# P4.2 Node Addition: Dual-Write + Paginated Shard Migration - Verification

## Summary

Verified that the P4.2 node addition implementation is complete and fully functional. All acceptance criteria tests pass.

## Implementation Verified

### Core Components (Already Implemented)

1. **`crates/miroir-core/src/rebalancer.rs`**:
   - `add_node()` method creates node addition operation
   - `compute_shard_moves_for_new_node()` determines affected shards
   - `run_migration_task()` executes paginated migration with dual-write
   - `HttpMigrationExecutor` implements actual HTTP-based document migration
   - Delta pass support for catching stragglers

2. **`crates/miroir-core/src/migration.rs`**:
   - Complete migration state machine with all phases
   - `MigrationCoordinator` for state transitions
   - `begin_dual_write()` starts dual-write phase
   - `begin_cutover()`, `complete_drain()`, `complete_cleanup()` for cutover sequence
   - Safety validation (anti-entropy required if delta pass skipped)

3. **`crates/miroir-core/src/rebalancer_worker/mod.rs`**:
   - Background worker with leader lease
   - `on_node_added()` handles topology events
   - `drive_migrations()` advances migration state machine
   - Job persistence via TaskStore

4. **`crates/miroir-proxy/src/routes/admin_endpoints.rs`**:
   - `add_node` API endpoint
   - Integration with rebalancer worker via event channel

### Integration Tests (All Pass)

**`crates/miroir-core/tests/p42_node_addition.rs`**:
1. `p42_node_addition_3_to_4_migration_10k_docs` - 10K docs migration test
2. `p42_chaos_writes_during_migration_dual_write` - Dual-write chaos test
3. `p42_performance_document_count_bounds` - Performance bounds verification
4. `p42_log_inspection_old_node_not_queried_after_migration` - Log inspection test
5. `p42_verify_dual_write_during_migration` - Dual-write verification
6. `p42_pagination_limit_offset` - Pagination verification

**`crates/miroir-core/tests/cutover_race.rs`**:
- 14 chaos tests for cutover boundary conditions
- All pass with anti-entropy and delta pass

**`crates/miroir-core/tests/p4_topology_chaos.rs`**:
- 8 topology chaos tests
- All pass

## Acceptance Criteria Status

- [x] Integration test: 3-node → 4-node migration, 10K docs, each doc still retrievable by ID after migration
- [x] Chaos: toggle writes on/off during migration; dual-write window catches all late writes
- [x] Performance: migrating `~S/(Ng+1)` shards moves ≤ `total_docs / (Ng+1) × 1.1` docs (10% slack for dual-write dupes)
- [x] The old node is not queried for the migrated shards after step 8 (verified via log inspection)

## Test Results

```
running 6 tests (p42_node_addition)
test p42_chaos_writes_during_migration_dual_write ... ok
test p42_node_addition_3_to_4_migration_10k_docs ... ok
test p42_log_inspection_old_node_not_queried_after_migration ... ok
test p42_performance_document_count_bounds ... ok
test p42_verify_dual_write_during_migration ... ok
test p42_pagination_limit_offset ... ok
test result: ok. 6 passed

running 14 tests (cutover_race)
test result: ok. 14 passed

running 8 tests (p4_topology_chaos)
test result: ok. 8 passed
```

Total: 28 tests passed, 0 failed.