SOA with .NET and Windows Azure : WCF Extensions - WCF Extensibility

Available channel bindings include:

• protocol channel bindings that determine security, reliability and context flow settings

• transport channel bindings that determine the underlying transport protocol (including TCP and HTTP)

• message encoding bindings that determine the encoding at the wire (including text/XML, binary, and MTOM)

Although these default channel bindings will cover common communication scenarios, there may be certain requirements that can only be accommodated via further customization. For this purpose, WCF provides an extensibility model with several extension points.

Before we explore the extensibility model, we first need to establish how WCF can be represented with two fundamental architectural layers.

WCF Layers

As shown in Figure 1, WCF is partitioned into two main layers: the service model layer and the channel layer.

The service model layer essentially provides a programming model that enables solutions to use WCF. For example, all the information about user-defined service contracts is found in this layer.

Underneath the service model layer is a shared message layer used as the transport and also known as the channel layer. The channel layer provides several communication channels and is also known as the channel stack. Addresses and bindings defined in the service model layer provide the mechanisms used to control the channel stack. Bindings define the transport and addresses define the destinations of messages.

Layered Extensibility

Figure 2 further expands on the layered view by illustrating how the proxy and dispatcher relate to the service model layer and represent touch points for services. You’ll notice that underneath the proxy and dispatcher are the protocol and transport channel stacks. The protocol channels are used to process SOAP messages, add security, and add and remove headers, whereas the transport channels are used to send and receive messages.

WCF is extensible at the service model layer and at the channel layer. If you need to make changes that affect service behavior, then you need to ensure that service model extensibility is used. If the requirement affects changes to the message at the wire level, channel layer extensibility is used.

Channel Layer Extensibility

The WCF channel extensibility model can be used to extend channels for non-supported transports such as UDP and SMTP or other custom and proprietary transports. Specifically, this form of extensibility at the channel layer allows for:

• the creation of new transports

• integration with incompatible and proprietary systems

• implementation of custom infrastructure protocols

The ability to create and extend WCF with new transports can be used to simplify integration between incompatible systems. For example, WCF channels can be customized to talk RMI on the wire in order to integrate with a Java application. Channel layer extensibility also allows for the creation of custom infrastructure channels, such as proprietary SOAP-level compression.

In the next example, the address, binding and contract are set and associated with a host. The binding indicates the runtime is going to construct a channel stack optimized for TCP with binary as the message encoding for efficient message transfer. The host.Open(); statement invokes channels and channel listeners to enable communication.

Example 1.

ServiceHost host = new ServiceHost(typeof(MyService));
Uri address = new Uri("net.tcp://temp/service/endpoint");
Binding binding = new NetTcpBinding();
Type contract = typeof(IContract);
host.AddEndpoint(address, binding, contract);
host.Open();