Interpolation tokens, variables, and lookup functions

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Important: This is old documentation; read the new version instead.

Puppet 4.9 and later use Hiera 5, and its documentation is in the Puppet reference manual. See the following pages for more info:

We are preserving this Hiera 3 documentation for historical reference, but the Hiera 5 documentation is a complete replacement for it.

When writing Hiera’s settings and data, you can instruct it to look up values at run-time and insert them into strings. This lets you make dynamic data sources in the hierarchy, and avoid repeating yourself when writing data.

Interpolation tokens

Interpolation tokens look like %{variable}, %{variable.subkey}, or %{function("input")}. That is, they consist of:

  • A percent sign (%)
  • An opening curly brace ({)
  • One of:
    • A variable name, optionally including any number of subkeys
    • A lookup function and its input
  • A closing curly brace (})

If any setting in the config file or value in a data source contains an interpolation token, Hiera will replace the token with the value it refers to at run time.


  • Hiera can interpolate values of any of Puppet’s data types, but the value will be converted to a string. For arrays and hashes, this won’t fully match Puppet’s rules for interpolating non-string values, but it will be close.
  • In YAML files, any string containing an interpolation token must be quoted.

Interpolating normal variables

Hiera receives a set of variables whenever it is invoked, and it can insert them by name into any string. This is the default behavior; if the content of the interpolation token doesn’t match one of the lookup functions listed below, Hiera will treat it as a variable name.

smtpserver: "mail.%{::domain}"

See “Passing Variables to Hiera” below for details on how Hiera receives these variables.

Interpolating hash or array elements

If a variable is an array or a hash, you can interpolate a single element of it by putting a dot and a subkey after the variable name, like %{trusted.certname}.

A subkey can be an integer if the value is an array, or a key name if the value is a hash.

You can also chain subkeys together to access nested data structures. For example, to interpolate the current hours of uptime you could use %{facts.system_uptime.hours}.

Using lookup functions

Hiera currently has two lookup functions: scope() and hiera(). These are described in their own sections below.

To use a lookup function in an interpolation token, write the name of the function plus a pair of parentheses containing a quoted input value:

wordpress::database_server: "%{hiera('instances::mysql::public_hostname')}"

Notes on this syntax:

  • The input value must be surrounded by either single quotes (') or double quotes (").
    • In YAML or JSON, the quotes may be escaped with a backslash if they are embedded in the same kind of quotes; however, the unescaping is handled by the parser and not by Hiera itself.
  • The parentheses surrounding the quoted input are mandatory.
  • There must be no spaces between:
    • The function and the interpolation token’s curly braces
    • The function name and the opening parenthesis
    • The parentheses and the quoted input value
    • The quotes and the input value itself

The hiera() lookup function

The hiera() lookup function performs a Hiera lookup, using its input as the lookup key. The result of the lookup must be a string; any other result will cause an error.

This can be very powerful in Hiera’s data sources. By storing a fragment of data in one place and then using sub-lookups wherever it needs to be used, you can avoid repetition and make it easier to change your data.

wordpress::database_server: "%{hiera('instances::mysql::public_hostname')}"

The value looked up by the hiera() function may itself contain a hiera() lookup. (The function will detect any circular lookups and fail with an error instead of looping infinitely.)

Note: Using recursive lookups in Hiera’s config file is untested and can potentially cause infinite recursion. (See HI-220.) You should only use the hiera() function in data sources.

The scope() lookup function

The scope() lookup function interpolates variables; it works identically to variable interpolation as described above. The function’s input is the name of a variable surrounded by single or double quotes.

The following two values would be identical:

smtpserver: "mail.%{::domain}"
smtpserver: "mail.%{scope('::domain')}"

The alias() lookup function

The alias() function allows you to make one key in Hiera data act as an alias for another. This is different than just interpolating another key, since that results in a string.

  - 'a'
  - 'b'
aliased: "%{alias('original')}"

This returns an array with ['a', 'b'] for both the original and the aliased keys.

The data entry that uses the alias function cannot combine it with post/prefix text — that is, aliased: "%{alias('original')} -c" would be an illegal value.

The literal() lookup function

The literal lookup function allows you to escape ‘%{}’ in Hiera data. This is useful when you have data containing this literal string, as with some Apache variables like %{SERVER_NAME} or %{QUERY_STRING}.

With this function, %{literal('%')}{secret} returns %{secret}, without attempting to interpolate a variable named secret.

The only value you should pass to literal() is a single % sign.

Where to interpolate data

In data sources

The main use for interpolation is in the config file, where you can set dynamic data sources in the hierarchy:

  - "nodes/%{trusted.certname}"
  - "environment/%{server_facts.environment}"
  - "virtual/%{facts.is_virtual}"
  - common

In this example, every data source except the final one will vary depending on the current values of $trusted[certname], $server_facts[environment], and $facts[is_virtual].

In other settings

You can also interpolate variables into other settings, such as :datadir (in the YAML and JSON backends):

  :datadir: "/etc/puppetlabs/code/environments/%{environment}/hieradata"

This example would let you use completely separate data directories for your production and development environments.

In data

Within a data source, you can interpolate values into any string, whether it’s a standalone value or part of a hash or array value. This can be useful for values that should be different for every node, but which differ predictably:

# /etc/puppetlabs/code/environments/production/hieradata/common.yaml
smtpserver: "mail.%{::domain}"

In this example, instead of creating a %{::domain} hierarchy level and a data source for each domain, you can get a similar result with one line in the common data source.

You can also interpolate values into hash keys:

# /etc/puppetlabs/code/environments/production/hieradata/common.yaml
    fileset: MailServer
    bacula_schedule: 'CycleStandard'
    fileset: LDAP
    bacula_schedule: 'CycleStandard'

This generally only useful when building something complicated with the create_resources function, as it lets you interpolate values into resource titles.

Note: With YAML data sources, interpolating into hash keys means those hash keys must be quoted.

Note: This only works for keys that are part of a value; that is, you can’t use interpolation to dynamically create new Hiera lookup keys at the root of a data source.

# /etc/puppetlabs/code/environments/production/hieradata/common.yaml
# This isn't legal:
    fileset: MailServer
    bacula_schedule: 'CycleStandard'

Passing Variables to Hiera

Hiera’s variables can come from a variety of sources, depending on how Hiera is invoked.

From Puppet

When used with Puppet, Hiera automatically receives all of Puppet’s current variables. This includes facts and built-in variables, as well as local variables from the current scope. Most users will almost exclusively interpolate facts and built-in variables in their Hiera configuration and data.

  • Remove Puppet’s $ (dollar sign) prefix when using its variables in Hiera. (That is, a variable called $::clientcert in Puppet is called ::clientcert in Hiera.)
  • Puppet variables can be accessed by their short name or qualified name.

Best practices

  • Avoid referencing user-set local variables from Hiera. Instead, use facts, built-in variables, top-scope variables, node-scope variables, or variables from an ENC whenever possible.
  • When possible, reference variables by their fully-qualified names (e.g. %{::environment} and %{::clientcert}) to make sure their values are not masked by local scopes.

These two guidelines will make Hiera more predictable, and can help protect you from accidentally mingling data and code in your Puppet manifests.

From the command line

When called from the command line, Hiera defaults to having no variables available. You can specify individual variables, or a file or service from which to obtain a complete “scope” of variables. See command line usage for more details.

From Ruby

When calling Hiera from Ruby code, you can pass in a complete “scope” of variables as the third argument to the #lookup method. The complete signature of #lookup is:

hiera_object.lookup(key, default, scope, order_override=nil, resolution_type=:priority)
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