Three pieces, all read-only or additive — no shard mutation, no
schema-version bump:
1. shard_for_document(document_root, M) in arborist/document.py.
Pure function: int(document_root[:8], 16) % M. 22 tests cover
determinism, range-bounds, near-uniform distribution (±5pp at
N=20k), and seven lock-in fixtures so peers will disagree
loudly if anyone changes the formula.
2. corpus_shard_count meta field + get/set helpers in store.py.
Lives in the existing key/value meta table; SCHEMA_VERSION
stays at v9.8.0 (the DDL doesn't change and source_root is
layout-independent, so cache records survive a reshard).
Legacy shards (without the field) return None; reshard tool
populates it on every target shard at migration time.
3. Pre-migration snapshot captured to
bench/results/pre-migration-snapshot.json:
docs 3,468,392 (3,468,226 globally unique)
chunks 6,235,764
edges 90,593,537
audit 3,468,403
This is the reference set post-reshard row counts must match.
4. Audit-event extraction script writes all 3.47M events from
all 4 shards to /tmp/audit-events.ndjson (2.0 GB) for the
Option-A canonical-chain consolidation step. Verifies chain
integrity on extract — all 4 source chains report 0 breaks.
5. Fixed a wrong chunk count in docs/corpus-history.md
(had ~3.54M/shard; actual is ~1.56M/shard) and added the
edge-count column (~22.6M/shard, 90.6M total). 6.24M chunks
total, not 14.12M.
Tests: 29 new pass (22 routing + 7 meta). No existing tests
touched.
