»template Stanza

Placementjob -> group -> task -> template

The template block instantiates an instance of a template renderer. This creates a convenient way to ship configuration files that are populated from environment variables, Consul data, Vault secrets, or just general configurations within a Nomad task.

job "docs" {
  group "example" {
    task "server" {
      template {
        source        = "local/redis.conf.tpl"
        destination   = "local/redis.conf"
        change_mode   = "signal"
        change_signal = "SIGINT"
      }
    }
  }
}

Nomad utilizes a tool called Consul Template. Since Nomad v0.5.3, the template can reference Nomad's runtime environment variables. Since Nomad v0.5.6, the template can reference Node attributes and metadata. For a full list of the API template functions, please refer to the Consul Template README. Since Nomad v0.6.0, templates can be read as environment variables.

»template Parameters

  • change_mode (string: "restart") - Specifies the behavior Nomad should take if the rendered template changes. Nomad will always write the new contents of the template to the specified destination. The possible values below describe Nomad's action after writing the template to disk.

    • "noop" - take no action (continue running the task)
    • "restart" - restart the task
    • "signal" - send a configurable signal to the task
  • change_signal (string: "") - Specifies the signal to send to the task as a string like "SIGUSR1" or "SIGINT". This option is required if the change_mode is signal.

  • data (string: "") - Specifies the raw template to execute. One of source or data must be specified, but not both. This is useful for smaller templates, but we recommend using source for larger templates.

  • destination (string: <required>) - Specifies the location where the resulting template should be rendered, relative to the task working directory. Only drivers without filesystem isolation (ex. raw_exec) or that build a chroot in the task working directory (ex. exec) can render templates outside of the NOMAD_ALLOC_DIR, NOMAD_TASK_DIR, or NOMAD_SECRETS_DIR. For more details on how destination interacts with task drivers, see the Filesystem internals documentation.

  • env (bool: false) - Specifies the template should be read back in as environment variables for the task. (See below) Note that if noop mode is used for the change_mode, the task will not be restarted and the environment variables wwon't be changed in the task. If you set env = true, then the change_mode should be one of restart or signal.

  • left_delimiter (string: "{{") - Specifies the left delimiter to use in the template. The default is "{{" for some templates, it may be easier to use a different delimiter that does not conflict with the output file itself.

  • perms (string: "644") - Specifies the rendered template's permissions. File permissions are given as octal of the Unix file permissions rwxrwxrwx.

  • right_delimiter (string: "}}") - Specifies the right delimiter to use in the template. The default is "}}" for some templates, it may be easier to use a different delimiter that does not conflict with the output file itself.

  • source (string: "") - Specifies the path to the template to be rendered. One of source or data must be specified, but not both. This source can optionally be fetched using an artifact resource. This template must exist on the machine prior to starting the task; it is not possible to reference a template inside of a Docker container, for example.

  • splay (string: "5s") - Specifies a random amount of time to wait between 0 ms and the given splay value before invoking the change mode. This is specified using a label suffix like "30s" or "1h", and is often used to prevent a thundering herd problem where all task instances restart at the same time.

  • vault_grace (string: "15s") - Deprecated

»template Examples

The following examples only show the template stanzas. Remember that the template stanza is only valid in the placements listed above.

»Inline Template

This example uses an inline template to render a file to disk. This file watches various keys in Consul for changes:

template {
  data        = "---\nkey: {{ key \"service/my-key\" }}"
  destination = "local/file.yml"
}

It is also possible to use heredocs for multi-line templates, like:

template {
  data = <<EOH
  ---
    bind_port:   {{ env "NOMAD_PORT_db" }}
    scratch_dir: {{ env "NOMAD_TASK_DIR" }}
    node_id:     {{ env "node.unique.id" }}
    service_key: {{ key "service/my-key" }}
  EOH

  destination = "local/file.yml"
}

»Remote Template

This example uses an artifact stanza to download an input template before passing it to the template engine:

artifact {
  source      = "https://example.com/file.yml.tpl"
  destination = "local/file.yml.tpl"
}

template {
  source      = "local/file.yml.tpl"
  destination = "local/file.yml"
}

»Node Variables

As of Nomad v0.5.6 it is possible to access the Node's attributes and metadata.

template {
  data = <<EOH
  ---
    node_dc:    {{ env "node.datacenter" }}
    node_cores: {{ env "attr.cpu.numcores" }}
    meta_key:   {{ env "meta.node_meta_key" }}
  EOH

  destination = "local/file.yml"
}

»Environment Variables

Since v0.6.0 templates may be used to create environment variables for tasks. Env templates work exactly like other templates except once the templates are written, they are parsed as KEY=value pairs. Those key value pairs are included in the task's environment.

For example the following template stanza:

template {
  data = <<EOH
# Lines starting with a # are ignored

# Empty lines are also ignored
LOG_LEVEL="{{key "service/geo-api/log-verbosity"}}"
API_KEY="{{with secret "secret/geo-api-key"}}{{.Data.value}}{{end}}"
EOH

  destination = "secrets/file.env"
  env         = true
}

The task's environment would then have environment variables like the following:

LOG_LEVEL=DEBUG
API_KEY=12345678-1234-1234-1234-1234-123456789abc

This allows 12factor app style environment variable based configuration while keeping all of the familiar features and semantics of Nomad templates.

Secrets or certificates may contain a wide variety of characters such as newlines, quotes, and backslashes which may be difficult to quote or escape properly.

