Introducing resources, parameters, and properties

Each of the manifests you wrote in the previous section declared one respective resource. Resources are the elementary building blocks of manifests. Each has a type (in this case, notify and service, respectively) and a name or title (Hello, world! and puppet). Each resource is unique to a manifest, and can be referenced by the combination of its type and name, such as Service["puppet"]. Finally, a resource also comprises a list of zero or more attributes. An attribute is a key-value pair, such as "enable => false".

Attribute names cannot be chosen arbitrarily. They are part of the Puppet resource type. Puppet differentiates between two different attributes: parameters and properties. Each resource type supports a specific set of attributes. Parameters describe the way that Puppet should deal with a resource type. Properties describe a specific setting of a resource. Certain parameters are available for all resource types (metaparameters), and some names are just very common, such as ensure. The service type supports the ensure property, which represents the status of the managed process. Its enabled property, on the other hand, relates to the system boot configuration (with respect to the service in question).

We have used the terms attribute, property, and parameter in a seemingly interchangeable fashion. Don't be deceived-there are important distinctions. Property and parameter are the two different kinds of attributes that Puppet uses.

You have already seen two properties in action. Let's look at a parameter:

service { 'puppet':
  ensure   => 'stopped',
  enable   => false,
  provider => 'upstart',
}

The provider parameter tells Puppet that it needs to interact with the upstart subsystem to control its background service, as opposed to systemd or init. If you don't specify this parameter, Puppet makes an educated guess. There is quite a multitude of supported facilities to manage services on a system. You will learn more about providers and their automatic choosing later on.

The difference between parameters and properties is that the parameter merely indicates how Puppet should manage the resource, not what a desired state is. Puppet will only take action on property values. In this example, these are ensure => 'stopped' and enable => false. For each such property, Puppet will perform the following tasks:

• Test whether the resource is already in sync with the target state
• If the resource is not in sync, it will trigger a sync action

A property is considered to be in sync when the system entity that is managed by the given resource (in this case, the upstart service configuration for Puppet) is in the state that is described by the property value in the manifest. In this example, the ensure property will be in sync only if the puppet service is not running. The enable property is in sync if upstart is not configured to launch Puppet at system start.

As a mnemonic concerning parameters versus properties, just remember that properties can be out of sync, whereas parameters cannot.

Puppet also allows you to read your existing system state by using the puppet resource command:

root@puppetmaster:~# puppet resource user root
user { 'root':
  ensure           => 'present',
  comment          => 'root',
  gid              => '0',
  home             => '/root',
  password         => '$6$17/7FtU/$TvYEDtFgGr0SaS7xOVloWXVTqQxxDUgH.
  eBKJ7bgHJ.hdoc03Xrvm2ru0HFKpu1QSpVW/7o.rLdk/9MZANEGt/',
  password_max_age => '99999',
  password_min_age => '0',
  shell            => '/bin/bash',
  uid              => '0',
}  

Please note that some resource types will return read-only attributes (for example, the file resource type will return mtime and ctime). Refer to the appropriate type's documentation.