Service Oriented Architecture and WCF 【转】

http://www.codeproject.com/Articles/515253/Service-Oriented-Architecture-and-WCF

Introduction

This article will not talk about how to create and host a WCF service rather we stress on importance of SOA and WCF.
If
you are a advanced WCF developer then article is not for you, if you
are one who is trying to learn WCF then article is not for you, this is
for them who is completely new to WCF and trying to understand why WCF
is there.

Index

Evolution of SOA

Evolution is the nature’s law. Just like Human beings
who evolved from Primate to Modern age Humans just to adapt the moving
environment and competitions, Programing style or we can say technique
also evolved to overcome the challenges in the changing programming
world.

Procedural Programming

Initially programmers used this approach for developing applications where Functions were everything.
In this approach Functionalities will be
encapsulated inside one or more functions, which can call each other,
pass some parameters and get some return value. Here one of the
functions will be made as entry point (like in C programming we had main
method).

Why it need to be evolved.

The biggest challenges with this approach were,

How to reuse the same code?
Difficulties in code Management.

Object oriented Programming

To overcome the problems in the Procedural
programming, object oriented era comes into the picture where people
start talking in terms of objects and classes.
Everything is treated and considered as real world objects created from
the blue print that is class.

Lots of Object oriented principles like Abstraction,
Encapsulation, Inheritance, polymorphism and solid has been introduced
are introduced in this era.

Why it need to be evolved.

OOP increased the reusability and thus improved the
code management. But one thing which was not addressed here was how two
or more applications will communicate each other,
especially when they have been written using different
languages or technologies.
For instance inventory module which is written using Java
will not be able to call function inside classes of accounting module
written in .NET.

Service oriented Programming

We have progressed from functions, to objects and now to services.
In SOA functions and tasks are created as loosely connected independent services communicating via messages.

Service provider publishes the service via standard interfaces in a publicly accessible directory called

Service Repository and Service consumer make a service request and in response gets the Service Response.

Architecture Terminologies

Before we go in depth and talk about SOA (Service
oriented architecture), it’s must to understand some terminologies
related to architecture.

Patterns

In simple words Pattern is a solution to a problem in a context.

Architectural Style

When we say architectural style, it’s a very high
level thing, it’s a concept. It’s up to us how we take this and
implement. For example REST is an architectural style which tells us how
we can use standard web features in effective way. It’s not standard or
specification, in a style which we can understand and design web
services in that style. In my perspective n-Tier is also an
architectural style, it says divide your systems into different tiers
but middle tier may be anything, and developer can make it a simple
class library or a WCF service. In face the value of ‘n’ is also not
sure.

Architectural Patterns

Architectural Pattern is something which solves our
problem at sub system level or in short module level. It deals with the
problem related to architecture of a project. We make Class libraries,
Components, Web services to solve the problem. For example - In MVC we
break the application into Model, View and Controller. Here we know how
View interacts with model and so on.

Design Patterns

Design pattern talk about problems at class or function level.

What is SOA?

SOA or Service oriented architecture is an
architecture style for building business applications by means of
services. Application comprises collection of services which communicate
through messages.

Service

Services are logical encapsulation of self-contained business functionality
Every Service encapsulates one action such as Register User, Send Email etc.

Messages

Services communicate with each other using messages.
Messages are standard formats which everyone (every service) can read
and understand.

Characteristics of SOA

In SOA, Services should be independent of other services.

Altering a service should not affect calling service.
Services should be self-contained.

When we talk about a RegisterCusomer service it
means, service will do all the necessary work for us, we are not
required to care about anything.
Services should be able to define themselves.
Services should be able to answer a question
what is does? It should be able to tell client what all operations it
does, what all data types it uses and what kind of responses it will
return.
Services should be published into a location (directory) where anyone can search for it.
As I said, SOA comprises of collection services which communicate via standard Messages.
Standard messages make them platform independent.

(Here standard doesn’t mean standard across
Microsoft it means across all programming languages and technologies.)
Services should be able to communicate with each other asynchronously.
Services should support reliable messaging.
Means there should be a guarantee that request
will be reached to correct destination and correct response will be
obtained.
Services should support secure communication.

