chrevanthreddy commented on code in PR #19309: URL: https://github.com/apache/hudi/pull/19309#discussion_r3896850783
########## rfc/rfc-109/rfc-109.md: ########## @@ -0,0 +1,594 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +# RFC-109: Native Vector Search Support in Apache Hudi + +## Proposers + +@chrevanthreddy + +## Approvers + +- TBD + +## Status + +Umbrella issue: [apache/hudi#19094](https://github.com/apache/hudi/issues/19094) + +Related: [apache/hudi#18676](https://github.com/apache/hudi/issues/18676) + +State: UNDER REVIEW + +--- + +## Table of Contents + +- [Abstract](#abstract) +- [1. Goals and Non-Goals](#1-goals-and-non-goals) +- [2. Architecture](#2-architecture) +- [3. IVF + RaBitQ Index Algorithm](#3-ivf--rabitq-index-algorithm) +- [4. Metadata Table Storage Model: the Posting Block](#4-metadata-table-storage-model-the-posting-block) +- [5. Bootstrap and Write Path](#5-bootstrap-and-write-path) +- [6. Read Path](#6-read-path) +- [7. Maintenance, Rebalancing, and Cleaner](#7-maintenance-rebalancing-and-cleaner) +- [8. Spark API Surface](#8-spark-api-surface) +- [9. Correctness and Consistency](#9-correctness-and-consistency) +- [10. Test Plan](#10-test-plan) +- [11. Rollout and MVP Scope](#11-rollout-and-mvp-scope) +- [12. References](#12-references) + +--- + +## Abstract + +This RFC proposes native approximate nearest-neighbor (ANN) vector search in Apache Hudi. +Tables increasingly carry embedding columns (`ARRAY<FLOAT>` produced by ML models) next to +their business data, and users want to ask *"find the K rows most similar to this query +vector"* — for semantic search, recommendations, RAG, and deduplication — without copying +data into a separate vector database. + +Today the only option on a Hudi table is a brute-force scan: read every vector, compute +every distance. That is correct but scales linearly with table size (tens of seconds at a +billion rows). This RFC adds an index so that vector queries read only a small, targeted +fraction of the index and the table, return results with high recall, and stay +transactionally consistent with the table under upserts and deletes — all with **no new +storage system**. The index lives in the Hudi Metadata Table (MDT), like Hudi's existing +record-level and secondary indexes, and is maintained by the same table services. + +The design combines three well-understood pieces — IVF clustering, RaBitQ quantization, and +exact re-ranking — with one storage innovation that makes them practical on an immutable, +columnar, object-store-resident lakehouse: + +> **The posting block.** Instead of one MDT record per indexed vector, the index packs +> ~1–4K vectors into a single MDT record laid out column-wise (structure-of-arrays), keyed +> so that one IVF cluster forms one contiguous, prefix-scannable key range. This reduces MDT +> record count by roughly three orders of magnitude, turns "scan a cluster" into a single +> contiguous range read, and lets a query touch only the columns a given scan pass needs. + +The base table remains the source of truth for exact vector values. The MDT stores only +routing, pruning, and approximate-scoring metadata; final ranking always reads exact vectors +from the base table. + +Prototype measurements on a **1-billion-row, 128-dimensional table** show exact-reranked +recall@10 = 0.985 at nprobe=128 with query latency in low single-digit seconds on a modest +Spark cluster, versus ~15s for brute force. + +--- + +## 1. Goals and Non-Goals + +### 1.1 Goals + +1. Keep authoritative vector values in the base table (`ARRAY<FLOAT>` / `VECTOR(D)` column). +2. Store the vector index in the MDT, maintained by Hudi metadata-table commits, compaction, + and cleaning — no hidden or generated columns in base-table files. +3. Make candidate discovery cheap and targeted: probe a few clusters, scan contiguous key + ranges, score on compressed codes with a provable pruning bound. +4. Make results trustworthy: approximate math selects candidates; **exact** distance on + base-table vectors ranks them. +5. Stay transactionally consistent: snapshot-pinned reads, correct behavior under inserts, + updates, deletes, and clustering. +6. Be engine-neutral in design; Spark is the first implementation. +7. Maintain the index incrementally (no global rebuild for normal churn) and support + versioned, zero-downtime rebuilds. + +### 1.2 Non-Goals (initial landing) + +- ANN families beyond IVF + RaBitQ (e.g. HNSW, DiskANN). +- Filtered search (arbitrary predicate + kNN) as a first-class planned operation. +- Time-travel-consistent index reads for historical snapshots. +- Engines beyond Spark, and GPU-accelerated encoding. +- Workload-specific auto-tuning of `nprobe` / refine factor. + +--- + +## 2. Architecture + + + +The design splits responsibilities the way Hudi already does between the data table and the +metadata table: + +```text +DATA TABLE (parquet/orc) METADATA TABLE (vector_index partition) + authoritative vectors + payload ←── the index: centroids, quantizer, posting blocks, + read only for final re-ranking cluster manifests, generation manifest + read for candidate generation +``` + +Each vector index is one MDT partition. Creating an index: + +```sql +CREATE INDEX embedding_idx +ON products +USING VECTOR (embedding) +OPTIONS ( + 'vector.dimension' = '768', + 'vector.metric' = 'cosine', + 'vector.quantizer' = 'IVF_RABITQ', + 'vector.num_clusters'= '4096' +); +``` + +creates: + +```text +.hoodie/metadata/vector_index_embedding_idx/ +``` + +and does not change the base-table schema: + +```text +products/category=electronics/<file-group>.parquet +├── _hoodie_record_key = p001 +├── _hoodie_partition_path = category=electronics +├── user columns = ... +└── embedding ARRAY<FLOAT> = [0.12, -0.08, ...] +``` + +The query path uses the MDT first to discover candidates, then reads base-table vectors for +exact re-ranking: + +```text +query vector + → compare to centroids, pick nprobe clusters (in-memory, ms) + → MDT prefix-scan those clusters' posting blocks (targeted range reads) + → two-pass RaBitQ scoring, keep refineFactor·K best (bit math + error bounds) + → validate candidate freshness via Record Level Index (batched point lookups) + → fetch ONLY those rows from the base table by position (page-level reads) + → exact distance on real vectors → final top-K +``` + +--- + +## 3. IVF + RaBitQ Index Algorithm + +Every practical