Web服务基础：XML-RPC, SOAP, UDDI与WSDL详解

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"O'Reilly.web services essentials - 一本关于XML Web服务的基础和参考书籍，涵盖了XML-RPC、SOAP、UDDI和WSDL等分布式应用技术。作者Ethan Cerami，由O'Reilly出版社于2002年出版。" 在当前的IT领域，Web服务是一种至关重要的技术，它允许不同的应用程序通过互联网进行通信和数据交换。《Web Services Essentials》这本书是针对程序员的一本实用指南，它深入浅出地介绍了XML Web服务的基础知识，并提供了创建服务的快速方法，同时探讨了四种关键的新兴技术：XML-RPC、SOAP、UDDI和WSDL。 1. **XML-RPC**（Remote Procedure Call）是一种简单的远程调用协议，允许程序通过HTTP进行跨平台的通信。XML-RPC使用XML作为数据交换格式，使得不同语言编写的系统能够相互通信。 2. **SOAP**（Simple Object Access Protocol）是基于XML的协议，用于在Web上交换结构化和类型化的信息。SOAP允许服务提供者定义复杂的操作，并且支持错误处理和安全性。 3. **UDDI**（Universal Description, Discovery, and Integration）是一个标准，用于发布和查找Web服务。它创建了一个目录服务，使得服务消费者可以找到并了解可用的服务。 4. **WSDL**（Web Services Description Language）是描述Web服务接口的语言，使用XML编写，定义了服务的位置、使用的消息格式以及如何调用这些服务。书中，作者首先介绍了Web服务的基本概念和架构，包括服务的分层模型，XML消息的使用，以及服务描述的重要性。WSDL作为一个关键组件，用于定义服务的接口、操作、输入和输出消息。接着，书中详细阐述了UDDI在服务发现中的角色，以及各种传输协议如HTTP在服务通信中的作用。此外，书中还讨论了Web服务的安全性问题，这在现代网络环境中是不可或缺的部分。安全考虑通常包括身份验证、授权和数据加密等方面。最后，作者强调了标准化和互操作性对于Web服务生态系统的重要性，以确保不同提供商的服务能够协同工作。《Web Services Essentials》不仅对初学者提供了清晰的引导，也对有经验的开发者提供了有价值的参考，帮助他们理解并利用这些技术构建分布式应用程序。无论你是希望打破Web服务的炒作，还是寻找实际的技术细节，这本书都是一个很好的起点。

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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