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Enterprise service bus offers streamlined integration

The enterprise service bus as a concept has increasingly gained currency in the IT marketplace, even as vendor camps have squabbled over what exactly an ESB is. As a result, many organizations remain uncertain about the need for and role of an ESB in their IT infrastructures. Is an ESB just gussied-up message-oriented middleware, or is it a genuinely new approach to integration?

In response to client inquiries regarding the definition of an ESB, Mike Gilpin, an analyst at Forrester Research Inc., published a report that described the technology as“software infrastructure that enables service-oriented architectures (SOA) by acting as an intermediary layer of middleware through which a set of reusable business services are made widely available.”

An ESB typically has some sort of “bus” messaging technology, such as Java Message Service or IBM’s MQSeries, and support for Web services standards. The standards support is designed to let enterprises map data from disparate systems, route messages, ensure that services are delivered in the correct order and enforce security rules automatically by using XML instead of changing code in the interfaces of services.

The ESB has evolved to meet users’ demands for a way to integrate applications that’s easier than traditional enterprise application integration (EAI). EAI systems require coding to link applications and can cost as much as 10 times more. Enterprises are looking to ESBs to provide the runtime infrastructure for making loosely coupled applications work. If you have a bunch of services doing different things, an ESB can compose them together. It allows you to run these processes over a long period of time. This bus must be very reliable, meaning that it can guarantee that your message has been received.

The largest group of companies using ESBs are those that need Web services for integration with existing message-oriented Common Object Request Broker Architecture (CORBA) or other integration technologies. Companies want to move toward a service-oriented approach, but they can’t throw away the investments they have made so far.

About 25% of companies using ESBs are replacing existing EAI platforms with the technology. They are saying that EAI was oversold and it didn’t fulfill all their expectations. If it turns out that 80% of their requirements are satisfied by one of these lighter-weight ESBs, they are using them.

Then,how it works?The heart of a ESB is an enterprise messaging backbone that can asynchronously transport data as messages. The messaging core could be proprietary messaging-oriented middleware, MOM technology based on JMS, MOM based on the WS-Reliable Messaging standards, or generic messaging engines. Each of the applications or services connected by the ESB becomes a service endpoint. The ESB provides a layer of abstraction from underlying protocols and defines a process flow that allows individual services be invoked across the transport layer.

企业服务总线提供完美的集成

尽管厂商们对ESB究竟是什么样还存在着争议，但企业服务总线(ESB)作为一个概念在IT市场日益受到追捧。结果，很多机构对是否需要ESB以及ESB在其IT基础设施起什么作用仍未确定。不知ESB到底只是一个被炒作起来的面向消息的中间件，还真是集成的新方法?

Forrester研究公司的分析师Mike Gilpin在回答客户有关ESB定义的询问时，发表了一篇报告，将此技术描写成“通过起到中间件的中间层作用而实现面向服务架构(SOA)的软件基础设施，通过这样的中间件，就能广泛利用一套可重复使用的商业服务。”

通常，ESB拥有某种“总线”传输技术(如Java Message Service(JMS)或者IBM的MQSeries)，以及支持Web服务标准。支持这种标准是为了让企业映射来自各个异型系统的数据、路由消息，确保以正确的次序提交服务，以及利用XML代替改变服务界面中的程序代码以自动实施安全规则。

ESB的进步已能满足用户对集成应用程序的需求，该集成方法比传统的企业应用程序集成(EAI)更为容易些。为联接应用程序EAI需要编程，其成本也要高出10倍之多。EAI系统企业正期待着ESB能提供运行时基础实施，以便使松散耦合的应用程序能工作。如果你有很多做不同事情的服务，那么ESB能将它们组合在一起。这就能让你在一段很长的时间内运行这些进程。此总线必须非常可靠，也就是说它能保证收到你的消息。

利用ESB最大的公司团体是那些需要用Web服务集成现有面向消息的CORBA(公共对象代理体系结构)或者其他集成技术的公司。公司需要朝面向服务的方法转移，但是他们不能扔掉已经投入的投资。

大约25%使用ESB的公司正在用此技术替代现有的EAI平台。他们说对EAI的宣传过头了，它未能达到预期。如果结果表明某个轻便的ESB能满足他们80%的要求，他们就用。

那么，它是如何工作的?ESB的核心是企业消息传输主干，它能异步地将数据作为消息传送。这种传送的内核可以是专有的面向消息中间件(MOM)，如基于JMS的MOM技术、基于WS-Reliable Messaging标准的MOM、或者一般性的传信引擎。由ESB联结的每个应用程序或服务成为了一个服务点。ESB提供了基础协议的抽象层并定义了进程流，它允许在传输层上调用各个服务。