ANN index answers two questions: **where to look** (avoid scanning +everything) and **how to compare cheaply** (avoid full-precision math on what is scanned). + +### 3.1 Where to look: IVF routing + +Inverted File (IVF) indexing clusters vectors with KMeans into `numClusters` groups (e.g. +~4K–64K). Each vector belongs to its nearest centroid. A query compares against the centroids +only (thousands, not billions), selects the `nprobe` nearest clusters, and scans only those +clusters' entries. `nprobe` is the recall dial. + +IVF is the right fit for a lakehouse-resident index because a cluster's entries can be stored Review Comment: Added §1.3, “Alternatives considered,” covering MDT versus dedicated/sidecar storage and the major routing/quantization alternatives with the reasons for this choice. ########## rfc/rfc-109/rfc-109.md: ########## @@ -0,0 +1,594 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +# RFC-109: Native Vector Search Support in Apache Hudi + +## Proposers + +@chrevanthreddy + +## Approvers + +- TBD + +## Status + +Umbrella issue: [apache/hudi#19094](https://github.com/apache/hudi/issues/19094) + +Related: [apache/hudi#18676](https://github.com/apache/hudi/issues/18676) + +State: UNDER REVIEW + +--- + +## Table of Contents + +- [Abstract](#abstract) +- [1. Goals and Non-Goals](#1-goals-and-non-goals) +- [2. Architecture](#2-architecture) +- [3. IVF + RaBitQ Index Algorithm](#3-ivf--rabitq-index-algorithm) +- [4. Metadata Table Storage Model: the Posting Block](#4-metadata-table-storage-model-the-posting-block) +- [5. Bootstrap and Write Path](#5-bootstrap-and-write-path) +- [6. Read Path](#6-read-path) +- [7. Maintenance, Rebalancing, and Cleaner](#7-maintenance-rebalancing-and-cleaner) +- [8. Spark API Surface](#8-spark-api-surface) +- [9. Correctness and Consistency](#9-correctness-and-consistency) +- [10. Test Plan](#10-test-plan) +- [11. Rollout and MVP Scope](#11-rollout-and-mvp-scope) +- [12. References](#12-references) + +--- + +## Abstract + +This RFC proposes native approximate nearest-neighbor (ANN) vector search in Apache Hudi. +Tables increasingly carry embedding columns (`ARRAY<FLOAT>` produced by ML models) next to +their business data, and users want to ask *"find the K rows most similar to this query +vector"* — for semantic search, recommendations, RAG, and deduplication — without copying +data into a separate vector database. + +Today the only option on a Hudi table is a brute-force scan: read every vector, compute +every distance. That is correct but scales linearly with table size (tens of seconds at a +billion rows). This RFC adds an index so that vector queries read only a small, targeted +fraction of the index and the table, return results with high recall, and stay +transactionally consistent with the table under upserts and deletes — all with **no new +storage system**. The index lives in the Hudi Metadata Table (MDT), like Hudi's existing +record-level and secondary indexes, and is maintained by the same table services. + +The design combines three well-understood pieces — IVF clustering, RaBitQ quantization, and +exact re-ranking — with one storage innovation that makes them practical on an immutable, +columnar, object-store-resident lakehouse: + +> **The posting block.** Instead of one MDT record per indexed vector, the index packs +> ~1–4K vectors into a single MDT record laid out column-wise (structure-of-arrays), keyed +> so that one IVF cluster forms one contiguous, prefix-scannable key range. This reduces MDT +> record count by roughly three orders of magnitude, turns "scan a cluster" into a single +> contiguous range read, and lets a query touch only the columns a given scan pass needs. + +The base table remains the source of truth for exact vector values. The MDT stores only +routing, pruning, and approximate-scoring metadata; final ranking always reads exact vectors +from the base table. + +Prototype measurements on a **1-billion-row, 128-dimensional table** show exact-reranked +recall@10 = 0.985 at nprobe=128 with query latency in low single-digit seconds on a modest +Spark cluster, versus ~15s for brute force. + +--- + +## 1. Goals and Non-Goals + +### 1.1 Goals + +1. Keep authoritative vector values in the base table (`ARRAY<FLOAT>` / `VECTOR(D)` column). +2. Store the vector index in the MDT, maintained by Hudi metadata-table commits, compaction, + and cleaning — no hidden or generated columns in base-table files. +3. Make candidate discovery cheap and targeted: probe a few clusters, scan contiguous key + ranges, score on compressed codes with a provable pruning bound. +4. Make results trustworthy: approximate math selects candidates; **exact** distance on + base-table vectors ranks them. +5. Stay transactionally consistent: snapshot-pinned reads, correct behavior under inserts, + updates, deletes, and clustering. +6. Be engine-neutral in design; Spark is the first implementation. +7. Maintain the index incrementally (no global rebuild for normal churn) and support + versioned, zero-downtime rebuilds. + +### 1.2 Non-Goals (initial landing) + +- ANN families beyond IVF + RaBitQ (e.g. HNSW, DiskANN). +- Filtered search (arbitrary predicate + kNN) as a first-class planned operation. +- Time-travel-consistent index reads for historical snapshots. +- Engines beyond Spark, and GPU-accelerated encoding. +- Workload-specific auto-tuning of `nprobe` / refine factor. + +--- + +## 2. Architecture + + + +The design splits responsibilities the way Hudi already does between the data table and the +metadata table: + +```text +DATA TABLE (parquet/orc) METADATA TABLE (vector_index partition) + authoritative vectors + payload ←── the index: centroids, quantizer, posting blocks, + read only for final re-ranking cluster manifests, generation manifest + read for candidate generation +``` + +Each vector index is one MDT partition. Creating an index: + +```sql +CREATE INDEX embedding_idx +ON products +USING VECTOR (embedding) +OPTIONS ( + 'vector.dimension' = '768', + 'vector.metric' = 'cosine', + 'vector.quantizer' = 'IVF_RABITQ', + 'vector.num_clusters'= '4096' +); +``` + +creates: + +```text +.hoodie/metadata/vector_index_embedding_idx/ +``` + +and does not change the base-table schema: + +```text +products/category=electronics/<file-group>.parquet +├── _hoodie_record_key = p001 +├── _hoodie_partition_path = category=electronics +├── user