Introduction to WCF Concepts - Fundamental WCF Concepts (Page 3 of 4 )
At its core, WCF is a development platform for service-oriented applications. As I mentioned earlier, WCF is part of the .NET Framework 3.0, which comprises a set of new assemblies that rely on the .NET Framework 2.0. System.ServiceModel is the assembly that contains core functionality for WCF, which explains why the WCF platform is often called the service model. Any project that exposes or consumes WCF services must reference the System.ServiceModel assembly, and possibly other supporting assemblies.
Before you can begin to do interesting things with the service model, it helps to understand the core features that make it possible to create, host, and consume services. In this section, I will briefly summarize some of the concepts that will be elaborated on in this chapter, to help you on your way.
Message Serialization
All enterprise applications at some point must make remote calls across process and machine boundaries. This is handled by sending messages between applications. The format of the message is what determines an applications ability to communicate with other applications. Remote procedure calls (RPC) and XML messaging formats are two common ways for applications to communicate.
RPC calls are used to communicate with objects (components) across boundaries--for example, calls from a client application to a remote object living in another process. RPC calls are marshaled by converting them to messages and sending them over a transport protocol such as TCP. When the message reaches its destination, it is un-marshaled and converted into a stack frame that invokes the object. This is all usually handled transparently through a client proxy and stub at the destination. Both proxy and stub know how to construct and deconstruct messages. This process is known as serialization and deserialization.
Figure 1-14 illustrates the serialization and deserialization process (often just called serialization) from a high level. As I mentioned, the transport carries a message according to the agreed-upon format of both the client and remote application. As far as the client is concerned, it is usually working with a proxy that looks like the remote object. When a method is invoked at the client, the proxy invokes underlying plumbing of the technology (for example, Enterprise Services or .NET Remoting) to serialize the outgoing message. The remote application usually listens for messages on a particular port, and as they arrive, deserializes those messages (using the same technology) to build a stack frame and invoke the appropriate method on the remote object. The method return is likewise serialized by the underlying plumbing and returned to the client, at which time the plumbing deserializes the message and constructs a stack frame for the response. RPC communication like this comes in many flavors, and each flavor is generally not compatible with another--that's why RPC is not interoperable.
Figure 1-14. RPC serialization and deserialization
To achieve interoperability, systems rely on a standard format for messages understood by both ends of the communication. Applications still exchange messages, but they are formatted in XML according to known protocols. The technology used to support this is traditionally associated with web services such as ASMX, WSE, or WCF. As Figure 1-15 illustrates, the serialization process is consistent with RPC, the key difference being the underlying plumbing, the format of the message, and the target object, which is usually a web service. The other difference is in the lifetime of the service. While remote objects are frequently kept alive for the duration of a client session, web services are typically constructed anew for each call.
Figure 1-15. Web service serialization and deserialization
WCF can be used to achieve both RPC-style messaging and web service messaging. In both cases, the type that ultimately processes messages is a service type, but its lifetime can be controlled to behave like traditional client-server components or like web services without the notion of a session. The service model handles all serialization activities based on configuration settings.
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 This article is excerpted from chapter 1 of the book Learning WCF A Hands-on Guide, written by Michele Leroux Bustamante (O'Reilly, 2007; ISBN: 0596101627). Check it out today at your favorite bookstore. Buy this book now.
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