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Phase 1 (miroir-cdo): Core Routing — Final verification

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Verified all Definition of Done items for Phase 1 Core Routing:

- Router: 96.20% coverage, all acceptance tests pass
- Topology: 100% coverage, state transitions validated
- Scatter: 100% coverage, stubbed implementation ready for Phase 2
- Merger: 94.67% coverage, global sort/facet/pagination tests pass
- Overall: 91.80% line coverage (exceeds 90% requirement)

All 151 unit tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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# Phase 1 — Core Routing Final Verification
# Phase 1 (miroir-cdo): Core Routing — Completion Summary
Bead ID: miroir-cdo
Date: 2026-05-09
## Completed
## Summary
Phase 1 Core Routing is **COMPLETE** with all Definition of Done items verified.
Phase 1 Core Routing implementation complete and verified. All 151 tests pass, covering deterministic, coordination-free routing primitives that everything else depends on.
### Implementation Status
## Verification Session (2026-05-09)
| Module | File | Coverage | Status |
|--------|------|----------|--------|
| Router | `router.rs` | 96.20% | ✓ Complete |
| Topology | `topology.rs` | 100.00% | ✓ Complete |
| Scatter | `scatter.rs` | 100.00% | ✓ Complete (stubbed) |
| Merger | `merger.rs` | 94.67% | ✓ Complete |
### What Worked
### Definition of Done — All Verified ✓
- All core routing primitives (router.rs, topology.rs, scatter.rs, merger.rs) verified as complete
- All 151 tests pass (18 router tests, 19 merger tests, 29 topology tests, 6 scatter tests, plus other modules)
- Rendezvous hashing (HRW) with XxHash64::with_seed(0) provides deterministic, minimal-reshuffling shard assignment
- Group-scoped assignment prevents both replicas of a shard from landing in the same group
- Node health state machine fully implemented with all required states
- Merger handles all edge cases: empty shards, degraded mode, pagination, facets
1. **Rendezvous determinism** — `test_rendezvous_determinism`, `acceptance_determinism_1000_runs`
2. **Minimal reshuffling on add** — `acceptance_reshuffle_bound_on_add` (≤ 2 × 1/4 × 64 = 32 shards)
3. **Uniform distribution** — `acceptance_uniformity_64_shards_3_nodes_rf1` (18-26 shards per node)
4. **Top-RF stability** — `acceptance_rf2_placement_stability`, `acceptance_reshuffle_bound_on_remove`
5. **write_targets correctness** — `test_write_targets_count` (RG × RF nodes)
6. **query_group distribution** — `test_query_group_distribution` (even round-robin)
7. **covering_set correctness** — `test_covering_set_one_per_shard`, `test_covering_set_replica_rotation`
8. **merger correctness** — All 20 merger tests pass (global sort, facets, pagination)
9. **Coverage ≥ 90%** — miroir-core: 91.80% line coverage
### What Didn't
- No issues encountered; the implementation was already complete and correct
### Surprise
- cargo-llvm-cov was available and showed 91.80% line coverage, exceeding the 90% requirement
- Phase 1 modules specifically have excellent coverage: router.rs (96.20%), topology.rs (100%), scatter.rs (100%), merger.rs (94.67%)
### Reusable Pattern
- Rendezvous hashing with XxHash64::with_seed(0) for deterministic shard assignment (matches Meilisearch Enterprise)
- Group-scoped assignment to ensure replica isolation
- Pure-function design for merger enables comprehensive unit testing without mocks
- Node health state machine: Healthy/Active (serving), Degraded (intermittent), Joining/Draining (transitional), Failed/Removed (unavailable)
- BTreeMap for stable JSON serialization in facet aggregation
## DoD Status
| Requirement | Status |
|------------|--------|
| Rendezvous assignment is deterministic | ✅ Verified |
| Adding 4th node moves ≤ 2×(1/4) of shards | ✅ Verified |
| 64 shards / 3 nodes / RF=1 → 18-26 shards each | ✅ Verified |
| Top-RF placement changes minimally | ✅ Verified |
| `write_targets` returns RG × RF nodes | ✅ Verified |
| `query_group` distributes evenly | ✅ Verified |
| `covering_set` returns one node per shard | ✅ Verified |
| Merger passes merge/facet/limit tests | ✅ Verified |
| miroir-core ≥ 90% line coverage | ✅ **91.80%** (via cargo-llvm-cov) |
## Coverage Report (cargo-llvm-cov)
### Test Results
```
Filename Regions Missed Regions Cover Functions Missed Functions Executed Lines Missed Lines Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
router.rs 1016 26 97.44% 60 1 98.33% 500 19 96.20%
topology.rs 776 0 100.00% 70 0 100.00% 421 0 100.00%
scatter.rs 214 0 100.00% 11 0 100.00% 121 0 100.00%
merger.rs 977 31 96.83% 49 4 91.84% 582 31 94.67%
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL 5672 423 92.54% 396 33 91.67% 3770 309 91.80%
test result: ok. 151 passed; 0 failed; 0 ignored
```
## Test Results
### Key Implementation Details
```
running 151 tests
test result: ok. 151 passed; 0 failed; 0 ignored; finished in 106.38s
```
- **Hash function**: XxHash64 with seed 0 (matches Meilisearch Enterprise)
- **Rendezvous scoring**: `score(shard_id, node_id)` — deterministic ordering
- **Group isolation**: Hashing scoped to intra-group node lists
- **Tie-breaking**: Lexicographic by node_id for identical scores
- **Merger optimization**: Binary min-heap for large fan-out (avoid keeping all hits in RAM)
### Files Modified (this session)
No new files — all Phase 1 work was already implemented in previous sessions.
This bead verified completeness and confirmed all DoD requirements.
### Next Steps
Phase 2 will wire the scatter orchestration and integrate with actual HTTP clients.