Balanced Coupling Model Reference

Based on Vlad Khononov's "Balancing Coupling in Software Design".

The Balance Rule

MODULARITY = STRENGTH XOR DISTANCE
COMPLEXITY = STRENGTH AND DISTANCE
BALANCE = (STRENGTH XOR DISTANCE) OR NOT VOLATILITY

• Modularity emerges when strength and distance counterbalance
• Complexity emerges when both are equal (both high or both low)
• Pragmatic: unbalanced coupling is tolerable if volatility is low

Three Dimensions

1. Integration Strength (Knowledge Shared)

From most to least intrusive:

Level	Knowledge Type	Implicit/Explicit
Intrusive	Implementation details, private interfaces	Implicit, fragile
Functional	Business rules, functional specifications	Often implicit
Model	Domain/business model, data structures	Explicit but broad
Contract	Integration contracts, facades	Most explicit, stable

Key insight: Intrusive and Functional coupling are often implicit — they exist without anyone realizing. Contract coupling is explicit by design.

2. Distance (Cost of Change)

Multiple dimensions contribute to distance:

• Code structure: methods → objects → modules → crates → services
• Organizational: same team vs different teams (Conway's Law)
• Runtime: synchronous (tight) vs asynchronous (loose)
• Lifecycle: shared deployments vs independent deployments

Distance is fractal: the same rules apply at every abstraction level.

3. Volatility (Probability of Change)

Determined by DDD subdomain classification, not just git history:

Subdomain	Volatility	Reason
Core	High	Competitive advantage, constantly optimized
Supporting	Low	Boring CRUD/ETL, rarely changes
Generic	Low	Solved problems, stable implementations

Important: distinguish essential vs accidental volatility. Accidental volatility comes from poor design, not business needs.

Configure this in .coupling.toml:

[subdomains]
core = ["src/balance.rs", "src/metrics.rs"]
supporting = ["src/analyzer.rs", "src/report.rs"]
generic = ["src/config.rs", "src/web/**"]

Issue Classification

Pattern	Strength	Distance	Volatility	Severity
High Cohesion	Strong	Close	Any	Ideal
Loose Coupling	Weak	Far	Any	Ideal
Acceptable	Strong	Far	Low	Minor
Global Complexity	Strong	Far	High	Critical
Local Complexity	Weak	Close	Any	Review
Hidden Coupling	Temporal co-change	No code edge	Medium/High	Medium/High
Accidental Volatility	Any	Any	Supporting/Generic churn	Medium

Hidden Coupling indicates strong co-change without a direct AST dependency, often duplicated business logic or connascence of meaning/algorithm. Accidental Volatility indicates high churn where the subdomain classification says volatility should be low.

Recognition Rules (avoid false positives)

The balance rule only condemns coupling when volatility is genuinely high. Apply these so the signal stays trustworthy:

• Essential vs accidental volatility: subdomain (.coupling.toml) volatility is authoritative for scoring. Raw git churn from a development sprint is accidental — it feeds AccidentalVolatility only, and must not create Cascading Change Risk on a low-essential-volatility target.
• Severity by volatility: Strong + Far + High = Global Complexity (act now); Strong + Far + Low = Acceptable (Minor — low volatility neutralizes the distance).
• Entrypoint (crate::main): high efferent fan-out and co-change with wired modules are expected by design → not a defect / not hidden coupling.
• Re-export facade (crate::crate_name = lib.rs): a stable Contract; coupling to it is not intrusive coupling to a volatile component.
• Stable central abstraction: high afferent coupling is good design; only risky when the hub is itself volatile (scale severity by the hub's essential volatility).

Analysis Manifest

cargo-coupling declares blind spots for static analysis. Text output can expand them with --blind-spots; --json and --ai include the full manifest. Use this when interpreting a clean report.

Connascence Refinement

Within each strength level, connascence types provide finer granularity:

Static (compile-time): Name → Type → Meaning → Position → Algorithm Dynamic (runtime): Execution → Timing → Values → Identity

Stronger connascence = harder to change = higher coupling cost.

Pragmatic Balancing

• Not all unbalanced coupling needs fixing — prioritize by volatility
• Low volatility neutralizes unbalanced coupling
• Focus refactoring on core subdomains (highest business value)
• Distance increases lifecycle coupling (deployment constraints)

For detailed reference: model-reference.md