Wednesday, April 30, 2014

Puppet Internals - Basic Modeling with RGen

Modeling Process - an example

In the previous post about modeling I covered the basic principles of models, i.e. - A model is an abstraction, a model that describes a model is a meta-model, and finally a model that defines the grammar for how meta-models are written is called a meta-meta model.

Since this may sound like hocus-pocus, I am going to show concrete examples of how this works. Before we can start doing tricks with meta-models, we must have something concrete to model, and we must learn the concepts to use to express these models, and that is the focus of this post.

I am going to use the RGen Metamodel Builder API, a Ruby DSL for creation of meta models to illustrate simply because it is the easiest way to create a model.

Modeling Process - an example

When we are modeling, we typically produce the model in several iterations - first just starting loosely with the nouns, verbs, and attributes of our problem domain. We recently did this exercise for a first version of Puppet's future Catalog Builder, and it went something like this:

  • We need to be able to build a Catalog
  • A Catalog contains Resources
  • A Resource describes a wanted state of a particular resource type in terms of Properties
  • A Resource also describes Parameters that are input to the resource provider performing the work
  • It would be great if we could define a catalog consisting of multiple catalogs to be applied in sequence - maybe Section is a good name for this
  • While the catalog is built we need to be able to make future references
  • We need to track where resources in the catalog originates (they may be virtual and exported from elsewhere)
  • ... etc.

Once we settled on a handful of statements, we could then whiteboard a tentative model, and reason about the implications. We continued with all the various sets of requirements, and we took pictures of the whiteboard, and made some written notes to remember what we did.

Next step was to document this more formally. I used the graphical modeling tool for Eclipse to make a diagram. Using this diagram, we then walked through it, continued discussing / testing use cases, and revising the diagram. The revised diagram after a couple of iterations looks like this:

At this point in the lifecycle of the model, it is fastest to make changes in the graphical tool, and it is much faster and easier to communicate what the model means than if everything is written in a programming language. After a while though, it becomes somewhat tedious to describe all the details in the diagram, and while we can use the output directly from the tool to get a version in Ruby, we really want to maintain the original model in Ruby source code form.

We then made an implementation of the model in Ruby that you can view on github.

The work of making the diagram and the implementation in Ruby took me something like 4 hours spread out over the breaks we took while discussing and white-boarding.

We expect to revise this model several times until it is done. If we want to generate a graph, we can now go in the other direction and create input to the graphical tool from the code we wrote in Ruby. This is basically a transformation from the Ruby code to Ecore in XMI since that is what the graphical tool understands.

The reason why I included this real life example is to show something that is relevant to the Puppet community. I am however going to switch to toy examples to demonstrate the various techniques when modeling since any real life model tends to overshadow the technique with domain specific issues - i.e. this post is not about the Catalog-model, it is about modeling in general.


The basic concepts used when modeling are:

  • We model classifiers / classes
  • Classes have named features
  • A feature is an attribute of a simple data type, or a relationship to another class
  • Attributes can be multi valued
  • A relationship can describe containment
  • A relationship can be uni-, or bi-directional (a bi-directional reference allows us to conveniently navigate to an object that is "pointing to" the object in question).
  • Relationships can be multi valued at one side (or both sides for non containment relationships).
  • The term multiplicity is sometimes used to denote optional/single/multi-value in relationships, e.g. 0:1, 1:1, 0:M, 1:M, or M:M
  • An abstract class can not be instantiated
  • Ecore also contains modeling of interfaces, this is mostly useful for Java and I am going to skip explaining these.


When using RGen's Meta Model Builder, the classes in a model are simply placed in a module and made to inherit the base class RGen::MetamodelBuilder::MMBase. It is common to let each meta model define an abstract class that signals that it is part of that model.

require 'rgen/metamodel_builder'

module MyModel
  # Let RGen know this module is a model
  extend RGen::MetamodelBuilder::ModuleExtension

  # An abstract class that makes it easier to check if a given
  # object is an element of "MyModel"
  class MyModelElement < RGen::MetamodelBuilder::MMBase
    # We make this class abstract to make it impossible to create instances of it

  class Car < MyModelElement
    # definition of Car

At this point we cannot really do much except create an instance of a Car

require 'mymodel'
a_car =

Attribute Features

Attribute features are used to define the attributes of the class that are represented by basic data types. Attributes have a name, a type, and multiplicity. A single value attribute is defined by calling has_attr, a multivalued attribute by calling has_many_attr.

The basic data types are:

  • String
  • Integer
  • Float
  • Numeric
  • Boolean
  • Enum

You can also use the completely generic Object type, but this also means that the model cannot be serialized, so this should be avoided. It is also possible to reference implementation classes, but this should also be avoided for serialization and cross platform reasons.

The Enum type can be specified separately, and given a name, or it may be defined inline.

