Software Architecture Modeling

Capturing the Architecture

Grady Booch

• Every system has an architecture
• Some architectures are manifest and visible, many others are not
• A system's (descriptive) architecture ultimately resides in its executable code
• Before a system is built, its (prescriptive) architecture should be made explicit
• A system's architecture may be visualized and represented using models that are somehow related to the code (existing or yet to be written)

What is modeling?

Richard Taylor

• An architectural model is an artifact that captures some or all of the design decisions that comprise a system’s architecture.
• Architectural modeling is the reification and documentation of those design decisions.

Abstraction and Interpretation

• The architecture models only some interesting aspects of a system.

Model Quality

• Ambiguity
  • A model is ambiguous if it leads to more than one interpretation
  • Incomplete models can be ambiguous: different people will fill in the gaps in different ways.
• Accuracy
  • A model is accurate if it is correct, conforms to fact, or deviates from correctness within acceptable limits.
• Precision
  • A model is precise if it is sharply exact or delimited.

Accuracy and Precision

Richard Taylor

Model Quality - Advice

• Make sure your architecture is accurate
  (a wrong, inconsistent or conflicting architectural decision is a recipe for disaster)
• Sometimes you can even make it complete
  (but it will be more expensive, so only do it for critical aspects of the system)
• Precision helps, but avoid over-specifying and over-designing the architecture, especially if the architecture is inaccurate, adding details will not fix it. (developers may be trusted to add missing details)

Why Modeling?

• Record decisions
  • Document the architecture and its rationale
  • Which decision? Why the decision?
• Communicate decisions
  • Visualize the architecture
  • Different roles have different perspectives
• Evaluate decisions
  • What is a good model?
  • What is a good decision?
  • Detect problems early
• Evolve decisions
  • Give constraints and a clear path to change the architecture
• Generate artifacts
  • Drive the development from very precise models

Canonical Models

Example Domain Model

• Music songs are organized in albums
• The same song can be authored by many artists
• Listening to each song costs 0.99 CHF, but short samples can be heard for free
• Songs can be downloaded and also live streamed
• Songs are stored in files of standard MP3 format
• Files contain embedded metadata and watermarks
• A music player can carry 10’000+ albums