Wire scatter (fan-out) directly into the RRF merger via scatter_gather_search(),
completing the full read path: plan → scatter → RRF merge. Add RRF simulation
mode to score-comparability benchmark for measuring rank correlation against
global BM25 ground truth.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
- Add NodeClient trait for HTTP calls to Meilisearch nodes (seam between pure miroir-core and networked miroir-proxy)
- Add ScatterPlan struct containing chosen_group, target_shards, shard_to_node mapping, deadline_ms, hedging_eligible
- Implement plan_search_scatter() pure function that constructs the covering set without I/O
- Implement execute_scatter() async function that fans out to nodes with partial-failure handling
- Add MockNodeClient for testing with pre-programmed responses/errors
- Add unit tests for plan construction, query group rotation, shard-to-node mapping, hedging eligibility, and scatter execution
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Research doc updated with precise 95% CIs per query type. compare.py
now computes and reports confidence intervals. Kendall τ = 0.79
(95% CI [0.7873, 0.8006]) confirms raw score merging is not viable;
RRF already implemented in merger.rs as mitigation. Follow-up bead
created (miroir-zfo) for RRF quality validation.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Completed Plan §15 Open Problem #4 research on cross-shard score comparability.
## Key Finding
Average Kendall tau: 0.79 vs. 0.95 threshold — FAIL
Cross-shard score comparability is a significant issue:
- Common-term queries: τ = 0.15 (catastrophic)
- Local IDF statistics cause score inflation on small shards
- Documents from 10-doc shards outrank 93K-doc shard results
## Recommendation
Implement Reciprocal Rank Fusion (RRF) for result merging.
Follow-up bead: miroir-nsu
## Artifacts Added
- Benchmark infrastructure: tests/benches/score-comparability/
- Corpus generator with extreme shard skew (100× variance)
- Query generator (10K random queries across 5 types)
- BM25-based simulation with global vs local IDF
- Kendall tau comparison tool
- Full experimental results (τ = 0.79 ± 0.01, 95% CI)
- Research writeup: docs/research/score-normalization-at-scale.md
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
