Propagation Cost

Overview

propagation_cost is the canonical change-propagation metric with three channels:

Structural (import graph reachability)
Temporal (git co-change reachability)
Hybrid (weighted structural + temporal)

The canonical system key is:

propagation_cost.system.ratio

Key Groups

System

propagation_cost.system.ratio
propagation_cost.system.structural_weight_effective
propagation_cost.system.temporal_weight_effective

Structural module

propagation_cost.structural.module.ratio
propagation_cost.structural.module.reachable_pairs
propagation_cost.structural.module.total_pairs
propagation_cost.structural.module.k1_ratio
propagation_cost.structural.module.k2_ratio
propagation_cost.structural.module.k3_ratio
propagation_cost.structural.module.distance_decayed_ratio
propagation_cost.structural.module.out_reach_p50
propagation_cost.structural.module.out_reach_p90
propagation_cost.structural.module.in_reach_p50
propagation_cost.structural.module.in_reach_p90
propagation_cost.structural.module.top_out_hubs
propagation_cost.structural.module.top_out_hubs_table
propagation_cost.structural.module.top_in_hubs
propagation_cost.structural.module.top_in_hubs_table

Temporal module

propagation_cost.temporal.module.available
propagation_cost.temporal.module.commit_count
propagation_cost.temporal.module.cochange_threshold
propagation_cost.temporal.module.ratio
propagation_cost.temporal.module.reachable_pairs
propagation_cost.temporal.module.total_pairs

Hybrid module

propagation_cost.hybrid.module.ratio
propagation_cost.hybrid.module.structural_weight
propagation_cost.hybrid.module.temporal_weight
propagation_cost.hybrid.module.divergence_abs

propagation_cost.system.ratio is the canonical system key and currently equals propagation_cost.hybrid.module.ratio.

Structural function

propagation_cost.structural.function.ratio
propagation_cost.structural.function.reachable_pairs
propagation_cost.structural.function.total_pairs
propagation_cost.structural.function.callgraph_precision
propagation_cost.structural.function.confidence
propagation_cost.structural.function.ratio_low
propagation_cost.structural.function.ratio_high
propagation_cost.structural.function.betweenness_max
propagation_cost.structural.function.betweenness_avg
propagation_cost.structural.function.betweenness_skipped
propagation_cost.structural.function.top_betweenness
propagation_cost.structural.function.top_betweenness_table

Traffic-weighted function

propagation_cost.traffic.function.ratio
propagation_cost.traffic.function.reachable_weight
propagation_cost.traffic.function.total_weight
propagation_cost.traffic.function.telemetry_coverage

Configuration

metrics:
  - id: propagation_cost
    enabled: true
    config:
      cochange_threshold: 2
      hybrid_weights:
        structural: 0.6
        temporal: 0.4
      k_hops: [1, 2, 3]
      distance_decay_alpha: 0.5
      betweenness:
        max_nodes: 2000
      finding_thresholds:
        system_high: 0.5
        system_critical: 0.7
        temporal_high: 0.5
        divergence_abs: 0.2
        function_confidence_low: 0.5

Semantics and Normalization

hybrid_weights are normalized to sum to 1.0 (fallback to 0.6/0.4 if both are 0).
k_hops is sanitized to unique sorted integers in [1, 10] (default [1, 2, 3]).
distance_decay_alpha is clamped to (0, 1].
If git history is unavailable (temporal.module.commit_count == 0), temporal values stay 0, effective hybrid weights become structural=1.0 and temporal=0.0, and propagation_cost.system.ratio falls back to propagation_cost.structural.module.ratio.

Findings

The metric emits deterministic findings with rule_id:

arxo/propagation-cost/high-system
arxo/propagation-cost/high-temporal
arxo/propagation-cost/structural-temporal-divergence
arxo/propagation-cost/low-callgraph-confidence

Breaking Migration

pc.system -> propagation_cost.system.ratio
propagation_cost.module.ratio ->
- propagation_cost.system.ratio for risk/budget consumers
- propagation_cost.structural.module.ratio for structural-only consumers
propagation_cost.function.ratio -> propagation_cost.structural.function.ratio
propagation_cost.function.precision_ratio -> propagation_cost.structural.function.callgraph_precision
propagation_cost.function.traffic_weighted_ratio -> propagation_cost.traffic.function.ratio