template Block

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 Go template and a tool called Consul Template, which adds a set of new functions that can be used to retrieve data from Consul and Vault. Nomad templates can reference Nomad's runtime environment variables, node attributes and metadata, Nomad service registrations, and Nomad variables. Templates can also be used to provide environment variables to your workload.

For a full list of the API template functions, please refer to the Consul Template documentation. For a an introduction to Go templates, please refer to the Learn Go Template Syntax guide.

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 following possible values 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
  • "script" - run a script

• 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.

• change_script (ChangeScript: nil) - Configures the script triggered on template change. This option is required if the change_mode is script.

• 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. Note that where possible, the NOMAD_SECRETS_DIR is mounted noexec, so rendered templates can't be used as self-executing scripts.

• env (bool: false) - Specifies the template should be read back in as environment variables for the task (example). To update the environment on changes, you must set change_mode to restart. Setting env when the change_mode is signal will return a validation error. Setting env when the change_mode is noop is permitted but will not update the environment variables in the task.

• error_on_missing_key (bool: false) - Specifies how the template behaves when attempting to index a map key that does not exist in the map.

  • When true, the template engine will return an error, which will cause the task to fail.

  • When false, the template engine will do nothing and continue executing the template. If printed, the result of the index operation is the string "<no value>".

• 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.

• uid (int: nil) - Specifies the rendered template owner's user ID. If negative or not specified (nil) the ID of the Nomad agent user will be used.

  Caveat: Works only on Unix-based systems. Be careful when using containerized drivers, such as docker or podman, as groups and users inside the container may have different IDs than on the host system. This feature will also not work with Docker Desktop.

• gid (int: nil) - Specifies the rendered template owner's group ID. If negative or not specified (nil) the ID of the Nomad agent group will be used.

  Caveat: Works only on Unix-based systems. Be careful when using containerized drivers, such as docker or podman, as groups and users inside the container may have different IDs than on the host system. This feature will also not work with Docker Desktop.

• 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 be fetched using an artifact resource. The template must exist in the task working directory prior to starting the task; it is not possible to reference a template whose source is inside 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.

• wait (Code: nil) - Defines the minimum and maximum amount of time to wait for the Consul cluster to reach a consistent state before rendering a template. This is useful to enable in systems where network connectivity to Consul is degraded, because it will reduce the number of times a template is rendered. This setting can be overridden by the client.template.wait_bounds. If the template configuration has a min lower than client.template.wait_bounds.min or a max greater than client.template.wait_bounds.max, the client's bounds will be enforced, and the template wait will be adjusted before being sent to the template engine.

  wait {
    min     = "5s"
    max     = "10s"
  }
  

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

template Examples

The following examples only show the template blocks. Remember that the template block 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 block 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"
}

Task meta values

To render values from a task's meta config, use the environment variable form of the meta variable name.

meta {
  mykey = "some_value"
}

template {
  data = <<EOH
  {{ env "NOMAD_META_mykey" }}
EOH
}

Node Variables

Use the env function to access the Node's attributes and metadata inside a template. Note the meta. syntax here applies only to node meta fields.

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

Templates may be used to create environment variables for tasks. These 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 block:

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 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 the 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 work around 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"
    mount {
      type   = "bind"
      source = "local"
      target = "/etc/redis.d"
    }
  }

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

Dependencies

For templates that read from Vault, Consul, or Nomad, each item read is called a "dependency". All the template blocks share the same internal runner which de-duplicates dependencies requesting the same item. You should avoid having large numbers of dependencies for a given task, as each dependency requires at least one concurrent request (a possibly blocking query) to the upstream server. If a task has more than 128 dependencies, a warn-level log will appear in the Nomad client logs which reports "watching this many dependencies could DDoS your servers", referring to the Vault, Consul, or Nomad cluster being queried.

Nomad Integration

Nomad Services

Nomad service registrations can be queried using the nomadService and nomadServices functions. The requests are tied to the same namespace as the job which contains the template block.

  template {
    data = <<EOF
# Configuration for a single NGINX upstream service.
upstream my_app {
  {{- range nomadService "my-app" }}
  server {{ .Address }}:{{ .Port }};{{- end }}
}

# Configuration for all services registered in Nomad as an NGINX upstream
# service.
{{ range nomadServices }}
# Configuration for service {{ .Name }}.
upstream {{ .Name | toLower }} {
  {{- range nomadService .Name }}
  server {{ .Address}}:{{ .Port }};{{- end }}
}
{{ end -}}
EOF

    destination = "local/nginx.conf"
  }

Simple Load Balancing with Nomad Services

The nomadService function now supports simple load balancing by selecting instances of a service via rendezvous hashing. To enable simple load balancing, the nomadService function requires 3 arguments.

• The number of services to select
• The hashing key (should be unique, but consistent per requester)
• The service name

By using NOMAD_ALLOC_ID as the hashing key, the selected instances will remain mostly stable for the allocation. Each time the template is run, nomadService will return the same set of instances for each allocation - unless N instances of the service are added or removed, in which case there is a 1/N chance of a selected instance being replaced. This helps maintain a more consistent output when rendering configuration files, triggering fewer restarts and signaling of Nomad tasks.

template {
  data        = <<EOH
# Configuration for 1 redis instances, as assigned via rendezvous hashing.
{{$allocID := env "NOMAD_ALLOC_ID" -}}
{{range nomadService 1 $allocID "redis"}}
  server {{ .Address }}:{{ .Port }};
{{- end}}
EOH
}

Nomad Variables

Nomad variables can be queried using the nomadVar, nomadVarList, nomadVarListSafe, and nomadVarExists functions.

nomadVarList and nomadVarListSafe

These functions can be used to list the paths of Nomad variables available to the task based on its workload identity. You can provide an optional prefix to filter the path list by as a parameter.

