The Dependency Inversion Principle
In this post I want to discuss the D of the S.O.L.I.D. principles and patterns. The principles and patterns subsumed in S.O.L.I.D. can be seen as the cornerstones of “good” application design. In this context D is the place holder for the dependency inversion principle. In a previous post I already have discussed the S which is the placeholder for single responsibility principle.
What is Bad Design?
Let’s first discuss a little bit about what bad design is. Is bad design when somebody claims
That’s not the way I would have done it…
Well, sorry, but this is not a valid measure for the quality of the design! This statement is purely based on personal preferences. So let’s find other, better criteria to define bad design. If a system exhibits any or all of the following three traits then we have identified bad design
- the system is rigid: it’s hard to change a part of the system without affection too many other parts of the system
- the system is fragile: when making a change, unexpected parts of the system break
- the system or component is immobile: it is hard to reuse it in another application because it cannot be disentangled from the current application.
An immobile design
Let’s now have a look a the latter of the traits of bad design mentioned above. A design is immobile when the desirable parts of the design are highly dependent upon other details that are not desired.
Imagine a sample where we have developed a class which contains a highly sophisticated encryption algorithm. This class takes as an input a source file name and a target file name. The content to encrypt is then read from the source file and the encrypted content is written to the target file.
Listing 1: Encryption Service depending on Details
The problem with the above class is that it is highly coupled to a certain input and output. In this case input and output are both files. But you might have invested quite some time and resources to develop the encryption algorithm which is the core responsibility of this service. It’s a shame that this encryption algorithm cannot be used in another context where the content to be encrypted is not present in a file but rather in a database and also where the encrypted content should not be written to a file but rather sent to a web service.
Certainly we could make the above service more flexible and change it’s implementation. We can put in place a switch for the content source type and one for the destination type of the encrypted content.
Listing 2: Slightly improved Encryption Service
However this adds new interdependencies to the system. As time goes on, and more and more source and/or destination types must participate in the encryption program, the “Encrypt” method will be littered with switch/case statements and will be dependent upon many lower level modules. It will eventually become rigid and fragile.
Comes the dependency inversion principle to the rescue.
The Dependency Inversion Principle
Theory: the dependency inversion principle states
a) High level modules should not depend upon low level modules. Both should depend upon abstractions
b) Abstractions should not depend upon details. Details should depend upon abstractions
One way to characterize the problem above is to notice that the method containing the high level policy, i.e. the Encrypt method , is dependent upon the low level detailed method that it controls. (i.e. GetFromFile and WriteToWebService). If we could find a way to make the Encrypt method independent of the details that it controls, then we could reuse it freely. We could produce other applications which used this service to encrypt content originating from any content source to any destination.
Consider the simple class diagram below.
Here we have an EncryptionService class which uses an abstract “Reader” class (identified by an interface IReader) and an abstract “Writer” class (identified by an interface IWriter). Note that the abstraction in this case is not achieved through inheritance but through the use of interfaces. We have separated the interface from the implementation.
The Encrypt method uses the “Reader” to get the content and sends the encrypted content to the “Writer”.
Listing 3: Encryption Service only depending upon Abstractions
The Encrypt method of the encryption service is now independent of a specific content reader or writer. The dependencies have been inverted; the EncryptionService class depends upon abstractions, and the detailed readers and writers depend upon the same abstractions.
The definition of the two interfaces used is
Listing 4: Reader and Writer Interfaces
Now we can reuse the encryption service. We can invent new kinds of “Reader” and “Writer” implementations that we can supply to the Encrypt method of the service. Moreover, no matter how many kinds of “Readers” and “Writers” are created, the encryption service will depend upon none of them. There will be no interdependencies to make the application fragile or rigid. And the encryption service can be used in many different contexts. The service is mobile.
Why call it dependency inversion?
The dependency structure of a well designed object oriented application is “inverted” with respect to the dependency structure that normally results from a “traditional” application which is implemented in a more procedural style. In a procedural application high level modules depend upon low level modules and abstractions depend upon details (as in listing 1 and 2).
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. 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.
When implementing an application the modules and components of a higher abstraction level should never directly depend upon the (implementation-) details of modules or components of a lower abstraction level. It’s the high level components that make an application unique. It’s the high level modules that contain most of the business value. Thus the high level components should dictate whether low level components have to change or not and not vice versa.
When a component does not depend on lower level components directly but only through abstractions this component is mobile – that is, the component is reusable in many different contexts.
Furthermore when the design is respecting the dependency inversion principle an application is less brittle or fragile. If some changes have to be made to a low level module there are no side effects to be expected that manifest themselves in other (possibly unexpected) parts of the system.