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miroir/notes/miroir-cdo.1.md
jedarden 400caa5483 P1.1 Rendezvous hash primitives - verification complete 
Verify that the three core rendezvous hash primitives (score, assign_shard_in_group, shard_for_key) are correctly implemented in miroir_core::router.

All implementations match the specification:
- score: Uses XxHash64::with_seed(0) with canonical (shard_id, node_id) order
- assign_shard_in_group: Group-scoped assignment with score sort and lexicographic tie-breaking
- shard_for_key: Uses XxHash64::with_seed(0) to hash primary_key

All 26 acceptance tests pass:
- Determinism across 1000 runs
- Reshuffle bounds on add/remove
- Uniformity distribution (15-27 shards per node)
- RF=2 placement stability
- shard_for_key fixture verification

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
2026-05-13 17:59:46 -04:00

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P1.1 Rendezvous Hash Primitives - Verification Complete

Task Status

COMPLETE - All three required functions were already implemented in /home/coding/miroir/crates/miroir-core/src/router.rs

Implementations Verified

1. score(shard_id: u32, node_id: &str) -> u64

  • Uses XxHash64::with_seed(0) (line 15)
  • Canonical concatenation order: (shard_id, node_id) (lines 16-17)
  • Returns u64 hash value (line 18)

2. assign_shard_in_group(shard_id: u32, group_nodes: &[NodeId], rf: usize) -> Vec<NodeId>

  • Computes score for each node in the group (line 30)
  • Sorts by score descending (line 33)
  • Breaks ties lexicographically on node_id (line 34)
  • Returns top rf nodes (lines 37-40)

3. shard_for_key(primary_key: &str, shard_count: u32) -> u32

  • Uses XxHash64::with_seed(0) (line 73)
  • Hashes primary_key (line 74)
  • Returns hash % shard_count as u32 (line 75)

Acceptance Tests (All Passed ✓)

  1. Determinism - 1000 randomized runs all produce identical assignments
  2. Reshuffle bound on add - 64 shards, 3→4 nodes: ≤ 32 edges differ
  3. Reshuffle bound on remove - 64 shards, 4→3 nodes: ~32 edges differ
  4. Uniformity - 64 shards, 3 nodes, RF=1: each node holds 15-27 shards
  5. RF=2 placement stability - Top-2 nodes change minimally on add/remove
  6. shard_for_key fixture - Verified against known test vectors
  7. Tie-breaking - Lexicographic on node_id for identical scores
  8. Canonical order - (shard_id, node_id) produces different hash than (node_id, shard_id)

Test Results

running 26 tests
test result: ok. 26 passed; 0 failed; 0 ignored

Key Design Constraints Verified

  • Hash function: twox-hash (XxHash family) ✓
  • Seed: 0 (matches Meilisearch Enterprise) ✓
  • Concatenation order: (shard_id, node_id) ✓
  • Group-scoped assignment: prevents same-group replicas ✓
  • Tie-breaking: lexicographic on node_id ✓

Files Modified

None - implementations were already correct and complete.