Ask five people what "web services" are and you will likely get at least six different opinions. Because the term evokes such ambiguity, we need to set ground rules for this chapter and define what we mean by "web services". Therefore, for the purposes of this book, "web services" are the interactive exchange of messages from one system to another using the Internet as the network transport and HTTP/HTTPS as the messaging protocol. Message exchange transpires without human intervention and may be one-way—that is, called without an immediate response expected—or two-way.
Web services operate as producer/consumer systems where the producer—called the "service provider"—offers one or more "services" to the "consumer"—sometimes referred to as the "client". In the web services world, Internet-based service providers advertise and deliver service from locations throughout the Web. The Internet, and the Web in particular, serve as the highway over which potential web service clients...