Configuration Templates

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Chef Habitat allows you to templatize your application’s native configuration files using Handlebars syntax. The following sections describe how to create tunable configuration elements for your application or service.

Template variables, also referred to as tags, are indicated by double curly braces: {{a_variable}}. In Chef Habitat, tunable config elements are prefixed with cfg. to indicate that the value is user-tunable.

Here’s an example of how to make a configuration element user-tunable. Assume that we have a native configuration file named service.conf. In service.conf, the following configuration element is defined:

recv_buffer 128

We can make this user tunable like this:

recv_buffer {{cfg.recv_buffer}}

Chef Habitat can read values that it will use to render the templatized config files in three ways:

1. default.toml - Each plan includes a default.toml file that specifies the default values to use in the absence of any user provided inputs. These files are written in TOML, a simple config format.
2. At runtime - Users can alter config at runtime using hab config apply. The input for this command also uses the TOML format.
3. Environment variable - At start up, tunable config values can be passed to Chef Habitat using environment variables.

Here’s what we’d add to our project’s default.toml file to provide a default value for the recv_buffer tunable:

recv_buffer = 128

All templates located in a package’s config folder are rendered to a config directory, /hab/svc/<pkg_name>/config, for the running service. The templates are re-written whenever configuration values change. The path to this directory is available at build time in the plan as the variable $pkg_svc_config_path and available at runtime in templates and hooks as {{pkg.svc_config_path}}.

All templates located in a package’s config_install folder are rendered to a config_install directory, /hab/svc/<pkg_name>/config_install. These templates are only accessible to the execution of an install hook and any changes to the values referenced by these templates at runtime will not result in re-rendering the template. The path to this directory is available at build time in the plan as the variable $pkg_svc_config_install_path and available at runtime in templates and install hooks as {{pkg.svc_config_install_path}}.

Chef Habitat not only allows you to use Handlebars-based tunables in your plan, but you can also use both built-in Handlebars helpers as well as Chef Habitat-specific helpers to define your configuration logic. See Reference for more information.