Whenever a templated variable may include special characters, use the toJSON function to ensure special characters are properly parsed by Nomad:

CERT_PEM={{ file "path/to/cert.pem" | toJSON }}

The parser will read the JSON string, so the $CERT_PEM environment variable will be identical to the contents of the file.

Likewise when evaluating a password that may contain quotes or #, use the toJSON function to ensure Nomad passes the password to task unchanged:

# Passwords may contain any character including special characters like:
#   \"'#
# Use toJSON to ensure Nomad passes them to the environment unchanged.
{{ with secret "secrets/data/application/backend" }}
DB_PASSWD={{ .Data.data.DB_PASSWD | toJSON }}
{{ end }}

For more details see go-envparser's README.

»Template Destinations

Templates are rendered into the task working directory. Drivers without filesystem isolation (such as raw_exec) or drivers that build a chroot in the task working directory (such as exec) can have templates rendered to arbitrary paths in the task. But task drivers such as docker can only access templates rendered into the NOMAD_ALLOC_DIR, NOMAD_TASK_DIR, or NOMAD_SECRETS_DIR. To workaround this restriction, you can create a mount from the template destination to another location in the task.

task "task" {
  driver = "docker"

  config {
    image = "redis:6.0"
    mounts = [
      {
        type   = "bind"
        source = "local"
        target = "/etc/redis.d"
      }
    ]
  }

  template {
    destination = "local/redis.conf"
  }
}

»Vault Integration

»PKI Certificate

Vault is a popular open source tool for managing secrets. In addition to acting as an encrypted KV store, Vault can also generate dynamic secrets, like PKI/TLS certificates.

When generating PKI certificates with Vault, the certificate, private key, and any intermediate certs are all returned as part of the same API call. Most software requires these files be placed in separate files on the system.

»As individual files

For templates, all dependencies are mapped into a single list. This means that multiple templates watching the same path return the same data.

template {
  data = <<EOH
{{ with secret "pki/issue/foo" "common_name=foo.service.consul" "ip_sans=127.0.0.1" }}
{{- .Data.certificate -}}
{{ end }}
EOH
  destination   = "${NOMAD_SECRETS_DIR}/certificate.crt"
  change_mode   = "restart"
}

template {
  data = <<EOH
{{ with secret "pki/issue/foo" "common_name=foo.service.consul" "ip_sans=127.0.0.1" }}
{{- .Data.issuing_ca -}}
{{ end }}
EOH
  destination   = "${NOMAD_SECRETS_DIR}/ca.crt"
  change_mode   = "restart"
}

template {
  data = <<EOH
{{ with secret "pki/issue/foo" "common_name=foo.service.consul" "ip_sans=127.0.0.1" }}
{{- .Data.private_key -}}
{{ end }}
EOH
  destination   = "${NOMAD_SECRETS_DIR}/private_key.key"
  change_mode   = "restart"
}

These are three different input templates, but when run under the Nomad job, they are compressed into a single call, sharing the resulting data.

»As a PEM formatted file

This example acquires a PKI certificate from Vault in PEM format, concatenates the elements into a bundle, and stores it into your application's secret directory.

template {
  data = <<EOH
{{ with secret "pki/issue/foo" "common_name=foo.service.consul" "ip_sans=127.0.0.1" "format=pem" }}
{{ .Data.certificate }}
{{ .Data.issuing_ca }}
{{ .Data.private_key }}{{ end }}
EOH
  destination   = "${NOMAD_SECRETS_DIR}/bundle.pem"
  change_mode   = "restart"
}

»Vault KV API v1

Under Vault KV API v1, paths start with secret/, and the response returns the raw key/value data. This secret was set using vault kv put secret/aws/s3 aws_access_key_id=somekeyid.

  template {
    data = <<EOF
      AWS_ACCESS_KEY_ID = "{{with secret "secret/aws/s3"}}{{.Data.aws_access_key_id}}{{end}}"
    EOF
  }

Note that if the name of a secret includes the - character, you must access it by index. This secret was set using vault kv put secret/app db-password=somepassword.

  template {
    data = <<EOF
      DB_PASSWORD = "{{with secret "secret/app"}}{{index .Data "db-password"}}{{end}}"
    EOF
  }

»Vault KV API v2

Under Vault KV API v2, paths start with secret/data/, and the response returns metadata in addition to key/value data. This secret was set using vault kv put secret/aws/s3 aws_access_key_id=somekeyid.

  template {
    data = <<EOF
      AWS_ACCESS_KEY_ID = "{{with secret "secret/data/aws/s3"}}{{.Data.data.aws_access_key_id}}{{end}}"
    EOF
  }

Notice the addition of data in both the path and the field accessor string. Additionally, when using the Vault v2 API, the Vault policies applied to your Nomad jobs will need to grant permissions to read under secret/data/... rather than secret/....

Similar to KV API v1, if the name of a secret includes the - character, you must access it by index. This secret was set using vault kv put secret/app db-password=somepassword.

  template {
    data = <<EOF
      DB_PASSWORD = "{{with secret "secret/data/app"}}{{index .Data.data "db-password"}}{{end}}"
    EOF
  }

»Client Configuration

The template block has the following client configuration options:

  • function_denylist ([]string: ["plugin"]) - Specifies a list of template rendering functions that should be disallowed in job specs. By default the plugin function is disallowed as it allows running arbitrary commands on the host as root (unless Nomad is configured to run as a non-root user).

  • disable_file_sandbox (bool: false) - Allows templates access to arbitrary files on the client host via the file function. By default templates can access files only within the task working directory.