Windows Communication Foundation (WCF)

What is service:
A service is some functionality that is exposed via a structured messaging scheme. This functionality could be shared on any transport protocol like HTTP, TCP, MSMQ. The structure messaging scheme could be somthing like what is defined in HTTP requests/responses.

Originally code named "Indigo", WCF is one of four APIs introduced with the .NET Framework 3.0, in December 2006. WCF unifies several other existing distributed communication APIs supported by the .NET Framework 2.0. Previously, separate APIs for SOAP-based communications for maximum interoperability (Web Services), binary-optimized communications between applications running on Windows machines (.NET Remoting), transactional communications (Distributed Transactions), and asynchronous communications (Message Queues) existed. WCF unified all these mechanisms into a single, common, general service-oriented programming model for communications.

The principles of Service orientation says “we don’t think in terms of objects (Object Oriented) or components (Component oriented) because these two have problems when they come in distrubuted environments. There are differences whether you talk locally (in case of objects or components developed locally) and when you are talking in a distributed environment. So we are thinking in terms of messages. We accept messages and send messages”

Four tenets of SOA and how is it implemented in WCF Technology in a way,how can WCF to avoid the well-known pitfalls of using current technologies such as (WSE):
1) Boundaries are explicit :
Applications is communicating to each other by sending and receiving messages to each other.
2) Services are autonomous: you can't control any service out of your organization boundaries.
Your solution should be loosely coupled ,so it can tolerate these changes and continue running if one or more services are un available.
3) Services share schema,contracts not classes or types: simply each service publish the schema of the data which will be exchanged between the service and clients and how can be binded to the clients,these information is used by clients to consume the published services.
4)Compatibility is based on policy:Schemas and contracts exposed by the service is define the "Shape" of the service.if you familiar with WS Enhancements,it has its specification on WS-Security,WS-Interoperability....etc which has been encapsulated into the WCF unified platform.

Additional Service Oriented principles (you should adhere to simply because it is the right thing to do) :
1.  Services are secure.
2.Services leave the system in a consistent state.
3.Services are thread-safe.
4.  Services are reliable & robust.
        Interoperable
        Scaleable
        Available
     Responsive
     Disciplined

 

WCF a part of the .NET Framework that provides a unified programming model for rapidly building service-oriented applications that communicate across the web and the enterprise. It is a microsoft platform for SOA (service oriented architecture). So service oriented is a key feature of WCF. It is an attempt to do interoperability with other platforms.

It is a single extendable object model.

WCF is a core component of the .NET Framework 3.0 (formerly WinFX) which is included with Windows Vista and will support Windows 2003 and Windows XP platforms as well as the future version of Windows Server.

As its name suggests, WCF provides the .NET Framework with a basis for writing code to communicate across components, applications, and systems.

Services expose one or more endpoints where messages can be sent by the clients in order to get some information from the service or submit some data to the service.

Each endpoint consists of 3 properties (ABC)

Address (Where) :  URI indicates where the service is located.

Binding (How) : Technical implementation of the communication. It defines how i

Contract (What) : It is a technical description or interface.

Services expose one or more endpoints where messages can be sent. Each endpoint consists of an address, a binding, and a contract. The address specifies where to send messages. The binding describes how to send messages. And the contract describes what the messages contain. Clients need to know this information before they can access a service.

 

Services can package up endpoint descriptions to share with clients, typically by using Web Services Description Language (WSDL). Then clients can use the provided service description to generate code within their environment capable of sending and receiving the proper messages.

WCF Architecture

 

ABCs of WCF : Address, Binding and Contact.

When we are creating a service library, we will be applying attributes on our interfaces and classes to build something called Data Contracts and Service Contracts.

Contracts

Contracts define various aspects of the message system. It is a platform-neutral and standard way of describing what the service does.

In WCF we have 4 types of contract:
· Service Contract: Service contracts define the operations that a service will perform when executed. They tell the outside world a lot about the service such as message data types, operation locations, the protocols the client will need in order to communicate with the service, and the operations the service provides. It maps CLR types to WSDL.

Service contracts uses two types of attributes:

ServiceContract - This attribute is used to define the Interface.
OperationContract - This attribute is used to define the method inside Interface.

· Data Contract: It describes a data structure. It defines which data types can be transferred in the service. A Data Contract marks a business entity (class) as a participant in a Service Contract and Service Operation. It maps CLR types to XSD.

Data contracts uses two types of attributes:
DataContract : is used to define the class.
DataMember : is used to define the  properties.

· Fault Contract: A Fault Contract is the definition of how errors are raised and handled by WCF clients. There is no concept of an exception in WCF (this is a .NET only idea). Fault Contracts describe error conditions and behavior for managing these conditions.

· Message Contract: It is an extra functionality allows you to send message in a customized way.It defines the structure of the message over the wire. It can be typed or untyped and useful when there is an existing message format to comply with. It maps CLR types to SOAP messages.

Steps to create and use a WCF service:

WCF Message Exchange Patterns
WCF service contracts can express three message exchange patterns that can be used in your
services:

1. Simplex (One way)
One-way is a “fire-and-forget” style of messaging as in this scenario, the client initiates communication and sends a message to the service without for a reply from the service. The service consumes the message and performs some action but does not communicate back to the client. The client in the Simplex message pattern suffers from short-term memory loss. When the client sends the message, it has no idea it sent a message because it is not expecting a response.

2. Duplex : It allows client and service to communicate openly by exchanging messages at both ends. It is a peer-to-peer connection.

3. Request-Reply: This is the default pattern of exchanging messages in WCF. This is the most common pattern in today’s web services (ASMX) world. In this the client sends a message(Request) and waits until a reply is sent back(Reply).