Dependency Inversion Principle

The Dependency Inversion Principle has been proposed by Robert C. Martin. Itstates that:
High level modules should not depend upon lowlevel modules. Both should depend upon abstractions.Abstractions should not depend upon details. Details should depend uponabstractions.
This principle seeks to "invert" the conventional notion that high levelmodules in software should depend upon the lower level modules. The principlestates that high level or low level modules should not depend upon each other,instead they should depend upon abstractions. This is best illustrated by anexample.
We will consider the design of a three layer protocol stack. The discussionis divided into three steps:
Original Design
Modified Design (Conforms to the Dependency Inversion Principle)
Advantages of Dependency Inversion
The following code shows the design of a threelayered radio link control (RLC) protocol stack. The three layers of theprotocol are:
RLC Physical Layer
RLC Datalink Layer
RLC Network Layer
All three layers are modeled as classes. A codeskeleton for these classes is presented below. This is not a good design. Ithas the following limitations:
All layers are dependent upon each other. The code for each layer needs to include the header files for all other layers.
Layer relationships are hard-coded. You cannot assemble new protocol by just assembling a new set of layers.
Design changes in one layer could impact the design of other layers.
The solution to these problems is to modify thedesign of layers by basing it on a common abstraction. This is the subject ofthenext section.
Layers
class RLC_Physical_Layer { RLC_Datalink_Layer *m_p_RLC_Datalink_Layer; public: void Device_Transmit(Datagram *p_Datagram) { Apply_Physical_Layer_Headers(p_Datagram); Write_Message_To_Device(p_Datagram); } void Handle_Device_Receive(Datagram *p_Datagram) { Remove_Physical_Layer_Header(p_Datagram); m_p_RLC_Datalink_Layer->Handle_Physical_Layer_Receive(p_Datagram); } }; class RLC_Datalink_Layer { RLC_Physical_Layer *m_p_RLC_Physical_Layer; RLC_Network_Layer *m_p_RLC_Network_Layer; public: void Datalink_Transmit(Datagram *p_Datagram) { Process_Datalink_Transmit(p_Datagram); m_p_RLC_Physical_Layer->Device_Transmit(p_Datagram); } void Handle_Physical_Layer_Receive(Datagram *p_Datagram) { Process_Datalink_Receive(p_Datagram); m_p_RLC_Network_Layer->Handle_Network_Layer_Receive(p_Datagram); } }; class RLC_Network_Layer { RLC_Datalink_Layer *m_p_RLC_Datalink_Layer; Application_Layer *m_p_Application_Layer; public: void Network_Transmit(Datagram *p_Datagram) { Process_Network_Layer_Transmit(p_Datagram); m_p_RLC_Datalink_Layer->Datalink_Transmit(p_Datagram); } void Handle_Datalink_Layer_Receive(Datagram *p_Datagram) { Process_Network_Layer_Receive(p_Datagram); m_p_Application_Layer->Handle_Application_Receive(p_Datagram); } };
We now apply the Dependency Inversion Principle to the the abovecode. We define an abstract class Protocol_Layer that represents ageneric layer. Important elements of the Protocol_Layer abstraction are:
The constructor allows you to associate a layer two more layers, viz. the upper layer and the lower layer.
A layer receives a message for transmission and invokes the layer specific processing. After layer specific processing, the message is passed to the lower layer.
When a message is received from the lower layer, layer specific processing is invoked. Once layer handling of the message is completed, the upper layer is invoked.
The three layers in the above example, will now inherit from theProtocol_Layer. This design completely decouples the three layers. All thelayers depend upon the abstraction but not on each other. The advantages aredescribed in thenext section.
Protocol_Layer Abstraction
// Protocol_Layer abstraction has been defined to decouple the // different protocol layers. Now the layers only depend upon // this abstraction. class Protocol_Layer { // Each layer can associate with an upper and lower layer. // This association is also in terms of the Protocol_Layer // class. Thus there is no dependency on the actual types // of individual layers. The only requirement is that they // should inherit from Protocol_Layer. Protocol_Layer *m_p_Lower_Layer; Protocol_Layer *m_p_Upper_Layer; // Each inheriting layer must override the following methods. // These methods handle the actual protocol processing. virtual void Process_Transmit(Datagram *p_Datagram) = 0; virtual void Process_Receive(Datagram *p_Datagram) = 0; public: // Create a protocol layer with the associated upper and // lower layers. Protocol_Layer(Protocol_Layer *p_Lower_Layer, Protocol_Layer *p_Upper_Layer) { m_p_Lower_Layer = p_Lower_Layer; m_p_Upper_Layer = p_Upper_Layer; } // Process and transmit the datagram passed by higher layer. void Transmit(Datagram *p_Datagram) { Process_Transmit(p_Datagram); if (m_p_Lower_Layer) { m_p_Lower_Layer->Transmit(p_Datagram); } } // Receive handler for a datagram received from lower layer void Handle_Receive(Datagram *p_Datagram) { Process_Receive(p_Datagram); if (m_p_Upper_Layer) { m_p_Upper_Layer->Handle_Receive(p_Datagram); } } }; class RLC_Physical_Layer : public Protocol_Layer { void Process_Transmit(); void Process_Receive(); public: RLC_Physical_Layer(Layer *p_Lower_Layer, Layer *p_Upper_Layer) : Protocol_Layer(p_Lower_Layer, p_Upper_Layer) { } }; class RLC_Datalink_Layer : public Protocol_Layer { void Process_Transmit(); void Process_Receive(); public: RLC_Datalink_Layer(Layer *p_Lower_Layer, Layer *p_Upper_Layer) : Protocol_Layer(p_Lower_Layer, p_Upper_Layer) { } }; class RLC_Network_Layer : public Protocol_Layer { void Process_Transmit(); void Process_Receive(); public: RLC_Network_Layer(Layer *p_Lower_Layer, Layer *p_Upper_Layer) : Protocol_Layer(p_Lower_Layer, p_Upper_Layer) { } };
Application of the Dependency Inversion Principle has given us the followingadvantages:
There is no coupling between the three layers. Each layer's implementation does not even need to include the header files for other layers.
You have flexibility in mixing and matching different layers. You could use the physical layer implementation with a different datalink layer. This can be accomplished without writing a single line of code in the physical layer class.
You can sandwich layers between two layers. For example, you can add a debugging layer between physical and datalink layers. This layer could trap messages between the two layers.
For more details about the design, please refer to theProtocol Layer Design Patternarticle.