Architecture Styles

Intro

• It is a pattern of component types and their interactions.
• Define constraints: how things are allowed to interact.
• Architectural pattern is synonym for style.
• Architectural patterns package tactics. A pattern that supports availability will use redundancy as a tactic.
• Defining features:
  • Component types: e.g data store, processes etc.
  • Component layout: e.g. which process is allowed to access data store.
  • Connectors: e.g how are they connected ? RPC, Data stream ?
  • Constraints: On components - e.g. data store cant modify itself, On connections - no P2P communication allowed.

Data Centred

• Characterised by the access/update of a central data store.
• The data store can be passive (e.g. a file) or be active (e.g. blackboard)
• Good for integration and extensibility. There is a low coupling - only the data is shared.
• E.g. Transactional DB, Blackboard, CASE Tools.

Blackboard

• If the current state of the central data is the main trigger for processes to execute, then the repository can be a blackboard.
• A common knowledge base, the "blackboard", is iteratively updated by a diverse group of specialist knowledge sources, starting with a problem specification and ending with a solution.
• Each knowledge source updates the blackboard with a partial solution when its internal constraints match the blackboard state.
• In this way, the specialists work together to solve the problem.
• Invocation of a KS is triggered by the state of the blackboard.

Data Flow Architectures

• Characterized by viewing the system as a series of transformations on successive pieces of input data.
• Orderly motion of data. Explicit pattern of data flow.
• No direct interactions b/w components.
• Objectives: Modifiability - plug in/out components. Reusability - sequence of components plugged together.

Batch Sequential

• Processing steps are independent programs.
• Each step runs to completion before next step can start.
• Batch of data is transmitted as a whole between components.

Pipe and Filter

• Similar to batch sequential, except that the computations (filters) work incrementally on the data.
• Data is immediately passed - i.e. it is streamed and not batched.
• No state is retained between computations.
• Pipes carry data from output of one computation (filter) to another.
• e.g. Unix Shell - ls | grep "java" | wc -l
• Advantages:
  • Simple, easy to reuse - a pipe and filter system can be made into another filter.
  • Support concurrency.
• Disadvantages:
  • Filters operate as a separate process (process overhead). They cannot cooperate.
  • Lowest common denominator of data has to be used for data stream - e.g. ASCII text.

Virtual Machine Architecture

• Main goal is to achieve portability.
• A virtual machine is a software style that simulates/provides interface to functionality of underlying hardware/software platform.
• biggest e.g JVM.
• Running program through an interpreter adds flexibility through the ability to interrupt and query program and introduce changes at runtime. But there is a performance penalty.
• Virtual machine structure. The interpreter selects an instruction, updates its internal state and based on instruction potentially updates the program's data.

Call and Return Architecture

• Classic programming structure. Decompose program into smaller structures.
• Goals of achieving modifiability and scalability.
• Many different styles:
  • Main program and subroutine
  • RPC

OO Style

• Encapsulate data and operations together.
• Encapsulation, Polymorphism and Inheritance.

Layered Style

• Components are assigned to layers to control component interaction.
• Each level communicates with only its immediate neighbours, Layer N can use services of only layer N-1 and delegates subtasks to it.
• Goal is to achieve qualities of modifiability and portability.
• There can be a relaxed layer - where a layer talks to layers other than immediate neighbours.

• Most common e.g. is the OSI protocol stack.
• Advantage
  • Increased levels of abstraction.
  • Confinement of change - change can max affect to other layers.
• Disadvantage:
  • Difficult to structure systems in layers.
  • Performance may require closer coupling between higher layers and lower layers.

Independent Component Architectures

• Consist of a no of independent comp's which that communicate through messages.
• Messages may be passed to named participants, or unnamed participants.
• Goal of modifiability by decoupling various portions of computations.

Event Systems

• Components announce data they wish to share (publish) with their environment - a set of unnamed components.
• Other components may register an interest in the class (subscribe).
• Typically a message manager manages communication - providing the decoupling between the publisher and the subscriber.

Communicating Processes

• Processes communicate directly.
• Single machine - signals.
• Multiple machines - RPC. TCP etc.
• classic e.g. client server architecture. Server works synchronously or asynchronously (meaning client has his own thread of control).

Heterogeneous Styles

• Systems are rarely built from a single style.
• Types of heterogeneity:
  • Locational: Runtime structure reveal patterns of different styles in different areas. Different style for different sub systems. e.g. one of the modules in call and return will have a shared data repository.
  • Hierarchical: A component is part of one style. When comp is decomposed, it is structured according to a different style. For e.g. a component in a pub-sub system might be layered.
  • Simultaneous: Any of several styles can be applicable.

Safety and Reliability Patterns

Homogeneous Redundancy

• Uses identical channels to increase reliability.
• All redundant channels are run in parallel and the output is compared.
• If odd no of channels are used, majority-wins policy can be used that can detect and correct failures in the minority channels.
• Advantage: reliability without additional development costs - since channels can be cloned.
• Disadvantage: Can detect random faults, not errors (which are systematic faults).
• Therefore protects against hardware failures.

Monitor Actuator

• Special type of redundancy.
• Channels are separated into monitoring and actuator channels.
• The actuator channel performs the action e.g. starting the motor, lowering the wing flap.
• The monitor channel keeps track of what the actuation and monitors the environment to ensure that the results of the actuator are appropriate.
• The actuator itself cannot rely on the monitor - they must use separate sensors.

Watchdog

• A watchdog is a subsystem that receives messages from other subsystems on a periodic or sequential basis.
• If msg arrives too late or out of sequence, watchdog takes corrective action.
• Watchdogs usually simple - implemented in hardware. Software also possible.
• Primary disadvantage - too simple to support complex error handling and fault recovery.

Safety Executive

• Uses a centralized coordinator for safety monitoring and control of system from faults.
• Advanced watchdog which tracks and coordinates all safety monitoring.
  • Watchdog timeouts.
  • Faults identified by monitors.
  • Periodic built in test suites.
  • Software error assertions.
• For larger and complex systems, safety executive provides a centralized point for safety processing.
• Makes it simpler to verify and validate safety measures.