columns = ... +└── embedding ARRAY<FLOAT> = [0.12, -0.08, ...] +``` + +The query path uses the MDT first to discover candidates, then reads base-table vectors for +exact re-ranking: + +```text +query vector + → compare to centroids, pick nprobe clusters (in-memory, ms) + → MDT prefix-scan those clusters' posting blocks (targeted range reads) + → two-pass RaBitQ scoring, keep refineFactor·K best (bit math + error bounds) + → validate candidate freshness via Record Level Index (batched point lookups) + → fetch ONLY those rows from the base table by position (page-level reads) + → exact distance on real vectors → final top-K +``` + +--- + +## 3. IVF + RaBitQ Index Algorithm + +Every practical ANN index answers two questions: **where to look** (avoid scanning +everything) and **how to compare cheaply** (avoid full-precision math on what is scanned). + +### 3.1 Where to look: IVF routing + +Inverted File (IVF) indexing clusters vectors with KMeans into `numClusters` groups (e.g. +~4K–64K). Each vector belongs to its nearest centroid. A query compares against the centroids +only (thousands, not billions), selects the `nprobe` nearest clusters, and scans only those +clusters' entries. `nprobe` is the recall dial. + +IVF is the right fit for a lakehouse-resident index because a cluster's entries can be stored +**contiguously**, which maps directly onto sorted key ranges in Hudi's MDT (§4). Graph +indexes (HNSW) give excellent in-memory recall but require random traversal of the whole +graph, which fights columnar, immutable, object-store storage. + +### 3.2 How to compare cheaply: RaBitQ quantization + +Inside a probed cluster there are still thousands of full vectors. Quantization stores a +small *code* per vector plus a few correction scalars, so most comparisons run on compressed +codes and only the best few hundred candidates are re-checked against real vectors. + +RaBitQ is chosen over scalar (SQ), product (PQ), and plain binary quantization for four +reasons: + +1. **Unbiased estimator with a provable per-vector error bound.** Each code carries scalars + that turn a cheap bit-level dot product into an *unbiased* estimate of the true distance, + plus a bound on how wrong it can be. The bound enables **safe pruning**: skip a vector + only when even its best plausible distance cannot make top-K. +2. **No codebooks.** RaBitQ needs only a random rotation (a seed) and the centroids — both + tiny, both versioned in the index metadata. Nothing to retrain when data drifts. +3. **Tunable precision.** B = 1 bit/dim is a fast coarse filter; B = 4 bits/dim gives + near-SQ quality at ~8× less space. Both are used in a two-pass scan (§6.2). +4. **Metric-flexible.** One stored code serves L2, cosine, and dot-product; the metric is + applied at query time. + +The four ideas, precisely: + +- **Residual.** Store each vector as its difference from its centroid, `r = v − c`. + Residuals are small and centered, which is what lets few bits go far. +- **Rotation.** Apply one fixed random orthonormal rotation `R` (derived from a per-generation + seed) to everything first: `x = R·r`. This spreads information evenly across dimensions, + which is what makes the error bound hold for any data distribution. Only the seed is stored. +- **Code.** Quantize `x` to B bits per dimension, stored as **bit planes** — plane 0 holds + bit 0 of every dimension, plane 1 bit 1, etc. The top plane alone is a 1-bit sign sketch. + Scoring a plane against the (transformed) query is `AND`/`XOR` + `popcount` — a few CPU + instructions per 64 dimensions. +- **Factors.** A handful of small scalars per vector (residual norm, two rescale factors, an + error term, a centroid correction) that convert plane math into an unbiased distance + estimate plus its confidence interval. + +At query time the query vector is transformed the same way (`R·(q − c)` per probed cluster), +and the per-plane popcounts combine with the stored factors into the estimate and its bound. +The estimate builds the shortlist; exact base-table distances produce the final ranking (§6). + +### 3.3 Why this fits Hudi + +- Centroids are small enough to load at planning time: `K × D` floats (~12 MB for K=4096, + D=768). +- Codes are compact: a 1B × 128-dim float table's raw vectors are ~512 GB; the RaBitQ index + including keys and locators is ~136 GB, and the scanned portion per query is tens of MB. +- Posting keys are prefix-scannable by generation, cluster, and shard (§4). +- Quantizer state is stable: a seed and centroids, no per-generation learned codebook. +- Exact re-ranking preserves correctness for returned candidates. + +--- + +## 4. Metadata Table Storage Model: the Posting Block + +This section describes the core storage contribution of this RFC. + +### 4.1 The posting block + +A naive index would write one MDT record per indexed vector. At a billion rows that is a +billion MDT records per generation — prohibitive to write, compact, scan, and clean. + +Instead, the index sorts entries by `(cluster, fileGroup, rowPosition)` and packs +~1–4K vectors' worth of codes and metadata into a single MDT record — a **posting block** — +laid out column-wise (structure-of-arrays) so each scan pass touches only the columns it +needs: + +```text +POSTING BLOCK (~512 KB target, one MDT record) +┌───────────────────────────────────────────────┐ +│ S1 sign planes ← pass 1 touches this │ +│ S2 extra bit planes ← pass 2, survivors │ +│ S3 factor arrays ← both passes │ +│ S4 row locators ← finalists only │ +│ S5 dictionaries (file groups, partitions) │ +│ S6 record keys ← finalists only │ +└───────────────────────────────────────────────┘ +key: 0x10 | generation | clusterId | shardId | blockId +``` + +Consequences: + +- **~1000× fewer MDT records.** One block record replaces ~1–4K per-vector records, cutting + write amplification, compaction cost, and cleaner load by three orders of magnitude. +- **Contiguous cluster scans.** The binary key scheme makes "scan cluster 12345" a single + contiguous HFile range read rather than thousands of point lookups. +- **Pay only for promise.