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From Wikitech

Hiera is a component of Puppet which allows storing configuration data outside of manifests. See Puppet coding for more general information about how, when, and where to use Puppet and Hiera on the Wikimedia cluster, and the upstream Hiera documentation for more general information about Hiera.


Every variable that puppet looks up in hiera will be searched via one or more backends according to what we configured in the hiera.yaml for that environment (cf. production, cloud VPS, and pontoon).

Depending on the kind of search (string, array or hash), hiera will search hierarchically within the sources using the configured Merge Behaviour. The default merge behaviour is first which means use the first entry found. Its also worth noting that the merge behaviour only affects hashes and arrays.

Our production hiera config searches for entries using the order shown in the following code snippet. Please note that the structure below is a simplified version of the main hiera file, showing the expanded paths and the hiera backend used. Check the file in git to see the actual content. The paths listed below refer to the paths on the the production puppetmaster. The hieradata directory referenced below is the hieradata directory from the puppet repo.

  - backend: standard
    path: "/etc/puppet/hieradata/hosts/%{::hostname}.yaml"
  - backend: standard
    path: "/etc/puppet/netbox/hieradata/hosts/%{::hostname}"
  - backend: wmflib::regex
    path: "/etc/puppet/hieradata/hosts/regex.yaml"
  - backend: wmflib::expand_path
    path:  "/etc/puppet/private/hieradata/%{::site}"
  - backend: wmflib::expand_path
    path: "/etc/puppet/hieradata/%{::site}"
  - backend: standard
    path: "/etc/puppet/hieradata/role/%{::site}/%{::_role}.yaml"
  - backend: standard
    path: "/etc/puppet/hieradata/role/common/%{::_role}.yaml"
  - backend: standard
    path: "/etc/puppet/private/hieradata/role/%{::site}/%{::_role}.yaml"
  - backend: standard
    path: "/etc/puppet/private/hieradata/role/common/%{::_role}.yaml"
  - backend: wmflib::expand_path
    path: "/etc/puppet/hieradata/common"
  - backend: standard
    path: "/etc/puppet/netbox/hieradata/common"
  - backend: wmflib::expand_path
    path: "/etc/puppet/private/hieradata/common"

And cloud VPS:

  - backend: cloudlib::httpyaml
    uri: "http://puppet-enc.cloudinfra.wmcloud.org:8100/v1/%{::labsproject}/node/%{facts.fqdn}"
  - backend: standard
    path: "/etc/puppet/hieradata/cloud/%{::wmcs_deployment}/%{::labsproject}/hosts/%{::hostname}.yaml"
  - backend: standard
    path: "/etc/puppet/hieradata/cloud/%{::wmcs_deployment}/%{::labsproject}/common.yaml"
  - backend: standard
    path: "/etc/puppet/hieradata/cloud/%{::wmcs_deployment}.yaml"
  - backend: standard
    path: "/etc/puppet/hieradata/cloud.yaml"
  - backend: standard
    path: "/etc/puppet/secret/hieradata/hosts/%{::trusted.certname}.yaml"
  - backend: standard
    path: "/etc/puppet/secret/hieradata/%{::labsproject}.yaml"
  - backend: standard
    path: "/etc/puppet/private/hieradata/labs/%{::labsproject}/common.yaml"
  - backend: standard
    path: "/etc/puppet/private/hieradata/%{::labsproject}.yaml"
  - backend: standard
    path: "/etc/puppet/private/hieradata/labs.yaml"
  - backend: standard
    path: "/etc/puppet/hieradata/common.yaml"
  - backend: standard
    path: "/etc/puppet/secret/hieradata/common.yaml"
  - backend: standard
    path: "/etc/puppet/private/hieradata/common.yaml"

The lookup hierarchy

This hierarchy is organized as follows:

