Wcf

What Is Windows Communication Foundation? The global acceptance of Web services, which includes standard protocols for application-toapplication communication, has changed software development. For example, the functions that Web services now provide include security, distributed transaction coordination, and reliable communication. The benefits of the changes in Web services should be reflected in the tools and technologies that developers use. Windows Communication Foundation (WCF) is designed to offer a manageable approach to distributed computing, broad interoperability, and direct support for service orientation. WCF simplifies development of connected applications through a new service-oriented programming model. WCF supports many styles of distributed application development by providing a layered architecture. At its base, the WCF channel architecture provides asynchronous, untyped messagepassing primitives. Built on top of this base are protocol facilities for secure, reliable, transacted data exchange and broad choice of transport and encoding options. The typed programming model (called the service model) is designed to ease the development of distributed applications and to provide developers with expertise in ASP.NET Web services, .NET Framework remoting, and Enterprise Services, and who are coming to WCF with a familiar development experience. The service model features a straightforward mapping of Web services concepts to those of the .NET Framework common language runtime (CLR), including flexible and extensible mapping of messages to service implementations in languages such as Visual C# or Visual Basic. It includes serialization facilities that enable loose coupling and versioning, and it provides integration and interoperability with existing .NET Framework distributed systems technologies such as Message Queuing (MSMQ), COM+, ASP.NET Web services, Web Services Enhancements (WSE), and a number of other functions. Problem Example The following example illustrates some of the problems that WCF addresses. A car rental company decides to create a new application for reserving cars. The creators of this rental car reservation application know that the business logic it implements must be accessible by other software running both inside and outside their company. Accordingly, they decide to build it in a service-oriented style, with the application’s logic exposed to other software through a well-defined set of services. To implement these services, and thus communicate with other software, the new application will use WCF.

Over its lifetime, the rental car reservation application will likely be accessed by a range of other applications. When it is designed, however, the architects of the rental car reservation application know that its business logic will be accessed, as shown in the preceding figure, by three other kinds of software:

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A call center client application running on the Windows desktops that are used by employees in the organization’s call center. Created specifically for the new reservations system, this application will also be built using the Microsoft .NET Framework and WCF. This application is not truly distinct from the new rental car reservation application, because its only purpose is to act as a client for the new system. From a service-oriented perspective, it is just another client for the reservation system’s business logic.

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An existing reservation application built on a J2EE server running on a non-Windows system. Due to a recent merger with another car rental firm, this existing system must be able to access the new application’s logic to provide customers of the merged firms with a unified experience.

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Partner applications running on a variety of platforms, each located within a company that has a business arrangement with the car rental firm. Partners might include travel agencies, airlines, and others that have a business requirement to make car rental reservations.

The diverse communication requirements for the new rental car reservation application are not simple. For interactions with the call center client application, for instance, performance is important, while interoperability is straightforward, because both are built on the .NET Framework. For communication with the existing J2EE-based reservation application and with the diverse partner applications, however, interoperability becomes the highest goal. The security requirements are also quite different,

varying across local Windows-based applications, a J2EE-based application running on another operating system, and a variety of partner applications coming in across the Internet. Even transactional requirements might vary, with only the internal applications being allowed to make transactional requests. How can these diverse business and technical requirements be met without exposing the creators of the new application to unmanageable complexity? WCF is designed for this diverse but realistic scenario and is the default technology for Windows applications that expose and access services. This topic provides an introduction to WCF, examining what it provides and showing how it is used. Throughout this introduction, the scenario just described will serve as an example. The goal is to make clear what WCF is, show what problems it solves, and illustrate how it solves those problems. Addressing the Problem The foundation for new Windows-based applications is the .NET Framework. Accordingly, WCF is implemented primarily as a set of classes on top of the .NET Framework CLR. Because it extends their familiar environment, WCF enables developers who create object-oriented applications using the .NET Framework today to also build service-oriented applications in a familiar way.

The figure shows a view of a WCF client and service. The two interact using SOAP, the WCF native message representation, so even though the figure shows both parties built on WCF, this is not required. WCF is built on .NET Framework 2.0. As the scenario described earlier suggests, WCF addresses a range of challenges for communicating applications. Three things stand out, however, as the most important aspects of WCF:

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Unification of existing .NET Framework communication technologies.

