Software Design Methodology 181
could go wrong when combining a number of architectural styles into one design to
form a simultaneous heterogeneous architecture which inherits all the
disadvantages of the styles without any benefit of their advantages.
7.2.3 Locationally heterogeneous styles
A software system seldom appears in the exact form of an architectural style as
theoretically defined in the previous chapter. For example, the Java virtual machine
described in Chapter 6 can be regarded as in the virtual machine architecture if we
focus on the subset of components and connectors within the boxed area in Figure
7.2, which is reproduced below.
Figure 7.2 Example of locationally heterogeneous style: JVM
A software system’s architecture is in a locationally heterogeneous style if it
can be considered as in one style when taking a subset of its components and
connectors, meanwhile it can also be viewed as in another style when taking a
different subset of the components and connectors.
For example, if we redraw the architecture diagram of the Java virtual
machine as in Figure 7.3 below, we can find that it is in the main-program-and-
subroutine with shared data style.
Class files
Class Loader
Method Area
Java Stacks
PC Registers
182 Chapter 7. Using Styles in Design
Figure 7.3 Alternative view to the architecture of JVM
Software systems’ architectures in locationally heterogeneous styles exist for
the following reasons.
First, the theoretical definitions of architectural styles are abstractions of
collections of software systems that have common features in their architecture
structures and behaviours. Those components and connectors have no significant
impact on the systems’ most important features hence do not necessarily appear in
all of the systems and are therefore hidden from the theoretical definition. This
enables the most important and dominating features and dynamic behaviours of the
systems to be clearly demonstrated and inferred from the definition without
interference of unnecessary and less important components. However, in practical
uses of a style, it is inevitable to add and/or modify some of the structural or
behavioural features into the architectural design of the systems. For example, the
theoretical definition of the virtual machine architectural style has ignored the issue
how programs and initial data are loaded on to the virtual machine. This issue
cannot be ignored in the design of Java virtual machine because it must allow the
program code for classes to be dynamically loaded to the system. It is, therefore, no
surprise to see a class loader component in the architecture of JVM in addition to
those relatively standard components of a virtual machine.
Second, the architectural styles that we have recognised are still far away
from a complete catalogue. In fact, it is doubtful if such a complete catalogue could
even be developed. Therefore, it is inevitable that sometimes we can only partially
recognise the architectural style of a system and have several different such partial
understandings of a software system’s architectural style. When there are several
different views of a system’s architectural style, a question remains for which style
Instruction A
Instruction B
Instruction C
Instruction Z
Class files
Class Loader
Method Area
Java Stacks
PC Registers

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