This article was contributed by Greg Moeck. Greg is a software craftsman who has been working with Ruby since 2004. When this article was published, he was working on mobile javascript development at Facebook.

Given that Ruby is an object oriented programming language, all Ruby programs are going to be composed of many objects. However, techniques for breaking the functionality of programs into objects can vary from programmer to programmer. In this article I’m going to walk through two common approaches to driving design at a high level: data-centric design and responsibility-centric design. I will briefly sketch the key ideas of each of the design methodologies, illustrating how one might structure parts of a simple e-commerce application using each of the methods. I’ll then follow up with some advice about where I’ve found the different approaches to be particularly helpful or unhelpful.

Data-centric design

In a data-centric design, the system is generally separated into objects based upon the data that they encapsulate. For example, in an e-commerce application you are likely to find objects that represent products, invoices, payments, and users. These objects provide methods which operate on that data, either returning its values, or mutating its state. A Product object might provide a method to determine how many of a given product are currently in stock, or possibly a method to add that product to the current shopping cart.

Names for data-centric objects are often nouns, because they frequently correspond to real-world objects. This real-worldliness is generally also true of the methods that these objects provide. The methods either represent accessors to the object’s data, relationships between objects, or actions that could be taken on the object. The following ActiveRecord object serves as a good example of this style of design:

class Product < ActiveRecord :: Base #relationships between objects has_many :categories #accessing objects data def on_sale? not ( sale_price . nil? ) end #action to take on the product def add_to_cart ( cart ) self . remaining_items -= 1 save! cart . items << self end end

Following along these lines, inheritance is generally used as a principle of classification, establishing a subtype relationship between the parent and the child. If B inherits from A, that is a statement that B is a type of A. This is generally described as an is-a relationship. For example, the classes LaborCharge and ProductCharge might both inherit from a LineItem base class which implements the features they have in common. The key thing to note about these classes is that they share at least some data attributes and the behavior around those attributes, even if some of that behavior might end up being overridden.

However, not everything can have a counterpart in the real world. There still needs to be some communication model that is created to describe the global or system level view of the interactions between objects. These controllers will fetch data from different parts of the system, and pipe it into actions in another part. Since these objects generally are very difficult to classify in a hierarchical way, it is a good idea to keep them as thin as possible, pushing as much logic into the actual domain model as you possibly can.

For those familiar with standard Rails architectures, you should see a lot of commonalities with the above description. Rails model objects are inherently structured this way because the ActiveRecord pattern tightly couples your domain objects to the way in which their data is persisted. And so all ActiveRecord objects are about some “encapsulated” data, and operations that can be done on that data. Rails controllers provide the global knowledge of control, interacting with those models to then accomplish some tasks.

Responsibility-centric design

In a responsibility-centric design, systems are divided by the collection of behaviors that they must implement. The goal of this division is to formulate the description of the behavior of the system in terms of multiple interacting processes, with each process playing a separate role. For example, in an e-commerce application with a responsibility-centric design, you would be likely to find objects such as a payment processor, an inventory tracker, and a user authenticator.

The relationships between objects become very similar to the client/server model. A client object will make requests of the server to perform some service, and a server object will provide a public API for the set of services that it can perform. This relationship is described by a contract - that is a list of requests that can be made of the server by the client. Both objects must fulfill this contract, in that the client can only make the requests specified by the API, and the server must respond by fulfilling those requests when told.

As an example, a responsibility-centric order processing service might look like what you see below:

class StandardOrderProcessor def initialize ( payment_processor , shipment_scheduler ) @payment_processor = payment_processor @shipment_scheduler = shipment_scheduler end def process_order ( order ) @payment_processor . debit_account ( order . payment_method , order . amount ) @shipment_scheduler . schedule_delivery ( order . delivery_address , order . items ) end end

The goal of describing relationships between objects in this way is that it forces the API for the server object to describe what it does for the client rather than how it accomplishes it. By its very nature the implementation of the server must be encapsulated, and locked away from the client. This means that the client object can only be coupled to the public API of its server objects, which allows developer to freely change server internals as long as the client still has an object to talk to that fulfills the contract.

