# Phase 1 — Core Routing: Final Verification Summary

## Overview
Phase 1 implements the deterministic, coordination-free routing primitives that form the foundation for all distributed operations in Miroir. The implementation uses rendezvous hashing (HRW) with twox-hash, matching the algorithm Meilisearch Enterprise uses internally.

## Implementation Summary

### Files Implemented
- `crates/miroir-core/src/router.rs` — Rendezvous hashing, shard assignment, write targets, covering sets
- `crates/miroir-core/src/topology.rs` — Node registry, replica groups, health state machine
- `crates/miroir-core/src/scatter.rs` — Fan-out orchestration primitives (stubbed execution for Phase 2)
- `crates/miroir-core/src/merger.rs` — Result merge primitives (RRF and score-based strategies)

## Definition of Done — All Verified ✅

1. **Determinism** — `test_determinism`, `prop_determinism` (1000 iterations, proptest with 1024 cases)
   - Same inputs always produce identical outputs
   - Verified across multiple runs

2. **Minimal Reshuffling** — `test_reshuffle_bound_on_add`, `prop_reshuffle_bound_on_add`
   - Adding a 4th node to 3-node group moves at most ~2 × (1/4) × 64 = 32 shard-node edges
   - Property-based tests verify bounds across 20-100 shards, 3-10 nodes, RF 1-3

3. **Uniform Distribution** — `test_uniformity`, `prop_uniformity`
   - 64 shards / 3 nodes / RF=1 → each node holds 17–26 shards (verified range)
   - Property-based tests verify even distribution across various configurations

4. **RF Placement Stability** — `test_rf2_placement_stability`, `test_reshuffle_bound_on_remove`
   - Top-RF placement changes minimally on add/remove
   - Verified with both unit and property-based tests

5. **Write Targets** — `test_write_targets_returns_rg_x_rf_nodes`, `test_write_targets_one_per_group`
   - Returns exactly RG × RF nodes, one from each replica group
   - Group isolation verified

6. **Query Distribution** — `test_query_group_uniform_distribution`
   - Chi-square test confirms even distribution (p < 0.05)
   - Round-robin by query counter

7. **Covering Set** — `test_covering_set_covers_all_shards`, `test_covering_set_rotates_replicas`
   - Returns exactly one node per shard within the chosen group
   - Intra-group replica rotation by query_seq verified

8. **Merger** — Comprehensive merge/facet/limit tests
   - Global sort by `_rankingScore`
   - Offset/limit handling
   - Facet aggregation (sum across shards)
   - `estimatedTotalHits` summation
   - `_miroir_*` field stripping
   - Both RRF and score-based merge strategies

9. **Coverage** — Line coverage for Phase 1 files
   - `router.rs`: 100% (65/65 lines)
   - `topology.rs`: 100% (130/130 lines)
   - `merger.rs`: 94.26% (148/157 lines)
   - `scatter.rs`: 77.29% (269/348 lines) — stub execution expected in Phase 2

## Test Results
- **Unit tests**: 516 passed, 0 failed
- **Property-based tests**: All proptest cases pass (1024 cases per property)
- **Integration**: Scatter-gather end-to-end tests pass

## Key Properties Verified

### HRW Rendezvous Hashing
- Deterministic: Same (shard, node) → same score
- Minimal reshuffling on topology changes
- Group-scoped assignment prevents both replicas in same group
- Tie-breaking by node_id for determinism

### Health State Machine
- Legal transitions: Joining → Active → Draining → Removed
- Failure paths: Active/Draining → Failed → Active
- Degraded state: Active ↔ Degraded
- Write eligibility respects shard migration state

### Result Merging
- RRF (Reciprocal Rank Fusion) with k=60 default
- Score-based merge for global-IDF preflight (OP#4)
- Deterministic tie-breaking on primary key
- Stable serialization (BTreeMap for facets)

## Notes
- Scatter execution stubs in `scatter.rs` are intentionally unimplemented pending Phase 2 wiring
- All core routing primitives are pure functions for easy testing
- The implementation is ready for Phase 2 (write path and read path integration)