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Support for cross-vendor interoperability, including reliability, security, and transactions.

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Explicit service orientation.

Unification of Microsoft Distributed Computing Technologies In the absence of WCF, the development team that implements the rental car application would need to choose the right distributed technology from the multiple choices offered by the .NET Framework. Yet given the diverse requirements of this application, no single technology would fit the requirements. Instead, the application would probably use multiple existing .NET Framework technologies, such as the following:

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ASP.NET Web services (ASMX). An option for communicating with the J2EE-based existing reservation application and with the partner applications across the Internet. Given that basic Web services are supported today on most platforms, this was the most direct way to achieve cross-vendor interoperability before the release of WCF.

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.NET Framework remoting. An option for communication with the call center application, because both are built on the .NET Framework. Remoting is designed expressly for tightly coupled .NET-to-.NET communication, so it offers a seamless and straightforward development experience for applications in the local network.

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Enterprise Services. Used by the rental car reservation application for managing object lifetimes and defining distributed transactions. These functions could be useful in communicating and integrating with any of the other applications in this scenario, but Enterprise Services supports only a limited set of communication options.

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WSE. Could be used along with ASMX to communicate with the J2EE-based reservation application and with the partner applications. Because it implements more recently defined Web services agreements, known collectively as the WS-* specifications, WSE allows for more flexible Web services security, as long as all applications involved support compatible versions of these new specifications.

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Microsoft Message Queuing (MSMQ). Used to communicate with Windows-based partner applications that require guaranteed data delivery as well as decoupling of workloads and application lifetimes. The durable messaging that Message Queuing provides is typically the best solution for intermittently connected applications.

Built on .NET Framework, the rental car reservation application must use more than one of these communication technologies to meet its requirements. Although this is technically possible, the resulting application would be complex to implement and challenging to maintain. With WCF, the solution is much easier to implement. As the figure shows, WCF can be used for all the situations previously described. Accordingly, the rental car reservation application can use this single technology for all of its application-to-application communication. The following shows how WCF addresses each of these requirements:

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Because WCF can communicate using Web services, interoperability with other platforms that also support SOAP, such as the leading J2EE-based application servers, is straightforward.

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You can also configure and extend WCF to communicate with Web services using messages not based on SOAP, for example, simple XML formats like RSS.

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Performance is of paramount concern for most businesses. WCF is developed with the goal of being one of the fastest distributed application platform developed by Microsoft. For a high-

level performance comparison between WCF and other Microsoft .NET distributed communication technologies, see http://go.microsoft.com/fwlink/?LinkId=94274.

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To allow optimal performance when both parties in a communication are built on WCF, the wire encoding used in this case is an optimized binary version of an XML Information Set. Messages still conform to the data structure of a SOAP message, but their encoding uses a binary representation of that data structure rather than the standard angle-brackets-and-text format of the XML 1.0 text encoding. Using this option makes sense for communicating with the call center client application, because it is also built on WCF, and performance is an important concern.

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Managing object lifetimes, defining distributed transactions, and other aspects of Enterprise Services are now provided by WCF. They are available to any WCF-based application, which means that the rental car reservation application can use them with any of the other applications it communicates with.

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Because it supports a large set of the WS-* specifications, WCF helps provide reliability, security, and transactions when communicating with any platform that also supports these specifications.

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The WCF option for queued messaging, built on Message Queuing, allows applications to use persistent queuing without using another set of application programming interfaces.

The result of this unification is greater functionality and significantly reduced complexity. Interoperability with Applications Built on Other Technologies While WCF introduces a new development environment for distributed applications, it is designed to interoperate well with the non-WCF applications. There are two important aspects to WCF interoperability: interoperability with other platforms, and interoperability with the Microsoft technologies that preceded WCF. The following section describes both. Interoperability with Other Web Services Platforms Enterprises today typically have systems and applications that they purchased from a range of suppliers. In the rental car application, for instance, communication is required with various other software applications written in various languages and running on various operating systems. Because WCF’s fundamental communication mechanism is SOAP-based Web services, WCF-based applications can communicate with other software running in a variety of contexts. An application built on WCF can interact with all of the following:

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WCF-based applications running in a different process on the same Windows machine.

