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Windows Communication Foundation, sometimes shortened to just WCF, previously codenamed Indigo, is a new communication subsystem to enable applications, in one machine or across multiple machines connected by a network, to communicate. WCF applications can be developed in any language which can target the .NET runtime.
It is one of the four major application programming interfaces introduced as part of .NET Framework 3.0, which is included with Windows Vista and Windows Server "Longhorn"; Windows XP and Windows Server 2003 are supported as well.
The WCF programming model unifies Web Services, .NET Remoting, Distributed Transactions, and Message Queues into a single Service-oriented programming model for distributed computing. It is intended to provide the rapid application development methodology to the development of web services, with a single API for inter-process communication in a local machine, LAN, or over the Internet. WCF runs in a sandbox and provides the enhanced security model all .NET applications provide.
WCF uses SOAP messages for communication between two processes, thereby making WCF-based applications interoperable with any other process that communicates via SOAP messages. When a WCF process communicates with a non–WCF process, XML-based encoding is used for the SOAP messages but when it communicates with another WCF process, the SOAP messages are encoded in an optimized binary format. Both the encodings conform to the data structure of the SOAP format, called Infoset.
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WCF is designed in accordance with Service oriented architecture principles to support Distributed computing where services are consumed by consumers. Clients can consume multiple services and services can be consumed by multiple clients. Services typically have a WSDL interface which any WCF client can use to consume the service, irrespective of which platform the service is hosted on. WCF implements many advanced WS* web services standards such as WS-Addressing, WS-ReliableMessaging and WS-Security. While Microsoft is a board member of WS-I it is not clear how many WS-I profiles they are committing to support fully.
A WCF Service is composed of three parts – a Service class that implements the service to be provided, a host environment to host the service, and one or more endpoints to which clients will connect. All communications with the WCF service will happen via the endpoints. The endpoints specify a Contract that defines which methods of the Service class will be accessible via the endpoint; each endpoint may expose a different set of methods. The endpoints also define a binding that specifies how a client will communicate with the service and the address where the endpoint is hosted.
WCF provides Windows Activation Services which can be used to host the WCF service. Otherwise the WCF service can also be hosted in IIS or in any process by using the Service Host class, which is provided by WCF. Services can also be self-hosted.
A WCF service class implements some service as a set of methods. In addition, it implements at least one Service Contract which defines the operations that the service can perform. It may also optionally implement Data Contracts which define what sort of data is worked upon by the exposed operations.
Contracts are defined using .NET attributes. Any class that is to be exposed as a WCF service must be marked with ServiceContract attribute, and all methods that a client can invoke using SOAP messages must be marked with OperationContract attribute. The attributes automatically generate WSDL descriptions for the exposed methods, which can then be accessed by clients, or advertised to clients.
A service can also employ multiple Service Contracts. This can be done by defining multiple .NET interfaces, each defining a Service Contract. The service class can then implement all the interfaces.
All Service Contracts have an associated Data Contract which defines the data that the service works on. If the data required by the service and the returned result is of simple types, like integers, doubles etc, then the Data Contract is defined automatically by WCF. But, if the data is of a complex type like an object or a struct, then the Data contract must be explicitly defined. Data contracts specify how the data is serialized and de-serialized.
A Data contract is defined by using a DataContract attribute on a class or structure. The members of the data structure which will be used by the service need to be marked with a DataMember attribute. Only the members marked with DataMember attribute will be transferred between the service and its client via SOAP messages.
The behavior of the Service in general and the operations in particular can be controlled using the ServiceBehavior and the OperationBehavior attributes respectively. The ServiceBehavior attribute has different properties which control the properties of the service. The ConcurrencyMode property specifies whether the service will be concurrent, i.e., whether it will support simultaneous clients or not. Similarly, the InstanceMode property specifies whether a new instance of the service will be created for each call or whether all calls from same client will be serviced by a single instance.
A WCF client connects to a WCF service via an endpoint. Each Service contract is exposed via an endpoint. An endpoint also has an address, which is a URL specifying where the endpoint can be accessed, and a binding that specifies how the data will be transferred between the service and its client. The mnemonic "ABC" can be used to remember Address / Binding / Contract. Binding specifies what communication protocols are used to access the service, whether security mechanisms are to be used, and the like. WCF includes predefined bindings for most common communication protocols such as SOAP over HTTP, SOAP over TCP, and SOAP over Message Queues etc. When a client wants to access the service via an endpoint, it has to adhere to the binding specified by the endpoint.
A client can communicate with a WCF service using any of the RPC-based mechanisms in which the service can be invoked as a method call. Any call to the service will be blocking - that is, it will halt the execution of the client until the service processes the request. The client has to connect to a service using a proxy object, which is connected to the specified endpoint of the service and abstracts the service as an object. All method calls to the proxy object will be routed to the service and the proxy will return the results returned by the service to the caller.
WCF handles creating the local proxy. It retrieves from the endpoint the WSDL definition of the endpoint and creates the proxy that will communicate the data for the service using the protocol specified by the binding of the endpoint. It also has to convert the data returned by the service into a form expected by the caller.
WCF also supports non-blocking calls as well as use of the services by passing messages containing the data to be used by the service. Communicating using messages does not require the use of proxy object. The returned data will also be returned using a message. The caller will be blocked until the call to the service returns. To make use of non-blocking messaging, Message Queues need to be used to handle the delivery and receipt of the messages. Message Queues will also allow an application to work even if the service is temporarily down, with the knowledge that the request will be serviced when the server comes up again.