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.

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

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.

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