Distributed Systems - Guide to Cloud Computing for Business and Technology Managers - page 85

to communicate with the server DCOM object. Instead of an ORB, DCOM
relies on a service control manager (SCM) to perform the various services of
locating and activating an object implementation. As in CORBA, the server is
responsible for invoking the method requested via interaction with the SCM.
Once the client-side device has received a reference to the server-side object,
it can access the exposed methods of the server.
In DCOM, the client side is called the proxy, while the server side is ref-
erenced as the stub. These stubs and proxies use an Interface Definition
Language similar in purpose to that of the CORBA IDL. Instead of maintain-
ing and using an interface repository as CORBA does, DCOM avails itself of
the Microsoft registry services, thereby enhancing flexibility at the cost of
increased complexity. Interestingly, DCOM server components can be built
in a number of languages, including C++, Java, and COBOL.
3.2.2.3 Java Component Technologies
In 1995, Sun Microsystems introduced the Java development technology. The
impetus behind Java was to create a development technology that could be
run on any platform. Platform neutrality has always been the challenge soft-
ware vendors faced. Prior to Java, software vendors had to develop several
versions of their products for the different platforms their customers were
using. The duplicated development efforts were a huge cost to the vendors.
Java's motto is “Write Once, Run Anywhere.” Instead of writing several ver-
sions of their programs, applications written in Java can be run on any plat-
form that supports a JVM.
The main elements of J2EE technology are components, container services,
and Web Services. J2EE offers two different component models, Servlet or
Java Server Page (JSP) for the Web component and EJB for the server com-
ponent. Servlet and JSP are primary J2EE component technology for Web
development, while EJB is the component model for application logic. JSP
is a text-based document that contains static content (i.e., images, text) and
dynamic data. The JSP static data could be expressed in HyperText Markup
Language (HTML) or XML code, while the dynamic data are controlled
by Java code. JSP is an extension of Servlet. When JSP is being run, the JSP
code is translated into a Java Servlet. Both Servlet and EJB operate in a sepa-
rate container; Servlet operates in a Web container and EJB operates in an
EJB container. A container is a runtime environment that hosts and man-
ages a component. The container provides services that allow the component
to operate, including transaction, messaging, remote access, security, and
other services. The container will automatically use a container service dur-
ing runtime depending on policies specified by the developer during design
time. For instance, a policy might be to use messaging for remote communi-
cation rather than RPC.
The main difference between the Servlet and EJB component is that the
Servlet specializes in communicating with Web browser. Thus, it takes