** Column-wise layout means pass 1 reads only sign planes + factors; + only survivors touch extra planes; only finalists touch locators and keys (§6.2). + +### 4.2 Row families + +The `vector_index_<name>` partition holds several record families under one binary-sorted key +scheme, so one prefix scan of a cluster returns its blocks and any fresh deltas together: + +| Key family | Cardinality | Purpose | +|---|---:|---| +| `__manifest__` | 1 | Active generation pointer used by readers. | +| `__centroids__` | 1 | Serialized `K × D` centroid matrix. | +| `__quantizer__` | 1 | RaBitQ type, bit width `B`, rotation seed, normalization flag. | +| `M\|<generation>` | generations | Immutable generation-level metadata. | +| `C\|<generation>\|<cluster>` | K per generation | Cluster manifest: shard count, vector count, candidate file groups, delta/tombstone counters. | +| `P\|<generation>\|<cluster>\|<shard>\|<blockId>` | blocks per generation | **Posting block** (packed codes, factors, locators, keys). | +| `P\|...\|<DELTA>` | deltas | Small per-record delta records appended to a cluster range between compactions. | + +### 4.3 Posting shards + +Large clusters are split into posting shards so one hot cluster does not become one oversized +prefix range. The cluster manifest stores `shardCount`; writers compute +`shardId = hash(record_key) % shardCount`. Within a shard, entries are packed into blocks by +`blockId`. Sharding changes physical layout only, never vector semantics. + +### 4.4 Delta records + +Between compactions, per-commit vector writes append small **delta records** (`blockId` +marked `DELTA`) at the end of the same cluster key range. Because they share the prefix, a +single cluster prefix scan sees packed blocks and fresh deltas in one pass (§6, §7). + +### 4.5 Generation model + +A generation is a consistent set of centroid, quantizer, cluster, and posting-block metadata: + +```text +__manifest__ -> active generation id +M|<gen> -> immutable generation metadata +__centroids__ -> centroid matrix for the active generation +__quantizer__ -> RaBitQ seed / width / normalization +C|<gen>|... -> cluster manifests +P|<gen>|... -> posting blocks + deltas +``` + +A query uses one generation consistently. Publishing a generation is atomic from a reader's +perspective: write the generation's rows, commit them to MDT, then flip `__manifest__`. Old +generations are retained until no retained table snapshot needs them. This enables +blue/green rebuilds with clean rollback. + +--- + +## 5. Bootstrap and Write Path + + + +### 5.1 Spark bootstrap + +Bootstrap builds a complete generation from a table snapshot in two distributed phases: Review Comment: Specified deterministic bootstrap keys, retry/rollback behavior, partial-generation invisibility, and pre-activation chunk/posting validation in §4.5 and §5.1; §10 now calls out bootstrap recovery tests. ########## rfc/rfc-109/rfc-109.md: ########## @@ -0,0 +1,594 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +# RFC-109: Native Vector Search Support in Apache Hudi + +## Proposers + +@chrevanthreddy + +## Approvers + +- TBD + +## Status + +Umbrella issue: [apache/hudi#19094](https://github.com/apache/hudi/issues/19094) + +Related: [apache/hudi#18676](https://github.com/apache/hudi/issues/18676) + +State: UNDER REVIEW + +--- + +## Table of Contents + +- [Abstract](#abstract) +- [1. Goals and Non-Goals](#1-goals-and-non-goals) +- [2. Architecture](#2-architecture) +- [3. IVF + RaBitQ Index Algorithm](#3-ivf--rabitq-index-algorithm) +- [4. Metadata Table Storage Model: the Posting Block](#4-metadata-table-storage-model-the-posting-block) +- [5. Bootstrap and Write Path](#5-bootstrap-and-write-path) +- [6. Read Path](#6-read-path) +- [7. Maintenance, Rebalancing, and Cleaner](#7-maintenance-rebalancing-and-cleaner) +- [8. Spark API Surface](#8-spark-api-surface) +- [9. Correctness and Consistency](#9-correctness-and-consistency) +- [10. Test Plan](#10-test-plan) +- [11. Rollout and MVP Scope](#11-rollout-and-mvp-scope) +- [12. References](#12-references) + +--- + +## Abstract + +This RFC proposes native approximate nearest-neighbor (ANN) vector search in Apache Hudi. +Tables increasingly carry embedding columns (`ARRAY<FLOAT>` produced by ML models) next to +their business data, and users want to ask *"find the K rows most similar to this query +vector"* — for semantic search, recommendations, RAG, and deduplication — without copying +data into a separate vector database. + +Today the only option on a Hudi table is a brute-force scan: read every vector, compute +every distance. That is correct but scales linearly with table size (tens of seconds at a +billion rows). This RFC adds an index so that vector queries read only a small, targeted +fraction of the index and the table, return results with high recall, and stay +transactionally consistent with the table under upserts and deletes — all with **no new +storage system**. The index lives in the Hudi Metadata Table (MDT), like Hudi's existing +record-level and secondary indexes, and is maintained by the same table services. + +The design combines three well-understood pieces — IVF clustering, RaBitQ quantization, and +exact re-ranking — with one storage innovation that makes them practical on an immutable, +columnar, object-store-resident lakehouse: + +> **The posting block.** Instead of one MDT record per indexed vector, the index packs +> ~1–4K vectors into a single MDT record laid out column-wise (structure-of-arrays), keyed +> so that one IVF cluster forms one contiguous, prefix-scannable key range. This reduces MDT +> record count by roughly three orders of magnitude, turns "scan a cluster" into a single +> contiguous range read, and lets a query touch only the columns a given scan pass needs. + +The base table remains the source of truth for exact vector values. The MDT stores only +routing, pruning, and approximate-scoring metadata; final ranking always reads exact vectors +from the base table. + +Prototype measurements on a **1-billion-row, 128-dimensional table** show exact-reranked +recall@10 = 0.985 at nprobe=128 with query latency in low single-digit seconds on a modest +Spark cluster, versus ~15s for brute force. + +--- + +## 1. Goals and Non-Goals + +### 1.1 Goals + +1. Keep authoritative vector values in the base table (`ARRAY<FLOAT>` / `VECTOR(D)` column). +2. Store the vector index in the MDT, maintained by Hudi metadata-table commits, compaction, + and cleaning — no hidden or generated columns in base-table files. +3. Make candidate discovery cheap and targeted: probe a few clusters, scan contiguous key + ranges, score on compressed codes with a provable pruning bound. +4. Make results trustworthy: