Client Server Architecture - Programming GPS and OpenStreetMap Applications with Java - page 145

Figure11.1. The four layers of a distributed real-object application: the client
(left) and the server (right). The code of the common library (top) defines the
ROAF laws, while the concrete application implementation (bottom) can add
rules.

tic know-how|a library for real-world knowledge. The RealObjectServer
should not be abstract, but rather should enable any ROAF developer to
launch the server \out of the box." In consecutive steps or cycles, the
RealObjectApplication extends the server and programs the scenario with
its API.
The left-hand side depicts the client and the right-hand side, the server.
We can also look at the architecture in terms of layers: the library layer
(top) does not have to be modified by the application developer layer (bot-
tom).

11.4 The ServerEngine

For the architect, the ServerEngine represents the external development
environment on the server side. The engine is not part of the library and
resides in roa.ldn.server , which is part of the first real-object application
to be rolled out step by step. Again, the elements of the MovingObjects
application can be recognized, although some of them are commented out.
For the time being, the server engine does not create a GUI or start an
observer. Prototyping is an important process to separate library and
application. Code can be shifted between these two layers to generalize
the library, while keeping the application lean.