The list functions return a slice of NomadVarMeta type:

type NomadVarMeta struct {
  Namespace, Path          string
  CreateIndex, ModifyIndex uint64
  CreateTime, ModifyTime   nanoTime
}

The nanoTime type contains Unix nanoseconds since the epoch. Its string method prints it out as a formatted date/time value. It also has a Time method that can be used to retrieve a Go time.Time for further manipulation.

NomadVarMeta objects print their Path value when used as a string. For example, these two template blocks produce identical output.

template {
  data        = <<EOH
{{ range nomadVarList }}
  {{ . }}
{{ end }}
EOH
}

template {
  data        = <<EOH
{{ range nomadVarList }}
  {{ .Path }}
{{ end }}
EOH
}

You can provide a prefix to filter the path list as an optional parameter to the nomadVarList function.

template {
  data        = <<EOH
{{ range nomadVarList "path/to/filter" }}
  {{ . }}
{{ end }}
EOH
}

By default, the nomadVarList will list variables in the same namespace as the task. The path filter can change the namespace by adding a suffix separated by the @ character:

template {
  data        = <<EOH
{{ range nomadVarList "path/to/filter@example_namespace" }}
  {{ . }}
{{ end }}
EOH
}

The nomadVarListSafe function works identically to nomadVarList, but refuses to render the template if the variable list query returns blank/empty data.

nomadVar

These functions can be used to a read Nomad variable, assuming the task has access rights to the variable based on the task's workload identity. If the path does not exist or the caller does not have access to it, the template renderer will block until the path is available. To avoid blocking, wrap nomadVar calls with nomadVarExists.

The nomadVar function returns a map of string to NomadVarItems structs. Each member of the map corresponds to a member of the variable's Items collection.

For example, given a variable exists at the path nomad/jobs/redis, with a single key/value pair in its Items collection—maxconns:15

template {
  data        = <<EOH
{{ with nomadVar "nomad/jobs/redis" }}{{ .maxconns }}{{ end }}
EOH
}

renders

15

Each map value also contains helper methods to get the variable metadata and a link to the parent variable:

• Keys: produces a sorted list of keys to this NomadVarItems map.

• Values: produces a key-sorted list.

• Tuples: produces a key-sorted list of K,V tuple structs, with accessors .K and .V for the key and value respectively.

• Metadata: returns this collection's parent metadata as a NomadVarMeta

• Parent: returns a parent object that has a Metadata field referring to the NomadVarMeta and an Items field that refers to this NomadVarItems object.

For example, given a variable exists at the path nomad/jobs/redis, you could render some of its metadata as follows:

template {
  data        = <<EOH
{{ with nomadVar "nomad/jobs/redis" }}
Path:       {{ .Metadata.Path }}
Namespace:  {{ .Metadata.Namespace }}
CreateTime: {{ .Metadata.CreateTime }}
ModifyTime: {{ .Metadata.ModifyTime }}
{{ end }}
EOH
}

By default, the nomadVar function reads a variable in the same namespace as the task. The path filter can change the namespace by adding a suffix separated by the @ character.

template {
  data        = <<EOH
{{ with nomadVar "nomad/jobs/redis@example_namespace" }}{{ .maxconns }}{{ end }}
EOH
}

nomadVarExists

This function can be used to check if a Nomad variable exists at the provided path, assuming the task has access rights to the variable based on the task's workload identity. If a variable exists, this will return true, false otherwise. Unlike nomadVar, this function will not block if the variable does not exist, which can be useful for controlling flow.

For example:

template {
  data        = <<EOH
{{ if nomadVarExists "app/beta_active" }}
  # ...
{{ else }}
  # ...
{{ end }}
EOH
}

Consul Integration

Consul KV

Consul KV values can be accessed using the key function to retrieve a single value from a key path. The ls function can be used to retrieve all keys in a path. For deeply nested paths, use the tree function.

  template {
    data = <<EOF
# Read single key from Consul KV.
APP_NAME = "{{key "app/name"}}"

# Read all keys in the path `app/environment` from Consul KV.
{{range ls "app/environment"}}
{{.Key}}={{.Value}}
{{end}}
    EOF

    destination = "local/env"
    env         = true
  }

Consul Services

The Consul service catalog can be queried using the service and services functions. For Connect-capable services, use the connect function.

  template {
    data = <<EOF
# Configuration for a single upstream service.
upstream my_app {
  {{- range service "my-app" }}
  server {{ .Address }}:{{ .Port }};{{- end }}
}

# Configuration for all services in the catalog.
{{ range services }}
# Configuration for service {{ .Name }}.
upstream {{ .Name | toLower }} {
  {{- range service .Name }}
  server {{ .Address}}:{{ .Port }};{{- end }}
}
{{ end -}}
    EOF

    destination = "local/nginx.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 pkiCert "pki/issue/foo" "common_name=foo.service.consul" "ip_sans=127.0.0.1" }}
{{- .Cert -}}
{{ end }}
EOH
  destination   = "${NOMAD_SECRETS_DIR}/certificate.crt"
  change_mode   = "restart"
}

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

template {
  data = <<EOH
{{ with pkiCert "pki/issue/foo" "common_name=foo.service.consul" "ip_sans=127.0.0.1" }}
{{- .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 pkiCert "pki/issue/foo" "common_name=foo.service.consul" "ip_sans=127.0.0.1" "format=pem" }}
{{ .Cert }}
{{ .CA }}
{{ .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/....

Like 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.