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WCF-based applications running on another Windows machine.

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Applications built on other technologies, such as J2EE application servers, that support standard Web services. These applications can be running on Windows machines or on machines running other operating systems.

To allow more than just basic communication, WCF implements Web services technologies defined by the WS-* specifications. All of these specifications were originally defined by Microsoft, IBM, and other vendors working together. As the specifications become stable, ownership often passes to standards bodies, such as the World Wide Web Consortium (W3C) or the Organization for the Advancement of Structured Information Standards (OASIS). These specifications address several areas, including basic messaging, security, reliability, transactions, and working with a service’s metadata. For more information, see Interoperability and Integration with Windows Communication Foundation. For more information about advanced Web services specifications, see http://go.microsoft.com/fwlink/? LinkId=86603. Grouped by function, those specifications cover:

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Messaging: SOAP is the foundation for Web services and defines a basic envelope that contains header and a body sections. WS-Addressing defines additions to the SOAP header for addressing SOAP messages, which frees SOAP from relying on the underlying transport protocol, such as HTTP, to carry addressing information. Message Transmission Optimization Mechanism (MTOM) defines an optimized transmission format for SOAP messages with large binary data contents based on the XML-binary Optimized Packaging (XOP) specification.

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Metadata: The Web Services Description Language (WSDL) defines a standard language for specifying services and various aspects of how those services can be used. WS-Policy allows specification of more dynamic aspects of a service’s behavior that cannot be expressed in WSDL, such as a preferred security option. WS-MetadataExchange allows a client to directly request descriptive information about a service, such as its WSDL and its policies, using SOAP.

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Security: WS-Security, WS-SecureConversation, WS-Trust, and WS-Federation all define additions to SOAP messages for providing authentication, data integrity, data privacy, and other security features.

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Reliability: WS-Reliable Messaging defines additions to the SOAP header that allow reliable end-to-end communication, even when one or more Web services intermediaries must be traversed.

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Transactions: Built on WS-Coordination, WS-Atomic Transaction allows coordinating two-phase commit transactions in the context of Web services conversations.

The rental car reservation application would likely use several of these more advanced technologies. For example, WS-Addressing is essential whenever SOAP is used over a transport mechanism other than HTTP, which might be the case for communication with the .NET Framework-based call center

client application. WCF relies on WS-Policy and WS-Metadata Exchange to discover whether the system it is communicating with is also using WCF and for other things. Reliable communication is essential for most situations, so it is likely that WS-Reliable Messaging would be used to interact with many of the other applications in this scenario. Similarly, you might also use WS-Security and the related specifications for securing the communication with one or more of the applications, because all would require some kind of protection against unauthorized access or message modification and interception. For the applications that require transaction integration with the rental car reservation system, WS-Atomic Transaction would be essential. Finally, MTOM could be used whenever an optimized wire format for binary data is necessary (for instance for pictures of fleet examples), and both sides of the communication supported this option. The key point is that WCF implements interoperable Web services complete with cross-platform security, reliability, transactions, and other services. To provide maximum throughput, WCF-to-WCF communication can be significantly optimized, but all other communication uses standard Web services protocols. In fact, it is possible for a single application to expose its services to both kinds of clients. Interoperability with Microsoft Technologies Many Microsoft customers have made significant investments in the .NET Framework technologies that WCF includes. Protecting those investments was a fundamental goal of WCF’s designers. Installing WCF does not break existing technology, so there is no requirement that organizations change existing applications to use it. A clear upgrade path is provided, however, and wherever possible, WCF interoperates with those earlier technologies. For example, both WCF and ASMX use SOAP, so WCF-based applications can directly interoperate with those built on ASMX. Existing Enterprise Services applications can also be wrapped with WCF interfaces, allowing them to interoperate with applications built on WCF. And because persistent queuing in WCF relies on MSMQ, WCF-based applications can interoperate directly with non-WCFbased applications built using native MSMQ interfaces. In the rental car reservations application, software built using any of these earlier technologies could directly connect to and use the new system’s WCF-based services. Interoperability is not always possible, however. For example, even though WSE 1.0 and WSE 2.0 implement some of the same WS-* specifications as WCF, these earlier technologies implement earlier versions of the specifications. Version 3.0 of WSE does allow interoperability with WCF, but earlier versions do not. For more information about interoperability, see Migrating WSE 3.0 Web Services to WCF. Interoperability with Other XML Protocols The future of the Internet is not predictable and the technologies used today may evolve or be replaced. Today, a popular trend in building Web-centric applications (called by many "Web 2.0"), is an application model based on communication using only simple XML formats that are not SOAP-based and exclusively rely on HTTP as a transport and as an application protocol. For example, the

