WFGY/ProblemMap/GlobalFixMap/RAG_VectorDB/update_and_index_skew.md

Update and Index Skew — Guardrails and Fix Pattern

🧭 Quick Return to Map

You are in a sub-page of RAG_VectorDB.
To reorient, go back here:

• RAG_VectorDB — vector databases for retrieval and grounding
• WFGY Global Fix Map — main Emergency Room, 300+ structured fixes
• WFGY Problem Map 1.0 — 16 reproducible failure modes

Think of this page as a desk within a ward.
If you need the full triage and all prescriptions, return to the Emergency Room lobby.

Use this page when recall flips or citations drift after a data or model update.
Skew appears when writers and readers see different corpus revisions, or when the index was rebuilt with changed params without a cutover plan.

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Open these first

• Visual map and recovery: RAG Architecture & Recovery
• Retrieval knobs: retrieval-playbook.md
• Vector store fragmentation: vectorstore_fragmentation.md
• Metric mismatch: metric_mismatch.md
• Normalization and scaling: normalization_and_scaling.md
• Tokenization and casing: tokenization_and_casing.md
• Chunking to embedding contract: chunking_to_embedding_contract.md

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Core acceptance

• Single INDEX_HASH is identical for writer, retriever, reranker, and LLM side prompts.
• ΔS(question, retrieved) ≤ 0.45 on 3 paraphrases and 2 seeds after the update.
• Coverage ≥ 0.70 to the target section, stable across shards and regions.
• λ remains convergent during the cutover window, no flip states at header reorder.

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Symptoms → likely cause → open this

• Results differ between two identical queries minutes apart
  → mixed corpus revisions or warm readers on stale index
  → retrieval-playbook.md

• Top-k looks similar in distance yet meaning is off after upgrade
  → metric or normalization changed during rebuild
  → metric_mismatch.md, normalization_and_scaling.md

• Some shards return old docs, others new ones
  → partial index redeploy or cache warmup skew
  → vectorstore_fragmentation.md

• After model switch, recall falls only on certain languages or code blocks
  → tokenizer or casing schema diverged
  → tokenization_and_casing.md

• Large jump in ΔS on long windows, citations no longer align
  → chunk schema changed but old vectors remain
  → chunking_to_embedding_contract.md

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Fix in 60 seconds

1. Pin the contract
   Compute INDEX_HASH = sha256(model_id + tokenizer_ver + chunk_schema + metric + dim + store_params + corpus_rev).
   Log it on writer, retriever, reranker, and in the LLM prompt header.

2. Shadow read
   Run a gold set against current index and a rebuilt index in parallel.
   Alert if ΔS variance > 0.05 or coverage drops below 0.70.

3. Freeze and rebuild
   Stop writes. Re-embed and rebuild offline with explicit dim, metric, and normalization.
   Verify tokenizer and casing are identical to the previous contract.

4. Cutover with warmup
   Warm the new index. Switch read traffic via percentage ramp.
   Abort if λ flips or ΔS exceeds 0.60 on any guardrail probe.

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Minimal checks you must script

• Contract echo
  Every query path must log INDEX_HASH, MODEL_ID, TOKENIZER_VER, CHUNK_SCHEMA_VER.

• Shard parity probe
  Run the same 25 queries to each shard or region.
  Flag if Jaccard(top-k) < 0.6 against the reference shard.

• Cache invalidation
  Clear reranker and query embedding caches when INDEX_HASH changes.

• Reader staleness
  Reject queries if reader_index_hash != router_index_hash. Fail fast, do not serve stale.

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Common gotchas

• Silent analyzer change in a search backend re-tokenizes text while vectors are unchanged.
• HNSW or IVF params differ between shards, causing order instability at k=10 but not at k=3.
• APM dashboards show healthy ingestion yet the retriever reads from a lagging replica.
• Reranker model upgraded without re-baselining acceptance targets.
• Partial re-embed of only new docs creates a semantic seam at time T.

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Verification

• Gold set of 100 questions, 3 paraphrases each.
• Require ΔS ≤ 0.45 and coverage ≥ 0.70 on both old and new indexes before cutover.
• After cutover, repeat on two seeds. If λ remains convergent and ΔS does not spike, close the change.

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