Enterprise Service Bus (ESB) - Guide to Cloud Computing for Business and Technology Managers - page 192

that interfaces with legacy applications, and external applications and data
sources using Web Services. In an ESB application, development tools allow
new or existing distributed applications to be exposed as Web Services and
be accessed via a portal. In general, resources in the ESB are modeled as
services that offer one or more business operations. Technologies like J2EE
Connector Architecture (JCA) may also be used to create services by inte-
grating packaged applications (like ERP systems), which would then be
exposed as Web Services.
An ESB enables the more efficient value-added integration of a number
of different application components, by positioning them behind a service-
oriented facade and by applying Web Service technology to the problem. For
instance, in the figure above, a distributed query engine, which is normally
based on XQuery or SQL, enables the creation of business data services,
for example, sales order data or available product sets, by providing uni-
form access to a variety of disparate business data sources or organization
repositories.
Endpoints in the ESB depicted in the Figure 9.1 above provide
abstraction of physical destination and connection information
(like TCP/IP host name and port number). In addition, they
facilitate asynchronous and highly reliable communication
between service containers using reliable messaging conventions.
Endpoints allow services to communicate using logical connection
names, which an ESB will map to actual physical network destinations
at runtime. This destination independence gives the services that are
part of the ESB the ability to be upgraded, moved, or replaced without
having to modify code and disrupt existing ESB applications. For
instance, an existing ESB invoicing service could be easily upgraded or
replaced with a new service without disrupting other applications.
Additionally, duplicate processes can be set up to handle failover if a
service is not available. The endpoints can be configured to use several
levels of QoS, which guarantee communication despite network fail-
ures and outages.
9.2 Elements of an ESB Solution
There are alternative ways to implement an ESB. The ESB itself can be a single
centralized service or even a distributed system consisting of peer and sub-
peer ESBs—in the form of an ESB federation—all working in tandem to keep
the SOA system operational. In small-scale implementations of integration