Web Services入门指南：原理与实战

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《Web Services Essentials》是一本由Ethan Cerami编著、O'Reilly出版社出版的实用教程，旨在为程序员提供Web服务的基础知识和实践经验。本书于2002年首次发行，共304页，适合希望深入了解分布式应用中XML-RPC、SOAP、UDDI和WSDL等技术的读者。该书以简洁明了的方式引导读者进入Web服务的世界，从第一章起逐步深入。第一章介绍Web服务的基本概念，包括Web服务的定义、架构以及其如何利用XML进行消息传递。1.2节详述了Web服务的架构，强调了服务提供者与消费者之间的松耦合和基于网络的交互模型。 1.3节重点讲解了XML在Web服务中的核心作用，它是数据交换的标准格式，使得服务可以跨越应用程序和平台界限。接下来的1.4章深入探讨了服务描述语言（WSDL），这是一种元数据标准，用于描述Web服务的功能、接口和操作，便于其他系统发现和集成服务。 UDDI（Universal Description, Discovery, and Integration）在1.5章中被介绍，作为服务目录系统，它允许服务提供者注册和查找其他服务，从而促进服务的发现和互操作性。服务的传输方式在1.6节中讨论，包括HTTP、SMTP等协议的选择和使用。安全问题是Web服务开发中不可忽视的部分，1.7节涉及对隐私、身份验证和数据加密等问题的考虑。1.8节通过实例展示了所有这些技术的整合，帮助读者理解它们如何共同构建一个完整的服务体系。最后，1.9节关注标准的统一性和一致性，确保不同供应商提供的服务能够无缝协作。第二部分专门讲解XML-RPC，这是一种早期的Web服务技术，尽管现在SOAP更受欢迎，但XML-RPC的理解对于理解整个Web服务生态系统至关重要。《Web Services Essentials》不仅是一本入门指南，还是一本实用的手册，它涵盖了从基础概念到实践操作的广泛内容，适合希望通过实际项目来掌握Web服务技术的开发人员。无论你是对Web服务技术感兴趣的新手，还是经验丰富的开发者寻求深化理解，这本书都是值得参考的资源。

Web Services Essentials

XML messaging

This layer is responsible for encoding messages in a common XML format so that

messages can be understood at either end. Currently, this layer includes XML-RPC

and SOAP.

Service description

This layer is responsible for describing the public interface to a specific web service.

Currently, service description is handled via the Web Service Description Language

(WSDL).

Service discovery

This layer is responsible for centralizing services into a common registry, and

providing easy publish/find functionality. Currently, service discovery is handled

via Universal Description, Discovery, and Integration (UDDI).

As web services evolve, additional layers may be added, and additional technologies may

be added to each layer. Figure 1-7 summarizes the current web service protocol stack.

Each layer is described in detail later in this book.

Figure 1-7. Web service protocol stack

1.2.3 Architectural Snapshot: The IBM Web Services Browser

To gain a high-level understanding of how the protocol stack actually works, try out the

IBM Web Services Browser. The browser enables you to search for existing services,

view their service descriptions, and automatically invoke those services. This lets you see

each layer within the protocol stack without actually writing any code.

To get started, open a browser and go to http://demo.alphaworks.ibm.com/browser/. You

should see the screen depicted in Figure 1-8.

In the righthand pane, you can search a centralized registry for existing web services. (The

registry actually uses UDDI, but don't get too caught up in the details just yet.) Within the

Search box, type "IBM Web Services" and click Search. IBM will search the centralized

directory for you and display all matching results in the left pane. Select the last folder,

entitled IBM Web Services TestArea, and you will see a list of available web services.

(See Figure 1-9.)

Web Services Essentials

The IBM browser does a good job of illustrating web services in action and highlighting

the main layers within the protocol stack. It also does a good job of illustrating the

potential of "just-in-time" application integration. Each service basically acts as an

individual building block, and you can continue stacking more and more services to the

same page. Best of all, you can do so without writing a single line of code!