approximate math selects candidates; **exact** distance on + base-table vectors ranks them. +5. Stay transactionally consistent: snapshot-pinned reads, correct behavior under inserts, + updates, deletes, and clustering. +6. Be engine-neutral in design; Spark is the first implementation. Review Comment: Added §5.4 to separate the engine-neutral index contract from engine adapters: Spark is the first implementation; Flink/Java must provide equivalent committed-record hooks before claiming write support. ########## rfc/rfc-109/rfc-109.md: ########## @@ -0,0 +1,594 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +# RFC-109: Native Vector Search Support in Apache Hudi + +## Proposers + +@chrevanthreddy + +## Approvers + +- TBD + +## Status + +Umbrella issue: [apache/hudi#19094](https://github.com/apache/hudi/issues/19094) + +Related: [apache/hudi#18676](https://github.com/apache/hudi/issues/18676) + +State: UNDER REVIEW + +--- + +## Table of Contents + +- [Abstract](#abstract) +- [1. Goals and Non-Goals](#1-goals-and-non-goals) +- [2. Architecture](#2-architecture) +- [3. IVF + RaBitQ Index Algorithm](#3-ivf--rabitq-index-algorithm) +- [4. Metadata Table Storage Model: the Posting Block](#4-metadata-table-storage-model-the-posting-block) +- [5. Bootstrap and Write Path](#5-bootstrap-and-write-path) +- [6. Read Path](#6-read-path) +- [7. Maintenance, Rebalancing, and Cleaner](#7-maintenance-rebalancing-and-cleaner) +- [8. Spark API Surface](#8-spark-api-surface) +- [9. Correctness and Consistency](#9-correctness-and-consistency) +- [10. Test Plan](#10-test-plan) +- [11. Rollout and MVP Scope](#11-rollout-and-mvp-scope) +- [12. References](#12-references) + +--- + +## Abstract + +This RFC proposes native approximate nearest-neighbor (ANN) vector search in Apache Hudi. +Tables increasingly carry embedding columns (`ARRAY<FLOAT>` produced by ML models) next to +their business data, and users want to ask *"find the K rows most similar to this query +vector"* — for semantic search, recommendations, RAG, and deduplication — without copying +data into a separate vector database. + +Today the only option on a Hudi table is a brute-force scan: read every vector, compute +every distance. That is correct but scales linearly with table size (tens of seconds at a +billion rows). This RFC adds an index so that vector queries read only a small, targeted +fraction of the index and the table, return results with high recall, and stay +transactionally consistent with the table under upserts and deletes — all with **no new +storage system**. The index lives in the Hudi Metadata Table (MDT), like Hudi's existing +record-level and secondary indexes, and is maintained by the same table services. + +The design combines three well-understood pieces — IVF clustering, RaBitQ quantization, and +exact re-ranking — with one storage innovation that makes them practical on an immutable, +columnar, object-store-resident lakehouse: + +> **The posting block.** Instead of one MDT record per indexed vector, the index packs +> ~1–4K vectors into a single MDT record laid out column-wise (structure-of-arrays), keyed +> so that one IVF cluster forms one contiguous, prefix-scannable key range. This reduces MDT +> record count by roughly three orders of magnitude, turns "scan a cluster" into a single +> contiguous range read, and lets a query touch only the columns a given scan pass needs. + +The base table remains the source of truth for exact vector values. The MDT stores only +routing, pruning, and approximate-scoring metadata; final ranking always reads exact vectors +from the base table. + +Prototype measurements on a **1-billion-row, 128-dimensional table** show exact-reranked +recall@10 = 0.985 at nprobe=128 with query latency in low single-digit seconds on a modest +Spark cluster, versus ~15s for brute force. + +--- + +## 1. Goals and Non-Goals + +### 1.1 Goals + +1. Keep authoritative vector values in the base table (`ARRAY<FLOAT>` / `VECTOR(D)` column). +2. Store the vector index in the MDT, maintained by Hudi metadata-table commits, compaction, + and cleaning — no hidden or generated columns in base-table files. +3. Make candidate discovery cheap and targeted: probe a few clusters, scan contiguous key + ranges, score on compressed codes with a provable pruning bound. +4. Make results trustworthy: approximate math selects candidates; **exact** distance on + base-table vectors ranks them. +5. Stay transactionally consistent: snapshot-pinned reads, correct behavior under inserts, + updates, deletes, and clustering. +6. Be engine-neutral in design; Spark is the first implementation. +7. Maintain the index incrementally (no global rebuild for normal churn) and support + versioned, zero-downtime rebuilds. + +### 1.2 Non-Goals (initial landing) + +- ANN families beyond IVF + RaBitQ (e.g. HNSW, DiskANN). +- Filtered search (arbitrary predicate + kNN) as a first-class planned operation. +- Time-travel-consistent index reads for historical snapshots. +- Engines beyond Spark, and GPU-accelerated encoding. +- Workload-specific auto-tuning of `nprobe` / refine factor. + +--- + +## 2. Architecture + + + +The design splits responsibilities the way Hudi already does between the data table and the +metadata table: + +```text +DATA TABLE (parquet/orc) METADATA TABLE (vector_index partition) + authoritative vectors + payload ←── the index: centroids, quantizer, posting blocks, + read only for final re-ranking cluster manifests, generation manifest + read for candidate generation +``` + +Each vector index is one MDT partition. Creating an index: + +```sql +CREATE INDEX embedding_idx +ON products +USING VECTOR (embedding) +OPTIONS ( + 'vector.dimension' = '768', + 'vector.metric' = 'cosine', + 'vector.quantizer' = 'IVF_RABITQ', + 'vector.num_clusters'= '4096' +); +``` + +creates: + +```text +.hoodie/metadata/vector_index_embedding_idx/ +``` + +and does not change the base-table schema: + +```text +products/category=electronics/<file-group>.parquet +├── _hoodie_record_key = p001 +├── _hoodie_partition_path = category=electronics +├── user columns = ... +└── embedding ARRAY<FLOAT> = [0.12, -0.08, ...] +``` + +The query path uses the MDT first to discover candidates, then reads base-table vectors for +exact re-ranking: + +```text +query vector + → compare to centroids, pick nprobe clusters (in-memory, ms) + → MDT prefix-scan those