Notes

Outline

Achieving Appropriate Values for Quality Attributes John D. McGregor Clemson University

Tactics A strategy is a high-level plan for achieving a goal. A tactic is the actual set of steps for achieving the goal.

General tactic Apply an architectural style that fits the overall problem This involves mapping the pattern onto the elements of the problem. “In a large research organization, research reports are produced, reports and papers from outside the organization are found and read by subscribing to journals and attending conferences. We need an information system which makes certain that each researcher sees the information he/she needs.”

General tactic - analysis Data is produced Data is consumed Should a researcher search for what they want at the moment or should it just show up? How would the researcher indicate what information they want to see?

Publish/Subscribe

Tactic description Do a domain analysis of the attribute What techniques affect the attribute Which of those techniques enhance the attribute

Availability Tactics A fault that causes a failure affects availability so… Remove the fault or find a work-around Fault detection exceptions – reactive ping - proactive Fault recovery hot swap Fault prevention operational guideline – reboot every day

Testability tactics How can we provide for more thorough testing Record/Playback supports future testing Special test interface very common in hardware Internal monitoring during testing the monitor checks state and throws      an exception

Usability tactics Separate user interface changes to user interface do not change functionality Support user initiative cancel/undo operations Support system initiative models of user, system, task properties files

Tactics and Architectural Patterns Architectural pattern involves several tactics ActiveObject pattern from Schmidt Main idea is introduction/management of concurrency Concurrency – performance tactic Information hiding – Modifiability tactic Intermediary – Modifiability tactic Binding time – Modifiability tactic Scheduling policy – performance tactic

Active Object Pattern

Patterns A pattern is “a solution to a problem in context” So a pattern is already validated as to the tradeoffs among the tactics within it, but … The tradeoffs only apply within the context for which the pattern has been validated

Binding times for tactics Concurrency – performance tactic – runtime Information hiding – Modifiability tactic – design time Intermediary – Modifiability tactic – design time Binding time of client/proxy – Modifiability tactic - open Scheduling policy – performance tactic – design time or later

Problem Want services but in as small a footprint as possible High reliability and security for services Modular system components Configurability

MicroKernel

MicroKernel

Activity diagram – system parts

System view

Known use – QNX - RTOS

Messaging in QNX

MicroKernel Database Engine