Description
${attr.cpu.arch}	CPU architecture of the client (e.g. amd64, 386)
${attr.consul.datacenter}	The Consul datacenter of the client (if Consul is found)
${attr.cpu.numcores}	Number of CPU cores on the client
${attr.driver.<property>}	See the [task drivers](/docs/drivers/index.html) for property documentation
${attr.unique.hostname}	Hostname of the client
${attr.unique.network.ip-address}	The IP address fingerprinted by the client and from which task ports are allocated
${attr.kernel.name}	Kernel of the client (e.g. linux, darwin)
${attr.kernel.version}	Version of the client kernel (e.g. 3.19.0-25-generic, 15.0.0)
${attr.platform.aws.ami-id}	AMI ID of the client (if on AWS EC2)
${attr.platform.aws.instance-type}	Instance type of the client (if on AWS EC2)
${attr.os.name}	Operating system of the client (e.g. ubuntu, windows, darwin)
${attr.os.version}	Version of the client OS

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.

Variable	Description
${NOMAD_ALLOC_DIR}	The path to the shared alloc/ directory. See [here](/docs/runtime/environment.html#task-directories) for more information.
${NOMAD_TASK_DIR}	The path to the task local/ directory. See [here](/docs/runtime/environment.html#task-directories) for more information.
${NOMAD_MEMORY_LIMIT}	The memory limit in MBytes for the task
${NOMAD_CPU_LIMIT}	The CPU limit in MHz for the task
${NOMAD_ALLOC_ID}	The allocation ID of the task
${NOMAD_ALLOC_NAME}	The allocation name of the task
${NOMAD_ALLOC_INDEX}	The allocation index; useful to distinguish instances of task groups. From 0 to (count - 1).
${NOMAD_TASK_NAME}	The task's name
${NOMAD_JOB_NAME}	The job's name
${NOMAD_IP_<label>}	The IP for the given port label. See [here](/docs/job-specification/network.html) for more information.
${NOMAD_PORT_<label>}	The port for the port label. See [here](/docs/job-specification/network.html) for more information.
${NOMAD_ADDR_<label>}	The ip:port pair for the given port label. See [here](/docs/job-specification/network.html) for more information.
${NOMAD_HOST_PORT_<label>}	The port on the host if port forwarding is being used for the port label. See [here](/docs/job-specification/network.html#mapped_ports) for more information.
${NOMAD_META_<key>}	The metadata value given by key on the task's metadata
${"env_key"}	Interpret an environment variable with key env_key set on the task.