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| The
Java Model (Continued...) |
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As a further implementation of decoupling, the interface between
the servlet that answers HTTP requests and the actual application
objects is governed by a set of XML definitions that express
the relationship between the object layer of the application
and the user interface mechanism (the marked-up HTML). This
configurable interface is managed by the PK4-proprietary
access manager, a low-overhead mechanism that concentrates on
managing site access and provides hooks into optional app servers.
To put it simply, the Access Manager is a servlet that behaves
like a traffic cop - it receives page requests from the user
and passes them on to the correct UDM. |
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In some business
applications, it may be necessary to build transaction management
and resource-pooling using app servers like IBM's WebSphere
or BEA's WebLogic. PK4's suggested architecture includes
mechanisms to interface with such app servers.
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In order to make
the architecture support extensibility on the server side,
the access manager is designed to work with function-specific
servlets that are integrated into the system. These
servlets can work with the application objects or the data
access objects directly. They could also be designed to work
with external non-Java components that may be needed in the
system.
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To fully decouple
the user interface from the business logic and database access
mechanism in the application, PK4 suggests the use of
macro-expansion engines like FreeMarker. While there
are other such utilities available, FreeMarker is a reputed,
extensible, freely-available environment that specifically
addresses the need for marrying HTML with object-driven architectures.
The HTML pages that are displayed to the user are recorded
in templates that include HTML statements marked up with simple
FreeMarker tags. FreeMarker could be easily replaced with
other macro-expansion utilities like WebMacro and, in future,
with XSL-based tools.
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The web server
that PK4 usually deploys is Apache, currently the single
most popular web server in the world. It includes the ability
to run Java-written servlets, a significant contributor to
PK4's open architecture.
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Conclusion
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The solutions that
PK4 delivers are highly maintainable, since it is based
on a strict set of coding rules for Java, D/HTML, VBScript,
database design etc., The architectures not only address the
web-centric, high-tech nature of most new systems but also
lends themselves by virtue of the de-coupling of various components,
to continuous improvement with newer and more effective technologies,
as they evolve.
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