WCF and Web services

WCF is a Microsoft framework for building Service-Oriented applications.
Comparing Web services with WCF will not be a good
idea. Can you compare yourself with yourself in past(let's say two year
before)? Obviously in two year you might have learned new things,
improved somewhat,right? WCF and Web service are related to each other
in same manner. WCF has evolved from Web services and so WCF can do all
which a web services is capable of, plus can do some more.

Features of WCF

WCF hosting - If we are working
with web services then only option we are having is hosting it inside
web server such as IIS using http or wshttp protocols. But WCF supports
four types of hosting
- IIS
- WAS (Windows process activation services)
- Self-hosting
- Windows services
Read more about WCF hosting at

http://msdn.microsoft.com/en-us/library/bb332338.aspx
Message transmission - Messages
can be transmitted between clients and service via various transport
protocols and encodings such as SOAP using http and binary data using
TCP.
Serialization - Web services uses XmlSerializer for transferring data between service calls whereas WCF supports multiple serializers
- DataContractSerializer(faster and supports versioning)
- NetDataContractSerializer(when it required to include CLR type information in the serialized XML)
- XmlSerializes(mostly to support backward compatibility).
Multiple technologies at one place - WCF unites following four technologies
- .NET remoting
- MSMQ
- Web Services
- COM+
Message Contract - In Web
services customizing the headers of the SOAP message was a tedious task.
For that we were supposed to derive a class from SoapHeader and then
SoapHeaderAttribute is used to indicate the presence of the header.

But with WCF we can make it easily with the help
of simple attributes like MessageContractAttribute,
MessageHeaderAttribute, and MessageBodyMemberAttribute.
Multiple Message Patterns - WCF supports three message patterns that describe how client and service pass messages
- Request-Reply Pattern – Client sends message to service and waits for reply.
- One-Way Message Pattern – Client sends message to service but service does not reply message to client.
- Duplex pattern – Both client and the
  service can initiate communication. The client calls a method of the
  service. The service can then use a client callback to call a method in
  the client.
Security - In WCF security can be implemented with the help of well-known standards such as SSL.
Reliable - WCF supports reliable messages with the help of Queues and reliable sessions.
REST - WCF can be extended to
support plain xml data that is not wrapped in a soap envelope, xml
formats such as ATOM and non xml standard such as JSON.
WCF Transaction - - WCF
supports to create distributed transactions for your service
application. Transaction is a collection of logical operations which
need to be run as a single logical unit.

(Either all operations will successfully execute
and completes or in case any of them fail others will rollback).
WCF instancing - In WCF we can
control the way WCF service objects are instantiated in the WCF server.
WCF Framework comes up with following instancing models
- Per Call - A new instance will be created for every client request.
- Per session - A new instance is created for each new client session and maintained for the lifetime of that session.
- Single - A single instance handles all client requests for the lifetime of the application.
WCF Concurrency - With WCF
Concurrency features we can control how service instances can serve
multiple requests at the same time. We have three choices
- Single – Only one request will be served at a time.
- Multiple - Multiple requests can be handled by the WCF service object at any given moment of time.
- Reentrant - A single request thread has
  access to the WCF service object, but the thread can exit the WCF
  service to call another WCF service or can also call a WCF client through
  callback and reenter without deadlock.

What is ABC in WCF

We had gone through the feature of WCF and
understood why its termed as advanced version of web services. Now it’s
time to answer a very basic question related to WCF i.e., what is ABC of
WCF?

When we say WCF, we came across end points. Service
endpoint can be a part of continuously hosted service hosted in IIS or
service hosted in an application.

ABC or Address, Binding and Contract are the three elements which constitutes and Service Endpoint.

Address - Where Service is residing (URL of the service.)
Binding – How to talk to the service?

Example – basicHttpBinding, wsHttpBinding, webHttpBinding etc.

Contract – What can the service do for me?

Conclusion

We have understood

What is Service oriented architecture?
Why SOA required?
What are the characteristics of SOA?

How WCF can be differentiated from Web service?
What are the characteristics of WCF?
What is mean by ABC of WCF?