clusters' posting blocks (targeted range reads) + → two-pass RaBitQ scoring, keep refineFactor·K best (bit math + error bounds) + → validate candidate freshness via Record Level Index (batched point lookups) + → fetch ONLY those rows from the base table by position (page-level reads) + → exact distance on real vectors → final top-K +``` + +--- + +## 3. IVF + RaBitQ Index Algorithm + +Every practical ANN index answers two questions: **where to look** (avoid scanning +everything) and **how to compare cheaply** (avoid full-precision math on what is scanned). + +### 3.1 Where to look: IVF routing + +Inverted File (IVF) indexing clusters vectors with KMeans into `numClusters` groups (e.g. +~4K–64K). Each vector belongs to its nearest centroid. A query compares against the centroids +only (thousands, not billions), selects the `nprobe` nearest clusters, and scans only those +clusters' entries. `nprobe` is the recall dial. + +IVF is the right fit for a lakehouse-resident index because a cluster's entries can be stored +**contiguously**, which maps directly onto sorted key ranges in Hudi's MDT (§4). Graph +indexes (HNSW) give excellent in-memory recall but require random traversal of the whole +graph, which fights columnar, immutable, object-store storage. + +### 3.2 How to compare cheaply: RaBitQ quantization + +Inside a probed cluster there are still thousands of full vectors. Quantization stores a +small *code* per vector plus a few correction scalars, so most comparisons run on compressed +codes and only the best few hundred candidates are re-checked against real vectors. + +RaBitQ is chosen over scalar (SQ), product (PQ), and plain binary quantization for four +reasons: + +1. **Unbiased estimator with a provable per-vector error bound.** Each code carries scalars + that turn a cheap bit-level dot product into an *unbiased* estimate of the true distance, + plus a bound on how wrong it can be. The bound enables **safe pruning**: skip a vector + only when even its best plausible distance cannot make top-K. +2. **No codebooks.** RaBitQ needs only a random rotation (a seed) and the centroids — both + tiny, both versioned in the index metadata. Nothing to retrain when data drifts. +3. **Tunable precision.** B = 1 bit/dim is a fast coarse filter; B = 4 bits/dim gives + near-SQ quality at ~8× less space. Both are used in a two-pass scan (§6.2). +4. **Metric-flexible.** One stored code serves L2, cosine, and dot-product; the metric is + applied at query time. + +The four ideas, precisely: + +- **Residual.** Store each vector as its difference from its centroid, `r = v − c`. + Residuals are small and centered, which is what lets few bits go far. +- **Rotation.** Apply one fixed random orthonormal rotation `R` (derived from a per-generation + seed) to everything first: `x = R·r`. This spreads information evenly across dimensions, + which is what makes the error bound hold for any data distribution. Only the seed is stored. +- **Code.** Quantize `x` to B bits per dimension, stored as **bit planes** — plane 0 holds + bit 0 of every dimension, plane 1 bit 1, etc. The top plane alone is a 1-bit sign sketch. + Scoring a plane against the (transformed) query is `AND`/`XOR` + `popcount` — a few CPU + instructions per 64 dimensions. +- **Factors.** A handful of small scalars per vector (residual norm, two rescale factors, an + error term, a centroid correction) that convert plane math into an unbiased distance + estimate plus its confidence interval. + +At query time the query vector is transformed the same way (`R·(q − c)` per probed cluster), +and the per-plane popcounts combine with the stored factors into the estimate and its bound. +The estimate builds the shortlist; exact base-table distances produce the final ranking (§6). + +### 3.3 Why this fits Hudi + +- Centroids are small enough to load at planning time: `K × D` floats (~12 MB for K=4096, + D=768). +- Codes are compact: a 1B × 128-dim float table's raw vectors are ~512 GB; the RaBitQ index + including keys and locators is ~136 GB, and the scanned portion per query is tens of MB. +- Posting keys are prefix-scannable by generation, cluster, and shard (§4). +- Quantizer state is stable: a seed and centroids, no per-generation learned codebook. +- Exact re-ranking preserves correctness for returned candidates. + +--- + +## 4. Metadata Table Storage Model: the Posting Block + +This section describes the core storage contribution of this RFC. + +### 4.1 The posting block + +A naive index would write one MDT record per indexed vector. At a billion rows that is a +billion MDT records per generation — prohibitive to write, compact, scan, and clean. + +Instead, the index sorts entries by `(cluster, fileGroup, rowPosition)` and packs +~1–4K vectors' worth of codes and metadata into a single MDT record — a **posting block** — +laid out column-wise (structure-of-arrays) so each scan pass touches only the columns it +needs: + +```text +POSTING BLOCK (~512 KB target, one MDT record) +┌───────────────────────────────────────────────┐ +│ S1 sign planes ← pass 1 touches this │ +│ S2 extra bit planes ← pass 2, survivors │ +│ S3 factor arrays ← both passes │ +│ S4 row locators ← finalists only │ +│ S5 dictionaries (file groups, partitions) │ +│ S6 record keys ← finalists only │ +└───────────────────────────────────────────────┘ +key: 0x10 | generation | clusterId | shardId | blockId +``` + +Consequences: + +- **~1000× fewer MDT records.** One block record replaces ~1–4K per-vector records, cutting + write amplification, compaction cost, and cleaner load by three orders of magnitude. +- **Contiguous cluster scans.** The binary key scheme makes "scan cluster 12345" a single + contiguous HFile range read rather than thousands of point lookups. +- **Pay only for promise.** Column-wise layout means pass 1 reads only sign planes + factors; + only survivors touch extra planes; only finalists touch locators and keys (§6.2). + +### 4.2 Row families + +The `vector_index_<name>` partition holds several record families under one binary-sorted key +scheme, so one prefix scan of a cluster returns its blocks and any fresh deltas together: + +| Key family | Cardinality | Purpose | +|---|---:|---| +| `__manifest__` | 1 | Active generation pointer used by readers. | +| `__centroids__` | 1 | Serialized `K × D` centroid matrix. | +| `__quantizer__` | 1 | RaBitQ type, bit width `B`, rotation seed, normalization flag. | +| `M\|<generation>` | generations | Immutable generation-level metadata. | +| `C\|<generation>\|<cluster>` | K per generation | Cluster manifest: shard count, vector count, candidate file groups, delta/tombstone counters. | +| `P\|<generation>\|<cluster>\|<shard>\|<blockId>` | blocks per generation | **Posting block** (packed codes, factors, locators, keys). | +| `P\|...