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.

Active Redundancy (Hot restart)

• There are N redundant components - all of which respond to events in parallel.
• Response/output from only one component is used though and rest are discarded.
• Downtime is minimal, because backups are current and time to recover is only the switching time.
• E.g. LAN with a number of parallel paths and redundant component in a separate path.
• Synch is done by ensuring that all msgs to any component are sent to all redundant components, therefore a reliable transmission protocol may be required.

Passive Redundancy (Warm restart)

• One component (the primary) responds to events and informs the other components (the standbys) of status updates.
• When a fault occurs, backup state on standby must be fresh before resuming services.

Spare

• Standby spare platform.
• Must be rebooted to the appropriate software config and the state must be initialized to the point where the failure occurs.
• Therefore checkpoints of the system state must be made regularly.

Repair Tactics / Component Reintroduction

• When a redundant comp fails, it may be reintroduced after it has been repaired.

Shadow operation

• The previously failed component may be made to run in shadow mode to mimic behaviour of working components for a short time before making it operational.