Alexander Nakhimovsky

Defining Web Services

This book is about consuming, creating, and deploying Web Services. To “consume” means to use a client that communicates with aWeb Service. What are Web Services, and how do their clients communicate with them? Most generally, aWeb Service is a distributed application that exposes public procedures whose input and output conform to a standard, language-independent and platform-independent protocol. The key feature of Web Services is interoperability: they can be invoked remotely over the network by client programs written in different languages and running on different platforms.

Web Services: Description, Interfaces, and Ontology

In this chapter, we compare the Semantic Web and Web Services, in the hope that such a comparison will shed light on each and help us understand the directions of their evolution. Both the Semantic Web and Web Services are young, rapidly changing technologies that overlap in significant ways. A better understanding of how they are similar and where they differ may add clarity to their development and mutual accommodation. If, as we believe is the case, they have complementary strengths and weaknesses, their mutual understanding can be of help to both.

Restructuring Results with XSLT

SOAP messages and REST query results come from the server to the client as XML data. In previous chapters, we either displayed them as raw XML (tags and all) or used Javascript to restructure the results. In this chapter, we’ll use XSLT to transform XML data into HTML presentations. This is probably the most common way to use XSLT, and it’s a good way to learn because you see the results of your code right away. We won’t go far into XSLT, but we will provide a self-contained introduction.

WSDL and Axis

SOAP helps automate access to the Web’s resources. This chapter goes up a level, presenting a framework and a technique to automate the creation of SOAP applications.

DBService and a Book Club

Now that we have created a few SOAP clients, we are going to create a SOAP server. The server will provide a database service, accept SQL queries wrapped in SOAP messages, and return query results (or SOAP faults). We will call it DBService.

The Plumbing: DOM and SOAP

In this chapter, we will go over the two APIs that regulate the critical junctures in aWeb Service implemented on top of the HTTP protocol. (This is the dominant implementation model, and the one used throughout this book.) To see what the junctures are, consider Figure 2-1.

WebDAV Client to Database via XML

In this chapter, we present two more filters. XsltFilter transforms the output of TidyFilter to build a more meaningful XML structure. DBFilter stores the resulting XML structure in a database to simplify searching and linking. Together, these two filters build a fairly complex system that is not just an example for this book but a simple version of an application that we believe is well motivated and potentially quite useful.

Authentication and REST

In this chapter, we continue working with DBService and the book club application; we will use standard HTTP facilities to extend them in two ways. First, we will add authentication so different groups of users have different access privileges. We will distinguish three groups of users: admins, who can grant privileges and create tables; members, who can submit reviews and otherwise change tables; and visitors, who can only browse and look things up. We will show you how to use HTTP-based authentication and indicate ways to provide more robust security. The revised service is called DBAuthService, DBService with authentication.

More Services: Java Applet

Enough google for now. In this chapter, we introduce two new Web Services: the weather report service from http://live.capescience.com/ccx/GlobalWeather and the Amazon Web Services from http://www.amazon.com. Initially, we approach them using the xmlhttp framework from Chapter 1. However, within that framework, we generalize doGoogle() as doSoapCall(). This function can invoke several different Web Services, including Google, the Cape Science weather service, and Amazon. In fact, you can use doSoapCall () to develop a Javascript client for any SOAP service for which you have sample SOAP requests to work from.

Tomcat, JSP, and WebDAV

Community-oriented Web Services for sharing or jointly authoring documents, such as blogs, RSS, and Wikis are becoming an increasingly important part of the Internet. These and other collaborative technologies are improving every day, but they still need greater standardization. Underneath reasonably uniform functionality, they are implemented in vastly different ways that are difficult to exchange or extend. The closest thing to a standard protocol for distributed authoring is WebDAV, an IETF-sponsored set of extensions to HTTP. In this chapter, we will develop aWebDAV-based community Web Service/client.