  • hieradata/hosts/${::hostname}.yaml. Here you should basically only include host-specific overrides. It is useful for things like testing new features on a single host of a cluster.
  • netbox/hosts/%{::hostname} This contains meta data populated by netbox
  • hieradata/regex.yaml If one of the entries in this file matches the $::fqdn of the host, hiera will lookup keys here. For the format, see below. This is useful if you need to set up a key for multiple hosts. Using this for cluster-wide configurations is deprecated, you should use the role keyword and the role backend.
  • hieradata/${::site}/$classpath.yaml If you need to configure something differently per-site (so, eqiad or codfw) globally it should go here. This is the place for global defaulks and should going forward should aonly have keys for the profile namespace. $classpath is computed as in the puppet autoload mechanism - so foo::params::param would be searched inside hieradata/${::site}/foo/params.yaml as the full key (foo::params::param).
  • role/${::site}/$rolepath.yaml where $rolepath is computed like $classpath above, but based on the role declared in site.pp for the current node. Attaching more than one role on a node is very heavily discouraged and is not guaranteed to work
  • private/hieradata/${::site}/$classpath.yaml where $classpath is evaluated as explained above. Since these data are in the private repository, this is the perfect place to store passwords into (without the need to define private classes at all, this should allow us to wipe most of the labs/private repo out in the end). This is the site-specific version.
  • hieradata/common/$classpath.yaml where $classpath is again computed as explained above. This is useful for global configurations that may be overridden at higher levels.
  • private/hieradata/common/$classpath.yaml
  • role/common/$rolepath.yaml where $rolepath is computed like above.
  • private/hieradata/role/${::site}/$rolepath.yaml and private/hieradata/role/common/$rolepath.yaml, which are in the private repository and work exactly as the other uses of role.


Role-based lookup

Role based lookups use the standard hiera 5 backend however we do use a custom role function to populate the $::_role variable. As well as creating the $::_role variable the function also includes the class role::$argument (this function can be used only at the node scope). when you use the following:

node 'foobar.eqiad.wmnet' {

you are doing the following:

  • including role::mediawiki::appserver
  • make hiera lookup the role/eqiad/mediawiki/appserver.yaml and role/common/mediawiki/appserver.yaml


Implemented using the wmflib::regex hiera backend Since we have large clusters of almost-identical servers, instead of having to write out hiera data for each server we added a special file called regex.yaml where variables matching a whole cluster can be assigned; the format of the file is:

  __regex: !ruby/regexp PATTERN
  var1: value1
  var2: value2

where LABEL is a unique identifier of the cluster, PATTERN is a Ruby regular expression that will be matched to the $::fqdn puppet fact. So, keep on with the preceding example, we have in regex.yaml

  __regex: !ruby/regexp /^mw1[0-2][0-9]{2}\.eqiad\.wmnet$/
  cluster: hhvm_appservers

to ensure the 'cluster' variable is defined consistently. As of now, the regex.yaml file should just be used seldom, and the role backend should be use instead.


Implemented using the wmflib::expand_path hiera backend. In order to minimise the size of the common.yaml and "${::site}.yaml" and thus reduce the number of merge conflicts we make use of the wmflib::expand_path backend. This is a simple backed which takes a hiera key and looks in a file relative to the hierarchy path matching the key prefix i.e. $path/$classpath.yaml, where $classpath equals the hiera key prefix with :: replaced with /. e.g. when searching for the hiera value profile::mysuper_cool_module::mysuper_cool_module::this_is_the_key, hiera will look for this key in:

 - {$::site}/profile/mysuper_cool_module/mysuper_cool_module.yaml
 - common/profile/mysuper_cool_module/mysuper_cool_module.yaml

It should also be noted that unqualified global values i.e. those without a ::, will be looked up in common.yaml and $site.yaml

Practical example

Say we are searching the value of profile::admin::always_groups for the node mw1017.eqiad.wmnet, which is defined as follows in site.pp:

node /^mw1(01[7-9]|0[2-9][0-9]|10[0-9]|11[0-3])\.eqiad\.wmnet$/ {
    role (mediawiki::appserver)

This will search for the value in the following files:

- hieradata/hosts/mw1017.yaml                            # $::hostname
- hieradata/regex.yaml                                   # where the $::fqdn will be matched to regexes, see above
- hieradata/eqiad/profile/admin.yaml                     # $::site, ''profile::admin'' expanded with wmflib::expand_path
- private/hieradata/eqiad/profile/admin.yaml             # $::site, ''profile::admin'' expanded with wmflib::expand_path
- hieradata/role/eqiad/mediawiki/appserver.yaml          # $::_role expands to ''mediawiki/appserver''
- hieradata/role/common/mediawiki/appserver.yaml         # $::_role expands to ''mediawiki/appserver''
- private/hieradata/role/eqiad/mediawiki/appserver.yaml  # $::_role expands to ''mediawiki/appserver''
- private/hieradata/role/common/mediawiki/appserver.yaml # $::_role expands to ''mediawiki/appserver''
- hieradata/common/profile/admin.yaml                    # profile::admin expanded with wmflib::expand_path
- private/hieradata/common/profile/admin.yaml            # profile::admin expanded with wmflib::expand_path

If it doesn't find any value in any of those files, puppet will use the class default value for that variable.

In Wikimedia Cloud Services (formerly known as Labs)

See main article Cloud VPS - Admin - Hiera.