Representational State Transfer (REST) architectural style has no notion of user-defined operations for dealing with data. Instead, application state is associated with HTTP URLs and HTTP methods (such as PUT, POST, DELETE, and GET). This approach is in contrast to the creation of user-defined procedures or functions that most developers are familiar with in an enterprise environment. However, the REST approach is of value in scenarios where services must function as the back end of Web 2.0 applications. REST is just one example of an evolving Web 2.0 technology. In this environment of experimental programming models and ongoing reinterpretation and refinement of standards, flexibility is required to cope with unforeseeable changes. WCF is flexible. For example, while WCF uses SOAP as an underlying structure, it is not bound to using SOAP for wire communication. In fact, WCF can be configured to process "plain" XML data that is not wrapped in a SOAP envelope. WCF can also be extended to support specific XML formats, such as ATOM (a popular RSS standard), and even non-XML formats, such as JavaScript Object Notation (JSON). This flexibility ensures that code written today will be valid in the future, even if protocols change or are replaced. Therefore, WCF was designed for the present and the future. Fundamental Windows Communication Foundation Concepts This document provides a high-level view of the Windows Communication Foundation (WCF) architecture. It is intended to explain key concepts and how they fit together. For a tutorial on creating the simplest version of a WCF service and client, see Getting Started Tutorial. To learn WCF programming, see Basic WCF Programming. WCF Fundamentals Windows Communication Foundation (WCF) is a runtime and a set of APIs for creating systems that send messages between services and clients. The same infrastructure and APIs are used to create applications that communicate with other applications on the same computer system or on a system that resides in another company and is accessed over the Internet. Messaging and Endpoints WCF is based on the notion of message-based communication, and anything that can be modeled as a message (for example, an HTTP request or an MSMQ message) can be represented in a uniform way in the programming model. This enables a unified API across different transport mechanisms. The model distinguishes between clients, which are applications that initiate communication, and services, which are applications that wait for clients to communicate with them and respond to that communication. A single application can act as both a client and a service. Messages are sent between endpoints. Endpoints are places where messages are sent or received (or both), and they define all the information required for the message exchange. A service exposes one or more application endpoints (as well as zero or more infrastructure endpoints), and the client generates an endpoint that is compatible with one of the service's endpoints.

An endpoint describes in a standard-based way where messages should be sent, how they should be sent, and what the messages should look like. A service can expose this information as metadata that clients can process to generate appropriate WCF clients and communication stacks. Communication Protocols One required element of the communication stack is the transport protocol. Messages can be sent over intranets and the Internet using common transports, such as HTTP and TCP. Other transports are included that support communication with Microsoft Message Queuing (MSMQ) applications and nodes on a Peer Networking mesh. More transport mechanisms can be added using the built-in extension points of WCF. Another required element in the communication stack is the encoding that specifies how any given message is formatted. WCF provides the following encodings:

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Text encoding, an interoperable encoding.

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Message Transmission Optimization Mechanism (MTOM) encoding, which is an interoperable way for efficiently sending unstructured binary data to and from a service.

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Binary encoding for efficient transfer.

