Software Architecture

Summary

• What is Software Architecture?
• Prescriptive v Descriptive Architecture
• Architectural Evolution
  • Architecture Degradation
  • Architectural Recovery
• Architectural Elements
  • Components, Connectors, and Configurations
• Architectural Styles
  • Type of Styles
    • P2P Styles
    • Napster
    • Skype
• Takeaways

What is Software Architecture?

Elements, Form, and Rationale

• What, how, and why
• Perry and Wolf

Shaw and Garland say that Software Architecture is a level of design that involves

• description of elements from which systems are built
• interactions among those elements
• patterns that guide their composition
• constraints on those patterns

Software Architecture := a set of principal design decisions about the system

Blueprint

• Structure
• Behavior
• Interaction
• Non-functional Properties

Temporal aspect

• SWA is not defined at once, but iteratively, overtime

Prescriptive v Descriptive Architecture

A prescriptive architecture captures the design decisions made prior to the system’s construction.

• “as-conceived” SWA

A descriptive architecture describes how the system has actually been built

• “as-implemented” SWA

Architectural Evolution

Ideally, when a system evolves, the prescriptive SWA should be modified first. This rarely happens.

Can happen for multiple reasons:

• Dev Sloppiness
• Short deadlines
• Lack of documented prescriptive SWA
• …

Architecture Degradation

Architectural Drift := introduction of architectural design decisions orthogonal to a system’s prescriptive architecture

Architectural Erosion := introduction of architectural design decisions that violate a system’s prescriptive architecture

Architectural Recovery

Drift and Erosion $\Rightarrow$ degraded architecture

• Keep tweaking code
• Fix it

Architectural Recovery := determining the SWA from implementation and fixing it.

Architectural Elements

3 main types of elements:

• Processing
  • Implement business logic
  • Perform transformations on data
• Data
  • Contain information to be used
• Interaction
  • Glue that holds everything together

Processing + Data = Components

Interaction elements are contained and controlled by system connectors

Components + Connectors = Configuration

Components, Connectors, and Configurations

Software Component := an architectural entity that encapsulates a subset of the system’s functionality and/or data

Components restrict access to that subset via explicit interface

Software Connector := an architectural entity effecting and regulating interactions

Architectural configuration := association between components and connectors of a software architecture

Architectural Styles

Styles capture idioms that work well together

A family of systems in terms of a pattern of structural organization; a vocabulary of components and connectors, with constraints on how they can be combined.

Named collection of architectural design decisions applicable in a given context

Type of Styles

• Pipes and Filters
  • Chain of processing elements
  • Output of one element is input of next element
• Event-driven
  • Event-Emitters and Event-Consumers
  • Like GUI’s reacting to push of a button
• PubSub
  • Senders send messages with tags, and without knowledge of who will receive them
  • Receivers listen for messages with certain tags
• Client-Server
  • Server provides functionality
  • Client requests functionality
• P2P
  • Decentralized
  • Nodes act as providers and requesters of functionality
• REST
  • Respresentational State Transfer
  • Hybrid architecture for distributed hypermedia systems
  • Uniform Connector Interface

P2P Styles

Decentralized resource sharing and discovery

Napster

P2P File sharing system

1. Peer A registers with content directory
2. Peer B registers with content directory
3. Peer A requests a song
4. Content directory returns a handle to peer that contains requested song
5. Peer A requests song
6. Peer B responds with song

Content Directory is SPOF and issue with scaling

Skype

Login server is only SPOF

Supernodes are NOT owned by skype. They are just regular nodes that are promoted by skype

• Requires node to be up for a long time
  • Microsoft killed Skype for 36 hours because they pushed a critical Windows Update that force-rebooted a bunch of machines

Peer 2 can talk to Peer 3 by going through Super Nodes A and B

Takeaways

A great architecture

• is a ticket to success
• reflects a deep understanding of the problem domain
• normally combines aspects of several simpler architectures