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