Software Architecture and Reflection
		 ------------------------------------

			      Dilip Soni
			dilip@scr.siemens.com
			755 College Road East
			 Princeton, NJ 08540
			    (609) 734-6545
				   
We are exploring the emerging area of software architecture. Software
architecture provides a unifying theme for the engineering and manage-
ment compromises that must be made to complete a product. By relating
form and function, software architecture provides the central element
around which many aspects of a project can revolve. The choice,
development, and refinement of a software architecture are thus
critical to the software development process.

We have taken an empirical approach [1], surveying the description,
analysis, and use of software architectures within Siemens. We are
also investigating the different strategies for system decomposition
and interconnection that are important within specific application
domains.

In our survey, we have observed that the software architecture of a
system can be viewed from many different perspectives [2]:

         - Code architecture organizes code into files, directories,
           class libraries, and program libraries.

	 - Module interconnection architecture describes the logical
	   structure of the system, organizing the system into
	   subsystems, modules, and layers with details of exported
	   and imported interfaces. 

	 - Execution architecture describes the dynamic structure of
	   the system with assignment of logical modules to processes
	   and processors.

	 - Conceptual architecture describes the system in terms of 
	   components representing its major design elements, and
	   connectors representing interfaces and interaction
	   protocols between components.

These architectural perspectives capture different yet inter-dependent
sets of design decisions made by different teams in different phases.
For example, the code architecture is influenced by the
implementation language and the development environment. The execution
architecture, on the other hand, is influenced by technological
support for communication, distribution, and integration.

The objective of our research project is to develop methods and tools
to support:

   	 - rigorous notations to describe architectures from a variety
	   of perspectives,
	  
	 - analysis techniques to determine and predict non-functional
	   properties (such as performance, schedulability,
	   configurability, and inter-operability,) and

	 - implementation techniques to assure such non-functional
	   properties.

Relevance of reflection
----------------------

In our opinion, there are at least three issues related to reflection:

         - what system aspects (such as structure or behaviour) are to be
           represented and operated upon,

	 - how a system can reflect upon its own aspects, and

	 - when a system reflects upon itself (e.g., design time,
	   compile time, installation time, start-up time, or run time).

Our work mainly addresses the first and the third issues. We have only
considered scenarios where the architectural perspectives are created,
operated upon, and modified as a part of the development process.
Tools supporting such scenarios improve the configurability of a
product in response to changes in requirements and technology.

However, their ability to support dynamic reconfiguration of a system
is limited, requiring the use of application-specific architectural
primitives and implementation techniques. We hypothesize that
reflection facilitate development of general techniques to improve
dynamic reconfigurability of software architectures.

References

[1]	Dilip Soni, Robert Nord, Liang Hsu, Paul Drongowski, "Many
        Faces of Software Architecture," in Proc. of Workshop on
	Studies of Software Design, Technical Report, Dept. of
	Computing and Information Sciences, Queens University, April
	1993. 

[2]	Dilip Soni, Robert Nord, and Liang Hsu, "An Empirical Approach
        to Software Architectures," to appear in the Proc. of the 7th 
	International Workshop on Software Specification and Design,
	Dec. 1993.