\|<DELTA>` | deltas | Small per-record delta records appended to a cluster range between compactions. | + +### 4.3 Posting shards + +Large clusters are split into posting shards so one hot cluster does not become one oversized +prefix range. The cluster manifest stores `shardCount`; writers compute +`shardId = hash(record_key) % shardCount`. Within a shard, entries are packed into blocks by +`blockId`. Sharding changes physical layout only, never vector semantics. + +### 4.4 Delta records + +Between compactions, per-commit vector writes append small **delta records** (`blockId` +marked `DELTA`) at the end of the same cluster key range. Because they share the prefix, a +single cluster prefix scan sees packed blocks and fresh deltas in one pass (§6, §7). + +### 4.5 Generation model + +A generation is a consistent set of centroid, quantizer, cluster, and posting-block metadata: + +```text +__manifest__ -> active generation id +M|<gen> -> immutable generation metadata +__centroids__ -> centroid matrix for the active generation +__quantizer__ -> RaBitQ seed / width / normalization +C|<gen>|... -> cluster manifests +P|<gen>|... -> posting blocks + deltas +``` + +A query uses one generation consistently. Publishing a generation is atomic from a reader's +perspective: write the generation's rows, commit them to MDT, then flip `__manifest__`. Old +generations are retained until no retained table snapshot needs them. This enables +blue/green rebuilds with clean rollback. + +--- + +## 5. Bootstrap and Write Path + + + +### 5.1 Spark bootstrap + +Bootstrap builds a complete generation from a table snapshot in two distributed phases: + +```text +1. Read latest base-table file slices; extract record key, partition path, file group, + base instant, and vector bytes. +2. Train IVF centroids with Spark ML KMeans over a bounded sample. +3. Broadcast centroids; in mapPartitions, assign each vector to its nearest cluster, + compute shard id, and encode the RaBitQ code + factors. +4. Sort entries by (cluster, fileGroup, rowPosition) and pack them into posting blocks. +5. Emit __centroids__, __quantizer__, __manifest__, M|, C| (with shard/vector counts and + candidate file groups), and P| posting-block records. +``` + +The training sample satisfies percentage and per-cluster floors, e.g.: + +```text +targetSample = min(N, max(1M, 256 * K, min(10M, 0.5%–1% of N))) +``` + +### 5.2 Incremental inserts and vector updates + +For an inserted row or a vector-changing update, the metadata writer's vector hook computes +the nearest centroid, cluster id, shard id, RaBitQ code, optional scalar, and base-table +location, and appends a posting **delta** to the active generation's cluster range. If an +update moves a record to a different cluster/shard, the stale entry is made invisible through +delta supersession and query-time arbitration (§6.3, §9.3). + +### 5.3 Non-vector updates and deletes + +If an update does not change the vector, the code and cluster placement stay valid; if the +record moved to a different base-table file group, its locator is refreshed at the next +compaction (before old file slices are cleaned). Deletes require **no index write**: the +Record Level Index already knows a key is gone, and query-time arbitration drops +stale/deleted candidates. + +--- + +## 6. Read Path + + + +### 6.1 Query planning + +```text +1. Validate the query dimension against the indexed vector schema. +2. Load the active generation: __manifest__, __centroids__, __quantizer__, and cluster + manifests (cacheable on the driver until the generation changes). +3. Probe centroids and select the top-nprobe clusters. +4. From cluster manifests, resolve shard prefixes and candidate file groups. +``` + +### 6.2 Two-pass scan over posting blocks + +For each selected `(cluster, shard)`, prefix-scan `P|<gen>|<cluster>|<shard>|*` (blocks + +deltas) and score column-wise: + +```text +Pass 1 (bound + prune): read S1 sign planes + S3 factors. For every vector compute an + optimistic bound — the best distance it could possibly achieve. Skip vectors whose + best case cannot beat the current K-th candidate (typically 85–95% pruned). +Pass 2 (refine): for survivors, read S2 extra bit planes and compute the full + multibit unbiased estimate; keep the refineFactor·K best. +Finalize: only the surviving finalists touch S4 locators and S6 keys to learn + where they live and who they are. +``` + +Cost is proportional to promise, not to table size. Delta records encountered in the same +scan are scored identically and supersede matching packed entries. + +### 6.3 Freshness arbitration and exact re-rank + +Posting locators are treated as *hints*. The finalists' record keys are validated against the +Record Level Index in batched point lookups on the same pinned snapshot: + +- moved file group → re-resolve location by key; +- missing → deleted → drop the candidate. + +Surviving finalists' exact vectors are then read from the base table using row-position / +page-level positional reads (the ~500 candidate rows cost megabytes, not a full slice scan), +exact distances are computed, and the final top-K is returned. + +### 6.4 Batched queries + +For a relation of query vectors, the plan shares MDT work: load generation metadata once, +encode all queries, probe per query, group by selected `(cluster, shard)`, scan each range +once while maintaining a per-query top-R heap, then read the union of candidate rows and +exact re-rank per query. + +### 6.5 Fallback + +If required vector metadata is missing, incompatible with the query dimension, or unavailable +for the requested snapshot, Hudi must not return incorrect ANN results — it falls back to an +exact table scan or fails per the configured failure mode. + +--- + +## 7. Maintenance, Rebalancing, and Cleaner + +Vectors change. Index health is kept without global rebuilds through three tiers of +increasing rarity (LIRE — Local Incremental Rebalancing). + +### 7.1 Per commit (continuous) + +The vector hook appends