The Tension between Interdependencies and Flexibility in Event-Based Infrastructures: managing variability in practice

In the past years, new levels of flexibility in software have been required. The need to support variability in software product lines, the multitude of applications in an organization and the fast evolution of software applications has pushed the development of flexible software infrastructures. Practitioners and researches sought for new ways of achieving reuse and variability in their domains. As part of this process, middleware, and more specifically, publish/subscribe infrastructures strive to provide the services demanded by different applications. This forces have resulted in growing research in the are of flexible middleware. As proposed by David Parnas in his seminal paper (Parnas 1978), in order to support this constant demand for change, software must be flexible, i.e. designed not as a single system but as a family of systems that can grow and shrink in order to accommodate new requirements. Publish/subscribe infrastructures are not different. As supporting infrastructures, they need to evolve to support the specific needs of applications that use their services. The implementation of Parnas’ flexibility in such infrastructures, however, has shown not to be a trivial task. In special, interdependencies between the many variability dimensions of a pub/sub infrastructures may pose a harsh limit in its flexibility. This paper analyses such dependencies in the context of a flexible architecture for pub/sub infrastructures. It also points out some strategies used to address some of those issues, discussing what was successful and what was not.