Chapter13.Inserting Ilities by Controlling Communications

Chapter 13. Inserting Ilities by Controlling Communications

ROBERT E. FILMAN, STUART BARRETT, DIANA D. LEE, AND TED LINDEN

For many applications, most code is not devoted to implementing the desired input-output behavior but to providing system-wide properties like reliability, availability, responsiveness, performance, security, and manageability. We call such qualities ilities. This article describes a system that enables a more complete separation of ility implementations from functional components, allowing ilities to be developed, maintained, and modified with minimal impact on functional implementations.

Ilities can seldom be entirely implemented simply as discrete services. For example, many replication algorithms require logging and distributed update on every object modification. Similarly, performance, security, and manageability enhancements demand systematic and widespread code changes, complicating a clean design. While object-oriented design and programming has provided effective ways to modularize functional requirements into separately maintainable components, it has been less successful in enabling programmers to modularize code devoted to ilities. Object orientation does not provide programming structures that allow ilities and functionality to evolve independently over the software lifecycle.

Separating ility support from functional components becomes significantly more important and complex in distributed applications. Distributed applications typically have more stringent ility requirements and need more complex ility algorithms. This article defines an approach that supplements standard object-oriented methods with a general mechanism for injecting ility implementations into the communications between functional components. Algorithms that support ilities are separated from functional components but may be invoked whenever functional components communicate. This allows ilities and functionality to be modified and maintained with minimal impact on each other.