More encoding mechanisms (for example, a compression encoding) can be added using the built-in extension points of WCF. Message Patterns WCF supports several messaging patterns, including request-reply, one-way, and duplex communication. Different transports support different messaging patterns, and thus affect the types of interactions that they support. The WCF APIs and runtime also help you to send messages securely and reliably. WCF Terms Other concepts and terms used in the WCF documentation include the following. message A message is a self-contained unit of data that may consist of several parts, including a body and headers. service A service is a construct that exposes one or more endpoints, with each endpoint exposing one or more service operations. endpoint An endpoint is a construct at which messages are sent or received (or both). It comprises a location (an address) that defines where messages can be sent, a specification of the communication mechanism (a binding) that described how messages should be sent, and a

definition for a set of messages that can be sent or received (or both) at that location (a service contract) that describes what message can be sent. An WCF service is exposed to the world as a collection of endpoints. application endpoint An endpoint exposed by the application and that corresponds to a service contract implemented by the application. infrastructure endpoint An endpoint that is exposed by the infrastructure to facilitate functionality that is needed or provided by the service that does not relate to a service contract. For example, a service might have an infrastructure endpoint that provides metadata information. address An address specifies the location where messages are received. It is specified as a Uniform Resource Identifier (URI). The URI schema part names the transport mechanism to use to reach the address, such as HTTP and TCP. The hierarchical part of the URI contains a unique location whose format is dependent on the transport mechanism. The endpoint address enables you to create unique endpoint addresses for each endpoint in a service, or under certain conditions share an address across endpoints. The following example shows an address using the HTTPS protocol with a non-default port: Copy Code HTTPS://cohowinery:8005/ServiceModelSamples/CalculatorService binding A binding defines how an endpoint communicates to the world. It is constructed of a set of components called binding elements that "stack" one on top of the other to create the communication infrastructure. At the very least, a binding defines the transport (such as HTTP or TCP) and the encoding being used (such as text or binary). A binding can contain binding elements that specify details like the security mechanisms used to secure messages, or the message pattern used by an endpoint. For more information, see Configuring Windows Communication Foundation Services. binding element A binding element represents a particular piece of the binding, such as a transport, an encoding, an implementation of an infrastructure-level protocol (such as WSReliableMessaging), or any other component of the communication stack. behaviors A behavior is a component that controls various run-time aspects of a service, an endpoint, a particular operation, or a client. Behaviors are grouped according to scope: common behaviors affect all endpoints globally, service behaviors affect only service-related aspects, endpoint behaviors affect only endpoint-related properties, and operation-level behaviors affect particular operations. For example, one service behavior is throttling, which specifies how a service reacts when an excess of messages threaten to overwhelm its handling capabilities. An endpoint behavior, on the other hand, controls only aspects relevant to endpoints, such as how and where to find a security credential.

system-provided bindings WCF includes a number of system-provided bindings. These are collections of binding elements that are optimized for specific scenarios. For example, the WSHttpBinding is designed for interoperability with services that implement various WS-* specifications. These predefined bindings save time by presenting only those options that can be correctly applied to the specific scenario. If a predefined binding does not meet your requirements, you can create your own custom binding. configuration versus coding Control of an application can be done either through coding, through configuration, or through a combination of both. Configuration has the advantage of allowing someone other than the developer (for example, a network administrator) to set client and service parameters after the code is written and without having to recompile. Configuration not only enables you to set values like endpoint addresses, but also allows further control by enabling you to add endpoints, bindings, and behaviors. Coding allows the developer to retain strict control over all components of the service or client, and any settings done through the configuration can be inspected and if needed overridden by the code. service operation A service operation is a procedure defined in a service's code that implements the functionality for an operation. This operation is exposed to clients as methods on a WCF client. The method may return a value, and may take an optional number of arguments, or take no arguments, and return no response. For example, an operation that functions as a simple "Hello" can be used as a notification of a client's presence and to begin a series of operations. service contract The service contract ties together multiple related operations into a single functional unit. The contract can define service-level settings, such as the namespace of the service, a corresponding callback contract, and other such settings. In most cases, the contract is defined by creating an interface in the programming language of your choice and applying the ServiceContractAttribute attribute to the interface. The actual service code results by implementing the interface. operation contract An operation contract defines the parameters and return type of an operation. When creating an interface that defines the service contract, you signify an operation contract by applying the OperationContractAttribute attribute to each method definition that is part of the contract. The operations can be modeled as taking a single message and returning a single message, or as taking a set of types and returning a type. In the latter case, the system will determine the format for the messages that need to be exchanged for that operation. message contract A message contract describes the format of a message. For example, it declares whether message elements should go in headers versus the body, what level of security should be applied to what elements of the message, and so on. fault contract

