Pi-SPACE: A Formal Architecture Description Language 
Based on Process Algebra For Evolving Software 
Systems






Reconfiguration, Architectural Evolution


A critical aspect of the design 
of any large software system is its 
global structure, that is, the high level 
organisation of computational elements and the interactions 
between those elements. The underlying idea of 
software architecture is that better software systems 
can be achieved by modelling this high 
level organisation during design. Large software systems 
are always at the design stage in 
the sense that there is always the 
potential for new requirements to trigger the 
need for the system design to evolve. 
A software architecture can assist by providing 
a high level understanding of the scope 
and nature of such evolutions. The architecture 
therefore must provide an understanding of the 
components in a system, and the connectors 
which link those components. Further the architecture 
must provide an understanding of how the 
system can be decomposed into constituent components 
and connectors, and how the system can 
be recomposed after components and connectors have 
been added, deleted or replaced. To provide 
effective support for such evolution, an architecture 
description language must cope with dynamism and 
evolution. It must be able to describe 
the changing configuration of components and connectors 
on the fly during run time. It 
must be possible to incorporate new components 
and connectors which were not defined when 
the system was initially created. This paper 
presents an architecture description language (ADL), called 
Pi-SPACE, designed to cope with requirements on 
dynamism and evolution. Pi-SPACE provides constructs for 
describing architectures based on the Pi-Calculus. It 
supports the adding, replacing and removing of 
components and connectors in an evolving software 
system. It provides facilities for reasoning about 
both functional and non-functional properties. Applications of 
Pi-SPACE are motivated by work on the 
relationships between business processes (including software development) 
and the software systems which support them. 
Businesses need flexible software that enhances their 
ability to adapt and survive in changing 
markets, rather than legacy systems that restrict 
their business decisions.