Architectural Styles

Question 1

ARCHITECTURAL STYLES

Answer 1

• An architectural style is a named collection of architectural design decisions that -are applicable in a given development context -elicit beneficial qualities in each resulting system
• A primary way of characterizing lessons from experience in software system design
• Reflect less domain specificity than architectural patterns
• Useful in determining everything from subroutine structure to top-level application structure

Question 2

PARTS OF A STYLE

Answer 2

• A set of component types with a given role/functionality
• A topology of relations (usually runtime relations)
• A set of connector types that handle communication, coordination, or collaboration
• A set of semantic constraints -I.e., what may components/connectors do or not do?

Question 3

Traditional, language-influenced styles

Answer 3

• Main program and subroutines
• Object-oriented

Question 4

Layered styles

Answer 4

• Virtual machines
• Client-server

Question 5

Data-flow styles

Answer 5

• Batch sequential
• Pipes and filters

Question 6

Shared memory styles

Answer 6

• Blackboard
• Rule based

Question 7

Interpreter Styles

Answer 7

• Interpreter
• Mobile code

Question 8

Implicit invocation Styles

Answer 8

• Event-based
• Publish-subscribe

Question 9

Styles classification

Answer 9

• Communication: Service-oriented architecture (SOA), Message Bus
• Deployment: Client/Server, N-tier, 3-tier
• Domain: Domain Driven Design
• Structure: Componend-Based, Object-Oriented, Layered-Architecture

Question 10

Blackboard Style

Answer 10

• Useful for problems for which no deterministic solution strategies are known
• several specialized subsystems assemble their knowledge to build a possibly partial or approximate solution
• Example: speech recognition, submarine detection, inference of 3D molecule structure
• All components have access to a shared data store (the blackboard)
• Components produce new data objects that are added to the blackboard
• Components look for particular kinds of data on the blackboard
• Find these by pattern matching

Question 11

Blackboard Style (2)

Answer 11

• Processes have to agree on the structure of the shared data space
• Adding new applications is easy
• Extending the structure of the data space is easy
• Modifying the structure of the data space is hard
• Possible need for synchronization, access control

Question 12

Pipes and Filters

Answer 12

Components (Filters)

• read streams of data on input producing streams of data on output
• local incremental transformation to input stream
• output usually begins before input is consumed

Connectors (Pipes)

• conduits for streams e.g. first-in-first-out buffer
• transmit outputs from one filter to input of other

Invariants

• filters must be independent, no shared state
• filters don’t know upstream or downstream filter identity
• correctness of output from network must not depend on order in which individual filters provide their incremental processing

Common specialisations

• pipelines: linear sequence of filters
• bounded and typed pipes …
• Common Example: lex/yacc -based compiler (scan, parse, generate code, ..)

Question 13

Pipes and Filters Strengths

Answer 13

• Overall behaviour is a simple composition of behaviour of individual filters.
• Reuse - any two filters can be connected if they agree on that data format that is transmitted.
• Ease of maintenance - filters can be added or replaced.
• Prototyping e.g. Unix shell scripts are famously powerful and flexible.
• Architecture supports formal analysis - throughput and deadlock detection
• Potential for parallelism - filters implemented as separate tasks, consuming and producing data incrementally.

Question 14

PIPES AND FILTERS WEAKNESSES

Answer 14

• Can degenerate to ‘batch processing’ - filter processes all of its data before passing on (rather than incrementally)
• Sharing global data is expensive or limiting
• Can be difficult to design incremental filters
• Not appropriate for interactive applications
• Synchronization of streams will constrain architecture.
• Error handling e.g. filter has consumed three quarters of its input and produced half its output and some intermediate filter crashes!
• Implementation may force lowest common denominator on data transmission e.g. Unix scripts everything is ASCII.

Question 15

Object-Oriented Style

Answer 15

• Components are objects
  
  • Data and associated operations
• Connectors are messages and method invocations
• Style invariants
  
  • Objects are responsible for their internal representation integrity
  • Internal representation is hidden from other objects
• Advantages
  
  • “Infinite malleability” of object internals
  • System decomposition into sets of interacting agents
• Disadvantages
  
  • Objects must know identities of servers
  • Side effects in object method invocations

Question 16

Implicit Invocation

Answer 16

• Event announcement instead of method invocation
  
  • “Listeners” register interest in and associate methods with events
  • System invokes all registered methods implicitly
• Component interfaces are methods and events
• Two types of connectors
  
  • Invocation is either explicit or implicit in response to events
• Style invariants
  
  • “Announcers” are unaware of their events’ effects
  • No assumption about processing in response to events

Question 17

Implicit Invocation Advantages

Answer 17

• Reuse: any component can be introduced by registering it for appropriate events
• Evolution: components may be replaced without affecting other interfaces

Question 18

Implicit Invocation Disadvantages

Answer 18

• Control: no way to know what will happen after event is announced
• Data: may lead to performance problems
• Correctness: meaning of procedure is context dependent

Question 19

Publish-Subscribe

Answer 19

Subscribers register/deregister to receive specific messages or specific content. Publishers broadcast messages to subscribers either synchronously or asynchronously.

Question 20

Publish-Subscribe (2)

Answer 20

• Components: Publishers, subscribers, proxies for managing distribution
• Connectors: Typically a network protocol is required. Content-based subscription requires sophisticated connectors.
• Data Elements: Subscriptions, notifications, published information
• Topology: Subscribers connect to publishers either directly or may receive notifications via a network protocol from intermediaries
• Qualities: Highly efficient one-way dissemination of information with very low-coupling of components

Question 21

Event-Based Style

Answer 21

• Independent components asynchronously emit and receive events communicated over message buses
• Components: Independent, concurrent event generators and/or consumers
• Connectors: Message buses (at least one)
• Data Elements: Events/messages – data sent as a first-class entity over the Message bus
• Topology: Components communicate with the Message buses, not directly to each other.
• Variants: Component communication with the Message bus may either be push or pull based.
• Highly scalable, easy to evolve, effective for highly distributed applications.

Question 22

Variations

Answer 22

• Enterprise Service Bus (ESB).
  
  • uses services for communication between the bus and components attached to the bus.
  • provides services that transform messages from one format to another, allowing clients that use incompatible message formats to communicate with each other
• Internet Service Bus (ISB).
  
  • applications hosted in the cloud instead of on an enterprise network.
  • uses Uniform Resource Identifiers (URIs) and policies to control the routing of logic through applications and services in the cloud.