A fault contract can be associated with a service operation to denote errors that can be returned to the caller. An operation can have zero or more faults associated with it. These errors are SOAP faults that are modeled as exceptions in the programming model. data contract The data types a service uses must be described in metadata to enable others to interoperate with the service. The descriptions of the data types are known as the data contract, and the types can be used in any part of a message, for example, as parameters or return types. If the service is using only simple types, there is no need to explicitly use data contracts. hosting A service must be hosted in some process. A host is an application that controls the lifetime of the service. Services can be self-hosted or managed by an existing hosting process. self-hosted service A self-hosted service is one that runs within a process application that the developer created. The developer controls its lifetime, sets the properties of the service, opens the service (which sets it into a listening mode), and closes the service. hosting process A hosting process is an application that is designed to host services. These include Internet Information Services (IIS), Windows Activation Services (WAS), and Windows Services. In these hosted scenarios, the host controls the lifetime of the service. For example, using IIS you can set up a virtual directory that contains the service assembly and configuration file. When a message is received, IIS starts the service and controls its lifetime. instancing A service has an instancing model. There are three instancing models: "single," in which a single CLR object services all the clients; "per call," in which a new CLR object is created to handle each client call; and "per session," in which a set of CLR objects are created, one for each separate session. The choice of an instancing model depends on the application requirements and the expected usage pattern of the service. client application A client application is a program that exchanges messages with one or more endpoints. The client application begins by creating an instance of a WCF client and calling methods of the WCF client. It is important to note that a single application can be both a client and a service. channel A channel is a concrete implementation of a binding element. The binding represents the configuration, and the channel is the implementation associated with that configuration. Therefore, there is a channel associated with each binding element. Channels stack on top of each other to create the concrete implementation of the binding: the channel stack. WCF client A WCF client is a client-application construct that exposes the service operations as methods (in the .NET Framework programming language of your choice, such as Visual Basic or Visual C#). Any application can host a WCF client, including an application that hosts a service. Therefore, it is possible to create a service that includes WCF clients of other services.

A WCF client can be automatically generated by using the ServiceModel Metadata Utility Tool (Svcutil.exe) and pointing it at a running service that publishes metadata. metadata The metadata of a service describes the characteristics of the service that an external entity needs to understand to communicate with the service. Metadata can be consumed by the ServiceModel Metadata Utility Tool (Svcutil.exe) to generate a WCF client and accompanying configuration that a client application can use to interact with the service. The metadata exposed by the service includes XML schema documents, which define the data contract of the service, and WSDL documents, which describe the methods of the service. When enabled, metadata for the service is automatically generated by WCF by inspecting the service and its endpoints. To publish metadata from a service, you must explicitly enable the metadata behavior. security Security in WCF includes confidentiality (encryption of messages to prevent eavesdropping), integrity (the means for detection of tampering with the message), authentication (the means for validation of servers and clients), and authorization (the control of access to resources). These functions are provided by either leveraging existing security mechanisms, such as TLS over HTTP (also known as HTTPS), or by implementing one or more of the various WS-* security specifications. transport security mode Security can be provided by one of three modes: transport mode, message security mode, and transport with message credential mode. The transport security mode specifies that confidentiality, integrity, and authentication are provided by the transport layer mechanisms (such as HTTPS). When using a transport like HTTPS, this mode has the advantage of being efficient in its performance, and well understood because of its prevalence on the Internet. The disadvantage is that this kind of security is applied separately on each hop in the communication path, making the communication susceptible to a "man in the middle" attack. message security mode Message security mode specifies that security is provided by implementing one or more of the security specifications, such as the specification named "Web Services Security: SOAP Message Security" (available at http://go.microsoft.com/fwlink/?LinkId=94684). Each message contains the necessary mechanisms to provide security during its transit, and to enable the receivers to detect tampering and to decrypt the messages. In this sense, the security is encapsulated within every message, providing end-to-end security across multiple hops. Because security information becomes part of the message, it is also possible to include multiple kinds of credentials with the message (these are referred to as claims). This approach also has the advantage of enabling the message to travel securely over any transport, including multiple transports between its origin and destination. The disadvantage of this approach is the complexity of the cryptographic mechanisms employed, resulting in performance implications. transport with message credential security mode