1.3 XML Messaging

XML has exploded onto the computing scene in recent years. It has gained rapid

acceptance because it enables diverse computer systems to share data more easily,

regardless of operating system or programming language. There are dozens of XML tools,

including parsers and editors that are available for nearly every operating system and every

programming language, including Java, Perl, Python, C#, C, C++, and Ruby. When

developers decided to build a web service messaging system, XML was therefore a natural

choice. There are two main contenders for XML messaging: XML-RPC and SOAP. The

following sections provide descriptions of both protocols.

1.3.1 XML-RPC

XML-RPC is a simple protocol that uses XML messages to perform RPCs. Requests are

encoded in XML and sent via HTTP POST. XML responses are embedded in the body of

the HTTP response. Because XML-RPC is platform-independent, it allows diverse

applications to communicate. For example, a Java client can speak XML-RPC to a Perl

server.

To gain a high-level understanding of XML-RPC, consider a simple weather service. The

service expects a zip code and returns the current temperature for the area. Here is a

sample XML-RPC request to the weather service (HTTP headers omitted):

<?xml version="1.0" encoding="ISO-8859-1"?>

<methodName>weather.getWeather</methodName>

</params>

</methodCall>

The request consists of a simple methodCall element that specifies the method name and

any method parameters.

Here is a sample XML-RPC response from the weather service:

<?xml version="1.0" encoding="ISO-8859-1"?>

<param>

</param>

</params>

</methodResponse>

The response consists of a single methodResponse element that specifies the return value.

In this case, the return value is specified as an integer.

Web Services Essentials

XML-RPC is the easiest way to get started with web services. In many ways, it is simpler

than SOAP and easier to adopt. However, unlike SOAP, XML-RPC has no corresponding

service description grammar. This prevents automatic invocation of XML-RPC services -

a key ingredient for enabling just-in-time application integration. More details of XML-

RPC are covered in Chapter 2.

1.3.2 SOAP

SOAP is an XML-based protocol for exchanging information between computers.

Although SOAP can be used in a variety of messaging systems, and can be delivered via a

variety of transport protocols, the main focus of SOAP is RPCs transported via HTTP.

Like XML-RPC, SOAP is platform-independent and therefore enables diverse applications

to communicate.

To gain a high-level understanding of SOAP, let's revisit our simple weather service. Here

is a sample SOAP request (HTTP headers omitted):

<?xml version='1.0' encoding='UTF-8'?>

<SOAP-ENV:Envelope

xmlns:SOAP-ENV="http://www.w3.org/2001/09/soap-envelope/"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:xsd="http://www.w3.org/2001/XMLSchema">

<SOAP-ENV:Body>

<ns1:getWeather

xmlns:ns1="urn:examples:weatherservice"

SOAP-ENV:encodingStyle="http://www.w3.org/2001/09/soap-encoding/">

</ns1:getWeather>

</SOAP-ENV:Body>

</SOAP-ENV:Envelope>

As you can see, the SOAP request is slightly more complicated than the XML-RPC

request. It makes use of both XML namespaces and XML Schemas. As in XML-RPC,

however, the body of the SOAP request specifies both a method name and a list of

parameters.

Here is a sample SOAP response from the weather service:

<?xml version='1.0' encoding='UTF-8'?>

<SOAP-ENV:Envelope

xmlns:SOAP-ENV="http://www.w3.org/2001/09/soap-envelope/"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:xsd="http://www.w3.org/2001/XMLSchema">

<SOAP-ENV:Body>

<ns1:getWeatherResponse

xmlns:ns1="urn:examples:weatherservice"

SOAP-ENV:encodingStyle="http://www.w3.org/2001/09/soap-encoding/">

</ns1:getWeatherResponse>

</SOAP-ENV:Body>

</SOAP-ENV:Envelope>

The response indicates a single integer return value. Full details of SOAP are discussed in

Chapter 3.

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