Integration Technologies - Guide to Cloud Computing for Business and Technology Managers - page 130

• Transaction specifications (WS-Atomic Transaction and WS-Business
Activity): These specify support for distributed transactions with
Web Services. Atomic Transaction specifies short duration, ACID
transactions, and Business Activity specifies longer running busi-
ness transactions, also called compensating transactions.
• WS-Reliable Messaging: This provides support for reliable commu-
nication and message delivery between Web Services over various
transport protocols.
• WS-Addressing: This specifies message coordination and routing.
• WS-Inspection: This provides support for dynamic introspection of
Web Service descriptions.
• WS-Policy: This specifies how policies are declared and exchanged
between collaborating Web Services.
• WS-Eventing: This defines an event model for asynchronous notifi-
cation of interested parties for Web Services.
5.11 Enterprise Service Bus (ESB)
An Enterprise Service Bus (ESB) is a software infrastructure acting as an
intermediary layer of middleware that addresses the extended requirements
that usually cannot be fulfilled by Web Services, such as integration between
Web Services and other middleware technologies and products, higher level
of dependency, robustness, and security, management, and control of ser-
vices and their communication. An ESB addresses these requirements and
adds flexibility to communication between services and simplifies the inte-
gration and reuse of services. An ESB makes it possible to connect services
implemented in different technologies (such as EJBs, messaging systems,
CORBA components, and legacy applications) in an easy way. An ESB can
act as a mediator between different, often incompatible, protocols and mid-
dleware products (see Chapter 9, “Enterprise Service Bus (EBS)”).
The ESB provides a robust, dependable, secure, and scalable communi-
cation infrastructure between services. It also provides control over the
communication and the use of services. It has message interception capa-
bilities, which allow us to intercept requests to services and responses from
services and apply additional processing to them. In this manner, the ESB
acts as an intermediary. An ESB usually provides routing capability to route
the messages to different services based on their content, origin, or other
attributes and transformation capability to transform messages before they
are delivered to services. For XML-formatted messages, such transforma-
tions are usually done using XSTLT (Extensible Style sheet Language for
Transformations) or XQuery engines.