This mode uses the transport layer to provide confidentiality, authentication, and integrity of the messages, while each of the messages can contain multiple credentials (claims) required by the receivers of the message. WS-* Shorthand for the growing set of Web Service (WS) specifications, such as WS-Security, WSReliableMessaging, and so on, that are implemented in WCF.

Windows Communication Foundation Architecture The following graphic illustrates the major layers of the Windows Communication Foundation (WCF) architecture. WCF Architecture

Contracts and Descriptions Contracts define various aspects of the message system. The data contract describes every parameter that makes up every message that a service can create or consume. The message parameters are defined by XML Schema definition language (XSD) documents, enabling any system that understands XML to process the documents. The message contract defines specific message parts using SOAP protocols, and allows finer-grained control over parts of the message, when interoperability demands such precision. The service contract specifies the actual method signatures of the service, and is

distributed as an interface in one of the supported programming languages, such as Visual Basic or Visual C#. Policies and bindings stipulate the conditions required to communicate with a service. For example, the binding must (at a minimum) specify the transport used (for example, HTTP or TCP), and an encoding. Policies include security requirements and other conditions that must be met to communicate with a service. Service Runtime The service runtime layer contains the behaviors that occur only during the actual operation of the service, that is, the runtime behaviors of the service. Throttling controls how many messages are processed, which can be varied if the demand for the service grows to a preset limit. An error behavior specifies what occurs when an internal error occurs on the service, for example, by controlling what information is communicated to the client. (Too much information can give a malicious user an advantage in mounting an attack.) Metadata behavior governs how and whether metadata is made available to the outside world. Instance behavior specifies how many instances of the service can be run (for example, a singleton specifies only one instance to process all messages). Transaction behavior enables the rollback of transacted operations if a failure occurs. Dispatch behavior is the control of how a message is processed by the WCF infrastructure. Extensibility enables customization of runtime processes. For example, message inspection is the facility to inspect parts of a message, and parameter filtering enables preset actions to occur based on filters acting on message headers. Messaging The messaging layer is composed of channels. A channel is a component that processes a message in some way, for example, by authenticating a message. A set of channels is also known as a channel stack. Channels operate on messages and message headers. This is different from the service runtime layer, which is primarily concerned about processing the contents of message bodies. There are two types of channels: transport channels and protocol channels. Transport channels read and write messages from the network (or some other communication point with the outside world). Some transports use an encoder to convert messages (which are represented as XML Infosets) to and from the byte stream representation used by the network. Examples of transports are HTTP, named pipes, TCP, and MSMQ. Examples of encodings are XML and optimized binary. Protocol channels implement message processing protocols, often by reading or writing additional headers to the message. Examples of such protocols include WS-Security and WS-Reliability. The messaging layer illustrates the possible formats and exchange patterns of the data. WS-Security is an implementation of the WS-Security specification enabling security at the message layer. The WSReliable Messaging channel enables the guarantee of message delivery. The encoders present a

variety of encodings that can be used to suit the needs of the message. The HTTP channel specifies that the HyperText Transport Protocol is used for message delivery. The TCP channel similarly specifies the TCP protocol. The Transaction Flow channel governs transacted message patterns. The Named Pipe channel enables interprocess communication. The MSMQ channel enables interoperation with MSMQ applications. Hosting and Activation In its final form, a service is a program. Like other programs, a service must be run in an executable. This is known as a self-hosted service. Services can also be hosted, or run in an executable managed by an external agent, such as IIS or Windows Activation Service (WAS). WAS enables WCF applications to be activated automatically when deployed on a computer running WAS. Services can also be manually run as executables (.exe files). A service can also be run automatically as a Windows service. COM+ components can also be hosted as WCF services.