Part 5: The Dependency Inversion Principle

Notes / Software Architecture / Principles of OOD / Part 5: The Dependency Inversion Principle

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What goes wrong with software?

Most of us have had the unpleasant experience of trying to deal with a piece of software that has a “bad design”. Some of us have even had the much more unpleasant experience of discovering that we were the authors of the software with the “bad design”. What is it that makes a design bad?

Most software engineers don’t set out to create “bad designs”. Yet most software eventually degrades to the point where someone will declare the design to be unsound.

The Definition of a “Bad Design”

Have you ever presented a software design, that you were especially proud of, for review by a peer? Did that peer say, in a whining derisive sneer, something like: “Why’d you do it that way?". Certainly this has happened to me, and I have seen it happen to many other engineers too. Clearly the disagreeing engineers are not using the same criteria for defining what “bad design” is. The most common criterion that I have seen used is the TNTWIWHDI or “That’s not the way I would have done it” criterion.

But there is one set of criteria that I think all engineers will agree with. A piece of software that fulfills its requirements and yet exhibits any or all of the following three traits has a bad design.

  1. It is hard to change because every change affects too many other parts of the system. (Rigidity)
  2. When you make a change, unexpected parts of the system break. (Fragility)
  3. It is hard to reuse in another application because it cannot be disentangled from the current application. (Immobility)

Moreover, it would be difficult to demonstrate that a piece of software that exhibits none of those traits, i.e. it is flexible, robust, and reusable, and that also fulfills all its requirements, has a bad design. Thus, we can use these three traits as a way to unambiguously decide if a design is “good” or “bad”.

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The Cause of “Bad Design”.

What is it that makes a design rigid, fragile and immobile? It is the interdependence of the modules within that design. A design is rigid if it cannot be easily changed. Such rigidity is due to the fact that a single change to heavily interdependent software begins a cascade of changes in dependent modules. When the extent of that cascade of change cannot be predicted by the designers or maintainers, the impact of the change cannot be estimated. This makes the cost of the change impossible to predict. Managers, faced with such unpredictability, become reluctant to authorize changes. Thus the design becomes officially rigid.

Fragility is the tendency of a program to break in many places when a single change is made. Often the new problems are in areas that have no conceptual relationship with the area that was changed. Such fragility greatly decreases the credibility of the design and maintenance organization. Users and managers are unable to predict the quality of their product. Simple changes to one part of the application lead to failures in other parts that appear to be completely unrelated. Fixing those problems leads to even more problems, and the maintenance process begins to resemble a dog chasing its tail.

A design is immobile when the desirable parts of the design are highly dependent upon other details that are not desired. Designers tasked with investigating the design to see if it can be reused in a different application may be impressed with how well the design would do in the new application. However if the design is highly interdependent, then those designers will also be daunted by the amount of work necessary to separate the desirable portion of the design from the other portions of the design that are undesirable. In most cases, such designs are not reused because the cost of the separation is deemed to be higher than the cost of redevelopment of the design.

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The Dependency Inversion Principle


Consider the implications of high level modules that depend upon low level modules. It is the high level modules that contain the important policy decisions and business models of an application. It is these models that contain the identity of the application. Yet, when these modules depend upon the lower level modules, then changes to the lower level modules can have direct effects upon them; and can force them to change.

This predicament is absurd! It is the high level modules that ought to be forcing the low level modules to change. It is the high level modules that should take precedence over the lower level modules. High level modules simply should not depend upon low level modules in any way.

Moreover, it is high level modules that we want to be able to reuse. We are already quite good at reusing low level modules in the form of subroutine libraries. When high level modules depend upon low level modules, it becomes very difficult to reuse those high level modules in different contexts. However, when the high level modules are independent of the low level modules, then the high level modules can be reused quite simply.

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According to Booch, ”…all well structured object-oriented architectures have clearly-defined layers, with each layer providing some coherent set of services though a well-defined and controlled interface.“ A naive interpretation of this statement might lead a designer to produce a structure similar to Figure 3. In this diagram the high level policy class uses a lower level Mechanism; which in turn uses a detailed level utility class. While this may look appropriate, it has the insidious characteristic that the Policy Layer is sensitive to changes all the way down in the Utility Layer. _Dependency is transitive._ The Policy Layer depends upon something that depends upon the Utility Layer, thus the Policy Layer transitively depends upon the Utility Layer. This is very unfortunate.

Figure 4 shows a more appropriate model. Each of the lower level layers are represented by an abstract class. The actual layers are then derived from these abstract classes. Each of the higher level classes uses the next lowest layer through the abstract interface. Thus, none of the layers depends upon any of the other layers. Instead, the layers depend upon abstract classes. Not only is the transitive dependency of Policy Layer upon Utility Layer broken, but even the direct dependency of Policy Layer upon Mechanism Layer is broken.

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Using this model, Policy Layer is unaffected by any changes to Mechanism Layer or Utility Layer. Moreover, Policy Layer can be reused in any context that defines lower level modules that conform to the Mechanism Layer interface. Thus, by inverting the dependencies, we have created a structure which is simultaneously more flexible, durable, and mobile.

Separating Interface from Implementation

One might complain that the structure in Figure 3 does not exhibit the dependency, and transitive dependency problems that I claimed. After all, Policy Layer depends only upon the interface of Mechanism Layer. Why would a change to the implementation of Mechanism Layer have any affect at all upon Policy Layer?

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Finding the Underlying Abstraction

What is the high level policy? It is the abstractions that underlie the application, the truths that do not vary when the details are changed. In the Button/Lamp example, the underlying abstraction is to detect an on/off gesture from a user and relay that gesture to a target object. What mechanism is used to detect the user gesture? Irrelevant! What is the target object? Irrelevant! These are details that do not impact the abstraction.

To conform to the principle of dependency inversion, we must isolate this abstraction from the details of the problem. Then we must direct the dependencies of the design such that the details depend upon the abstractions.

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The high level policy in Listing 6 is reusable with any kind of button, and with any kind of device that needs to be controlled. Moreover, it is not affected by changes to the low level mechanisms. Thus it is robust in the presence of change, flexible, and reusable.

Extending the Abstraction Further

Once could make a legitimate complaint about the design in Figure/Listing 6. The device controlled by the button must be derived from ButtonClient. What if the Lamp class comes from a third party library, and we cannot modify the source code.

Figure 7 demonstrates how the Adapter pattern can be used to connect a third party Lamp object to the model. The LampAdapter class simply translates the TurnOn and Turn-Off message inherited from ButtonClient, into whatever messages the Lamp class needs to see.

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The principle of dependency inversion is at the root of many of the benefits claimed for object-oriented technology. Its proper application is necessary for the creation of reusable frameworks. It is also critically important for the construction of code that is resilient to change. And, since the abstractions and details are all isolated from each other, the code is much easier to maintain.