Integration Technologies - Guide to Cloud Computing for Business and Technology Managers - page 129

using different software platforms, operating systems, and programming
languages. From the technological perspective, Web Services are the next
evolutionary step in distributed architectures. Web Services are similar to
their predecessors but also differ from them in several aspects.
Web Services are the first distributed technology to be supported by all
major software vendors. Therefore, it is the first technology that fulfills the
universal interoperability promise between applications running on dispa-
rate platforms. The fundamental specifications that Web Services are based on
are SOAP (Simple Object Access Protocol), WSDL (Web Services Description
Language), and UDDI (Universal Description, Discovery, and Integration).
SOAP, WSDL, and UDDI are XML based, making Web Services protocol mes-
sages and descriptions human readable (see Chapter 8, “Web Services”).
From the architectural perspective, Web Services introduce several impor-
tant changes compared to earlier distributed architectures. They support
loose coupling through operations that exchange data only. This differs
from component and distributed object models, where behavior can also be
exchanged.
Operations in Web Services are based on the exchange of XML-formatted
payloads. They are a collection of input, output, and fault messages. The
combination of messages defines the type of operation (one way, request/
response, solicit response, or notification). This differs from previous dis-
tributed technologies. For more information, please refer to WSDL and XML
schema specifications.
Web Services provide support for asynchronous as well as synchronous
interactions. They introduce the notion of end points and intermediar-
ies. This allows new approaches to message processing. Web Services are
stateless and utilize standard Internet protocols such as HTTP (hypertext
transfer protocol), SMTP (Simple Mail Transfer Protocol), FTP (File Transfer
Protocol), and MIME (Multipurpose Internet Mail Extensions). So, connec-
tivity through standard Internet connections, even those secured with fire-
walls, is less problematic.
In addition to several advantages, Web Services also have a few disad-
vantages. One of them is performance, which is not as good as distributed
architectures that use binary protocols for communication. The other is that
plain Web Services do not offer infrastructure and quality of service (QoS)
features, such as security and transactions, which have been provided by
component models for several years. Web Services fill this important gap by
introducing additional specifications:
• WS-Security: This addresses authentication and message-level secu-
rity and enables secure communication with Web Services.
• WS-Coordination: This defines a coordination framework for Web
Services and is the foundation for WS-Atomic Transaction and
WS -Bu s i n e s s Ac t iv it y.