Architecture Tactics

Intro

• How can we design an architecture that will achieve the desired quality attributes ?
• Sources of architecture
  • Theft: From previous systems, literature
  • Method: Systematic and conscious, derived from requirements via transformations and heuristics.
  • Intuition: Ability to conceive without conscious reasoning. Increased reliance on intuition increases the risk.
• Ratio of usage of above three methods varies according to architects experience and novelty.

• What is a tactic ? - A tactic is a design decision that influences the control of a quality attribute response.
• A collection of tactics is an architectural strategy.
• Each tactic is a design option for the architect.

Availability Tactics

• All approaches to maintaining availability involve some type of redundancy, some type of health monitoring and some type of recovery when a failure is detected.
• Availability tactics involve- Fault detection, fault recovery and fault prevention.

Fault Detection

• Ping/echo and hearbeat generally operate among distinct processes and the exception tactic operates within a single process.

Ping/Echo

• One component issues a ping to a component to be checked and expects to receive back an echo within a predefined time.
• Response time allows performance to be assessed.
• If bandwidth consumption of pings is an issue, then the ping/echo detectors can be organized in a hierarchy.
  • Low-level detector pings low level processes and higher level fault detectors ping lower level ones.

Heartbeat

• One component emits a heartbeat message periodically and another component listens for it.
• Absence of heartbeat means originating component has failed.
• Heartbeat messages can be combined with useful data.

Exceptions

• Exceptions encountered during an exception.
• Exception handler is invoked which typically executes in the same process that introduced the exception.

Fault Recovery

• Fault recovery consists of preparing for recovery and making the actual system repair.

= Voting

• Processes running on redundant processors each take equivalent input and compute a simple output value that is sent to a voter.
• Voter detects deviant behaviour from a single processor - then it fails it.
• Different choices of voting algorithm - "majority wins" or "preferred component".
• Often used in control systems to correct faulty algo's or processors.