EngineEnum =[:combustion, :electric, :hybrid])

class Car < MyModelElement
  has_attr 'engine', EngineEnum
  has_attr 'doors', Integer
  has_attr 'steering_wheel', String
  has_attr 'wheels', Integer

If we have an attribute that should be capable of holding many values, we use has_many_attr. In the example below, an enum for extra equipment is used - it does not have to be an enum, a multi valued attribute can have any basic data type.

ExtraEquipmentEnum =[
  :gps, :stereo, :sun_roof, :metallic
class Car < MyModelElement
  has_many_attr 'extras', ExtraEquipmentEnum
  # ...

The resulting implementation allows us to set and get values.

a_car =
a_car.engine = :combustion
a_car.extras = [:gps, :sun_roof]

For multi-valued attributes, we can add and remove entries

a_car.extras   # => [:gps, :metallic]

If we attempt to assign something of the wrong type, we get an error:


=> In MyModel::Car : Can not use a Symbol(:catapult_chair) where a
   [:gps,:stereo,:sun_roof,:metallic] is expected

We can also specify additional metadata for each attribute, but I will return to that later.

Relationship Features

All non basic data types are handled via references. There are two types of references; containment, and regular. A containment reference is used when the referenced element is an integral part of the object - e.g. when it is an attribute of the object, when something cannot and should not be shared between objects (one particular wheel can only be mounted on once car at the time), and when they should be serialized as part of the object that holds the reference. All other references requires that the referenced object is contained somewhere else (although this is not quite true as we will see later when we talk about more advanced concepts, it is a reasonable conceptual approximation for now).

In order to have something meaningful to model, lets expand the notion of Engine.

class Engine < MyModelElement

FuelTypeEnum =[:diesel, :petrol, :etanol])

class CombustionEngine < Engine
  has_attr 'fuel', FuelTypeEnum

class ElectricalEngine < Engine
  has_attr 'charge_time_h', Integer

# skipping HybridEngine for now


We can now change the Car to contain an Engine. When we do this, we must also decide if the Engine should explicitly know about which car it is mounted in or not. That is, if the containment relationship is uni- or bi- directional. Let's start with a uni-directional containment:

class Car < ModelElement
  contains_one_uni 'engine', Engine
  # ...

We can now create and assign an Engine to a Car.

a_car =
a_car.engine =

If we want to make the containment bi-directional:

class Car < MyModelElement
  contains_one 'engine', Engine, 'in_car'
  # ...

The assignment works as before, but we can now also navigate to the car the engine is mounted in. We achieved this, by defining the reverse role 'in_car' for the bi-directional containment. Now we can do this:

an_engine =
an_engine.in_car            # => nil
a_car =
a_car.engine = an_engine
an_engine.in_car            # => Car

The semantics of containment means that if we assign the engine to another car we will move it!

# continued from previous example
another_car =
a_car.engine                # => CombustionEngine
another_car.engine = an_engine
another_car.engine          # => CombustionEngine
a_car.engine                # => nil

This may seem scary, but it is actually quite natural. If we find that we want to contain something in more than one place at a given time our model (and thinking) is just wrong, and one of the references should not be a containment reference. Say if we have a ServiceOrder (imagine for the purpose of repairing an engine), the engine is not ever contained in the order (it is still mounted in the car). Model-wise we simply use a non-containment / regular reference from the ServiceOrder to an Engine.

We specify different kinds of containment with the methods:

  • contains_one
  • contains_many
  • contains_one_uni
  • contains_many_uni

Regular References

Regular references are defined with one of the methods:

  • has_one, uni-directional
  • has_many, uni-directional
  • one_to_many, bi-directional
  • many_to_one, bi-directional
  • many_to_many, bi-directional

We can now define a ServiceOrder for service of engines:

class ServiceOrder < MyModelElement
  has_one 'serviced_engine', Engine

And we can use this:

an_engine =
a_car =
a_car.engine = an_engine
a_car.engine                   # => CombustionEngine
so =
so.serviced_engine = an_engine
a_car.engine                   # => CombustionEngine

As you can see, since the relationship is non-containment, the engine does not move to become an integral part of the service order, it is still mounted in the car (which is exactly what we wanted).

For the bi-directional references, the reverse role is required. When using these, it is important to consider the cohesion of the system - we do not want every piece to know about everything else. Therefore, choose bi-directional references only where it really matters.

Testing the examples

You can easily try the examples in this gist. You need to have the rgen gem installed, and then you can run the examples in irb, and try things out.

In this Post

In this post you have seen examples of how to build a model with RGen, and how it can be used. As you probably have noticed, you get quite a lot of functionality with only a small amount of work. How much code would you have to write to correctly support fully typed many to many relationships? (And then discover that you want to model it differently).

I hope this post has showed the usefulness of modeling even when no fancy modeling tricks have been used simply because of the robust, type and referentially safe implementation that we get from a small an concise definition.


  1. shouldn't that be...

    another_car.engine = an_engine


  2. Yes, you are right - will update the post.
    Thank you!