WCF offers immensely valuable support for reliability, transactions, concurrency management, security, and instance activation, all of which rely on the WCF interception-based architecture. Having the client interact with a proxy means that WCF is always present between the service and the client, intercepting the call and performing pre-call and post-call processing.The interception starts when the proxy serializes the call stack frame to a message and sends the message down a chain of channels. The channel is merely an interceptor whose purpose is to perform a specific task. Each client-side channel does pre-call processing of the message. The exact structure and composition of the chain depend mostly on the binding. For example, one of the channels may be responsible for encoding the message (binary, text, or MTOM), another for passing the security call context, another for propagating the client transaction, another for managing the reliable session, another for encrypting the message body (if so configured), and so on. The last channel on the client side is the transport channel, which sends the message over the configured transport to the host.
On the host side, the message goes through another chain of channels that perform host-side pre-call processing of the message. The first channel on the host side is the transport channel, which receives the message from the transport. Subsequent channels perform various tasks, such as decryption of the message body, decoding of the message, joining the propagated transaction, setting the security principal, managing the session, and activating the service instance. The last channel on the host side passes the message to the dispatcher. The dispatcher converts the message to a stack frame and calls the service instance.
This sequence is depicted in Figure 1
The service has no way of knowing that it was not called by a local client. In fact, it was called by a local client—the dispatcher. The interception both on the client and the service sides ensures that the client and the service get the runtime environments they require to operate properly.
The service instance executes the call and returns control to the dispatcher, which then converts the returned values and error information (if any) into a return message. The process is then reversed: the dispatcher passes the message through the host-side channels to perform post-call processing, such as managing the transaction, deactivating the instance, encoding the reply, encrypting it, and so on. The returned message then goes to the transport channel, which sends it to the client-side channels for clientside post-call processing. This process in turn consists of tasks such as decryption, decoding, committing or aborting the transaction, and so on. The last channel passes the message to the proxy, which converts the returned message to a stack frame and returns control to the client.