Hiera lookup framework

Introduction

Hiera is a flexible, powerful tool for resolving values for variable lookups, which was first popularised by its use in Puppet.

This module is a "clean-room" Go implementation of the Hiera framework, suitable for use as a library from other tools.

Details

Hiera uses the concept of "managing by exception": you design a hierarchy of data sources, with the most specific source at the top and least-specific defaults at the bottom. Hiera searches for keys starting at the top, allowing more-specific sources to override defaults. Sources are usually YAML files stored on the filesystem, and layers usually use variable interpolation to find the right file, allowing the context of the lookup to pick the right file.

How to run it

Standalone execution

Install the module

To install the module under $GOPATH/src:

go get github.com/yanndegat/hiera

Install the lookup binary

Install the lookup binary under $GOPATH/bin:

go install github.com/yanndegat/hiera/lookup

Run the binary

lookup --help

Pass values for interpolation

If your hierarchy config contains variable interpolation, you can provide context for the lookup using the var query parameter. Repeated var parameters will create an array of available parameters. The values should be colon-separated variable-value pairs:

curl 'http://localhost:8080/lookup/aws.tags?var=environment:production&var=hostname:specialhost'

TODO: Nested variable lookups such like os.family are not yet working.

Hiera configuration and directory structure

Much of hiera's power lies in its ability to interpolate variables in the hierarchy's configuration. A lookup provides values, and hiera maps the interpolated values onto the filesystem (or other back-end data structure). A common example uses two levels of override: one for specific hosts, a higher layer for environment-wide settings, and finally a fall-through default. A functional hiera.yaml which implements this policy looks like:

---
version: 5
defaults:
datadir: hiera
data_hash: yaml_data

hierarchy:
- name: "Host-specific overrides"
    path: "hosts/%{hostname}.yaml"
- name: "Environmental overrides"
    path: "environments/%{environment}.yaml"
- name: "Fall through defaults"
    path: "defaults.yaml"

This maps to a directory structure based in the hiera subdirectory (due to the datadir top level key) containing yaml files like:

hiera
├── defaults.yaml
├── environments
│   └── production.yaml
└── hosts
    └── specialhost.yaml

Extending Hiera

When Hiera performs a lookup it uses a lookup function. Unless the function embedded in the hiera binary, it will make an attempt to load a RESTful Plugin that provides the needed function. Such plugins are enabled by using the API provided by the hierasdk library.

How Hiera finds its plugins

The resolution of a plugin can be controlled using a "plugindir" key in the Hiera configuration file. As with "datadir", the "plugindir" can be specified both in the defaults hierarchy or explicitly in a specific hierarchy. Unless specified, the "plugindir" is assumed to be the directory "plugin" present in the same directory as the configuration file.

In addition to "plugindir", a hierarchy may also specify a "pluginfile". Unless specified, the "pluginfile" is assumed to be equal to the name of the lookup function (with the extension ".exe" in case of Windows).

Environment Variables

The following environment variables can be set as an alternative to CLI options.

HIERA_CONFIGFILE - --config

Values passed as CLI options will take precendence over the environment variables.

The following environment variables can be set as an alternative to setting values in the defaults hash.

HIERA_DATADIR - datadir
HIERA_PLUGINDIR - plugindir

Values set in hiera.yaml will take precedence over the environment variables.

Containerized extension

In order to include an extension in a Hiera Docker image you need to:

Copy the source (or clone the git repository) of the desired extensions into the hiera plugin directory (don't worry, this directory will be ignored by git when doing commits to hiera).
For each extension, add a line like the following line to the Hiera Dockerfile below the comment # Add plugin builds here:
```
RUN (cd plugin/hiera_terraform && go build -o ../terraform_backend)
```
Run the docker build.

Useful extensions

Azure Key Vault lookup_key. Allows you to lookup single values stored as secrets from the Azure Key Vault.
Terraform Backend data_hash. Allows hiera to query data from a Terraform backend.

yanndegat/hiera