The practical benefit of this kind of design is that it makes certain kinds of changes very easy. For example, the following code could be used as a drop-in replacement for the StandardOrderProcessor , because it implements the same contract:

class OrderValidationProcessor def initialize ( order_processor , error_handler ) @order_processor = order_processor @error_handler = error_handler end def process_order ( order ) if is_valid_order ( order ) @order_processor . process_order ( order ) else @error_handler . invalid_order ( order ) end end private def is_valid_order ( order ) #does some checking for if the order is valid end end

The client does not know which sort of order processor it is talking to, it just knows how to request that an order gets processed. Validations are skipped when the client is provided with a StandardOrderProcessor , and they are run when it is provided with a OrderValidationProcessor , but the client does not know or care about these details. This allows for substantial changes in order processing behavior without requiring any modifications to the client object.

To make them easier to work with, these kinds of service objects would generally be composed with a factory that might look something like what you see below:

class OrderProcessor # ... def with_validation OrderValidationProcessor . new ( without_validation , error_handler ) end def without_validation StandardOrderProcessor . new ( payment_processor , shipment_scheduler ) end # ... end

The notion of client and server are related to what side of a contract each object is on, which means that individual objects frequently play both roles. For example, a payment processor object may consume the services of a credit card processor, while providing services for an order processor. From the perspective of the credit card processor, the payment processor is a client, but just the opposite is true for the ordering system. A key feature of this kind of design is that objects are coupled to an interface rather than an implementation, which makes the relationships between objects much more dynamic than what you can expect from a data-centric design.

As you’ve probably already noticed, because these kinds of objects represent the behavior of the system rather than the data, the objects are not generally named after real-world entities. The roles that an object plays often represent real-world processes, and the implementation of these roles are often named after how they implement the desired role. For example, within our system there might be two objects which can play the role of a shipment scheduler: a FedexDeliveryScheduler and UPSDeliveryScheduler . Despite the specificity of their names, the client consuming these objects would not know which of the two it was talking to as long as they implemented a common interface for scheduling deliveries. A natural consequence of role-based modeling is that method names become more important while class names become less important, and this example is no exception.

Another core concept of responsibility-centric designs is that data tends to flow through the system rather than being centrally managed within the system. As a result, data typically takes the form of immutable value objects. For example, in the above order processors, the processes were being passed an order object, which contained the data for a given order. The objects within the system are not mutating or persisting this data directly, but passing values around instead. With that in mind, an object responsible for tracking the current order might look like what you see below:

class CurrentOrderTracker def initialize @order = Order . new end def item_selected ( item ) @order = order . add_item ( item ) end class Order attr_accessor :items def initialize ( items ) @items = items || [] end def add_item ( item ) Order . new ( @items + item ) end end end

Because any reference to one of these values is guaranteed to be immutable, any process can read from it at any time without worrying that it might have been modified by another process. This is not to say however that this data is never persisted. When it is necessary to persist this data, an object playing the role of a persister must be created, and it must receive messages containing these values just like any other part of the system. In this way, the persistance logic generally lives on the boundaries of the system rather than in the center. Such an object might look something like this:

class SQLOrderPersister #assuming that AROrder is an active record object def persist_order ( order ) order = AROrder . find ( order . id ) if order order . update_attributes ( order . attributes ) else AR . Order . new ( order . attributes ). save end end end

The last thing to note is that in this sort of system using inheritance as a form of classification doesn’t really make much sense. Historically inheritance has taken the form of “plays the same role as” instead of is-a. Objects which play the same role have historically inherited from a common abstract base class which merely implements the role’s public API, and forces any class that inherits from it to do the same. This relationship expresses that an object implements a certain contract, rather than categorically claiming what the object is.

In Ruby, using inheritance for this sort of relationship isn’t strictly necessary. Due to duck typing, if something quacks like a duck (that is if it implements the same API as a duck), it is a duck, and there is no need to have the objects inherit from a common base class. That being said, it can still be nice to explicitly name these roles, and an abstract base class can often be used to do that.

Comparing and contrasting the two design styles

As with almost any engineering choice, it isn’t possible to say that either of these two approaches is always superior or inferior. That said, we can still walk through some strengths and weaknesses of each approach.

Strengths of data-centric design:

1) Because the code is broken into parts around real world entities, these entities are easy to find and tweak. All the code relative to a certain set of data lives together.

