Hitchhiker's Guide to Software Architecture and Everything Else - by Michael Stal

Tuesday, February 24, 2015


Recently, I discussed architecture design with some colleagues who are involved in Product Line Engineering projects. When the term "Reference Architecture" came up, it became obvious after a while that they used the term specifically for Product Line Architecture, while I had Reference Architectures in mind, which are typically created to steer and guide standardization. They might also present architecture styles that are commonly accepted within a domain. Think of compilers which are in most cases structured as a Pipes & Filters architecture which is composed of a lexer, a parser, a semantic evaluator, an optimizer, and a code generator.

A Reference Architecture (RA), on the other hand, is a coarse grained architecture template different organizations use for guiding their design activities. You may remember the OSI 7 Layers model or the OMG Reference Architecture for CORBA, both invented in the Stone Age of software engineering. A RA is a blueprint which doesn't include any further assets but documents. In fact, it is not very specific but rather abstract.

A Product Line Architecture (PLA) is an architecture for the product line of an organization. It captures the commonalities of a set of similar products and defines variation points using feature models or meta models. It contains different core assets, some of which are ready for use. A PLA is much more specific than a RA and defines a framework with (partially) implemented artifacts and variation mechanisms.

An RA can be used as the core of a PLA. For this purpose, the RA is concretized in Domain Engineering by addressing requirements and constraints of an organization. In the process, engineers may provide additional variation points or bind existing variation points. In the latter case the PLA transforms a variation point to a commonality. If many organizations do this for the same variation point in similar ways, a new extended RA is born.

A PLA can become the base of a RA if it is subject to abstraction and considered as an established practice in the domain, i.e. if most PLAs will end up in the same RA.

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