2) Because it has a global flow control, and the fact that it is it is centered around data (which people generally understand), it is relatively easy for programmers experienced with traditional procedural languages to adapt their previous experience into this style.

3) It is very easy to model things like create/read/update/destroy because the data is found in a single model for all real world objects.

4) For systems with many data types and a small amount of behavior, this approach evenly distributes the location of the code.

Weaknesses of data-centric design:

1) Because the structure of an object is a part of its definition, encapsulation is generally harder to achieve.

2) Because the system is split according to data, behavior is often hard to track down. Similar operations often span across multiple data types, and as such end spread out across the entirety of the system.

3) The cohesion of behavior within an object is often low since every object has to have all actions that could be taken upon it, and those actions often have very little to do with one another.

4) In practice it often leads to coupling to the structure of the object as one needs to violate the Law of Demeter to traverse the relationships of the objects. For example, think of often you in Rails you see something like the following code:

@post . comments . each do | comment | if comment . author . something ... end end

Strengths of responsibility-centric design:

1) Objects tend to be highly cohesive around their behavior, because roles are defined by behavior, not data.

2) Coupling to an interface rather than an implementation makes it easier to change behavior via composition.

3) As more behaviors are introduced into the system, the number of objects increases rather than the lines of code within model objects.

Weaknesses of responsibility-centric design:

1) It is often difficult to drop into the code and make simple changes as even the simplest change necessitates understanding the architecture of at least the module. This means that the on-ramping time for the team is generally fairly high.

2) Since there is generally no global control, it is often difficult for someone to grasp where things are happening. As Kent Beck, and Ward Cunningham have said, “The most difficult problem in teaching object- oriented programming is getting the learner to give up the global knowledge of control that is possible with procedural programs, and rely on the local knowledge of objects to accomplish their tasks.”

3) Data is not as readily available since the destructuring of the application is around behavioral lines. The data can often be scattered throughout the system. Which means changing the data structure is more expensive than changing the behavior.

Choosing the right design

Rails has proven how the data centric approach can lead to quickly building an application that can create, read, update and destroy data. And for applications whose domain complexity lies primarily in data types, and the actions that can be taken on those data types, the pattern works extremely well. Adding or updating data types is fast and easy since the system is cohesive around its data.

However as some large legacy Rails codebases show, when the complexity of the domain lies primarily in the behaviors or rules of the domain then organizing around data leads to a lot of jumbled code. The models end up needing to have many methods on them in order to process all of the potential actions that can be taken on them, and many of these actions end up being similar across data types. As such the cohesion of the system suffers, and extending or modifying the behavior becomes more and more difficult over time.

The opposite of course is true as well in my experience. In a system whose domain complexity lies primarily in its behavior, decomposing the system around those behaviors makes extending or modifying the behavior of the system over time to be much faster and easier. However the cost is that extending or modifying the data of the system can become more and more difficult over time.

As with most design methods, it comes down to an engineering decision, which often means you have to guess, and evolve over time. There is no magic system that will be the right way to model things regardless of the application. There might even be some subsets of an application that might be better modeled in a data-centric way, whereas other sections of the system might be better modeled in a behavior-centric way. The key thing I’ve found is to be sensitive to the “thrash” smell, where you notice that things are becoming more and more difficult to extend or modify, and be open to refactor the design based on the feedback you’re getting from the system.

Further references

1) Growing Object Oriented Software Guided By Tests, Steve Freeman, Nat Pryce

2) Object-oriented design: a responsibility-driven approach, R. Wirfs-Brock, B. Wilkerson, OOPSLA ‘89 Conference proceedings on Object-oriented programming systems, languages and applications

3) The object-oriented brewery: a comparison of two object-oriented development methods, Robert C. Sharble, Samuel S. Cohen, ACM SIGSOFT Software Engineering Notes, Volume 18 Issue 2, April 1993

4) Mock Roles, Not Objects, Steve Freeman, Tim Mackinnon, Nat Pryce, Joe Walnes, OOPSLA ‘04 Companion to the 19th annual ACM SIGPLAN conference on Object-oriented programming systems, languages, and applications

5) A Laboratory For Teaching Object-Oriented Thinking, Kent Beck, Ward Cunningham, OOPSLA ‘89 Conference proceedings on Object-oriented programming systems, languages and applications