posting deltas for written rows and increments per-cluster counters +(live / delta / tombstone) additively. Deletes need no index write. + +### 7.2 Compaction (periodic, counter-triggered) + +When a cluster's delta count crosses a threshold, MDT compaction rewrites that cluster's +range: fold deltas into packed blocks, drop dead entries, refresh locators for rewritten file +groups, and reset counters. This bounds read cost so deltas never accumulate past a few +percent of a cluster. + +### 7.3 Rebalancing (rare, statistics-triggered) + +Sustained skew triggers a **local** split/merge: fetch the affected cluster's raw vectors +(the same positional reader the query path uses), run a local 2-means, re-encode into two +clusters, and reassign a bounded set of boundary vectors from neighbors — committed under a +new centroid epoch so in-flight queries stay consistent. Each operation touches one cluster Review Comment: Reconciled this in §7.6: LIRE does not mutate an epoch inside the active generation. Split/merge builds and atomically publishes a new generation; the first implementation uses full generation rebuilds. ########## rfc/rfc-109/rfc-109.md: ########## @@ -0,0 +1,594 @@ +<!-- + Licensed to the Apache Software Foundation (ASF) under one or more + contributor license agreements. See the NOTICE file distributed with + this work for additional information regarding copyright ownership. + The ASF licenses this file to You under the Apache License, Version 2.0 + (the "License"); you may not use this file except in compliance with + the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. +--> + +# RFC-109: Native Vector Search Support in Apache Hudi + +## Proposers + +@chrevanthreddy + +## Approvers + +- TBD + +## Status + +Umbrella issue: [apache/hudi#19094](https://github.com/apache/hudi/issues/19094) + +Related: [apache/hudi#18676](https://github.com/apache/hudi/issues/18676) + +State: UNDER REVIEW + +--- + +## Table of Contents + +- [Abstract](#abstract) +- [1. Goals and Non-Goals](#1-goals-and-non-goals) +- [2. Architecture](#2-architecture) +- [3. IVF + RaBitQ Index Algorithm](#3-ivf--rabitq-index-algorithm) +- [4. Metadata Table Storage Model: the Posting Block](#4-metadata-table-storage-model-the-posting-block) +- [5. Bootstrap and Write Path](#5-bootstrap-and-write-path) +- [6. Read Path](#6-read-path) +- [7. Maintenance, Rebalancing, and Cleaner](#7-maintenance-rebalancing-and-cleaner) +- [8. Spark API Surface](#8-spark-api-surface) +- [9. Correctness and Consistency](#9-correctness-and-consistency) +- [10. Test Plan](#10-test-plan) +- [11. Rollout and MVP Scope](#11-rollout-and-mvp-scope) +- [12. References](#12-references) + +--- + +## Abstract + +This RFC proposes native approximate nearest-neighbor (ANN) vector search in Apache Hudi. +Tables increasingly carry embedding columns (`ARRAY<FLOAT>` produced by ML models) next to +their business data, and users want to ask *"find the K rows most similar to this query +vector"* — for semantic search, recommendations, RAG, and deduplication — without copying +data into a separate vector database. + +Today the only option on a Hudi table is a brute-force scan: read every vector, compute +every distance. That is correct but scales linearly with table size (tens of seconds at a +billion rows). This RFC adds an index so that vector queries read only a small, targeted +fraction of the index and the table, return results with high recall, and stay +transactionally consistent with the table under upserts and deletes — all with **no new +storage system**. The index lives in the Hudi Metadata Table (MDT), like Hudi's existing +record-level and secondary indexes, and is maintained by the same table services. + +The design combines three well-understood pieces — IVF clustering, RaBitQ quantization, and +exact re-ranking — with one storage innovation that makes them practical on an immutable, +columnar, object-store-resident lakehouse: + +> **The posting block.** Instead of one MDT record per indexed vector, the index packs +> ~1–4K vectors into a single MDT record laid out column-wise (structure-of-arrays), keyed +> so that one IVF cluster forms one contiguous, prefix-scannable key range. This reduces MDT +> record count by roughly three orders of magnitude, turns "scan a cluster" into a single +> contiguous range read, and lets a query touch only the columns a given scan pass needs. + +The base table remains the source of truth for exact vector values. The MDT stores only +routing, pruning, and approximate-scoring metadata; final ranking always reads exact vectors +from the base table. + +Prototype measurements on a **1-billion-row, 128-dimensional table** show exact-reranked +recall@10 = 0.985 at nprobe=128 with query latency in low single-digit seconds on a modest +Spark cluster, versus ~15s for brute force. + +--- + +## 1. Goals and Non-Goals + +### 1.1 Goals + +1. Keep authoritative vector values in the base table (`ARRAY<FLOAT>` / `VECTOR(D)` column). +2. Store the vector index in the MDT, maintained by Hudi metadata-table commits, compaction, + and cleaning — no hidden or generated columns in base-table files. +3. Make candidate discovery cheap and targeted: probe a few clusters, scan contiguous key + ranges, score on compressed codes with a provable pruning bound. +4. Make results trustworthy: approximate math selects candidates; **exact** distance on + base-table vectors ranks them. +5. Stay transactionally consistent: snapshot-pinned reads, correct behavior under inserts, + updates, deletes, and clustering. +6. Be engine-neutral in design; Spark is the first implementation. +7. Maintain the index incrementally (no global rebuild for normal churn) and support + versioned, zero-downtime rebuilds. + +### 1.2 Non-Goals (initial landing) + +- ANN families beyond IVF + RaBitQ (e.g. HNSW, DiskANN). +- Filtered search (arbitrary predicate + kNN) as a first-class planned operation. +- Time-travel-consistent index reads for historical snapshots. +- Engines beyond Spark, and GPU-accelerated encoding. +- Workload-specific auto-tuning of `nprobe` / refine factor. + +--- + +## 2. Architecture + + + +The design splits responsibilities the way Hudi already does between the data table and the +metadata table: + +```text +DATA TABLE (parquet/orc) METADATA TABLE (vector_index partition) + authoritative vectors + payload ←── the index: centroids, quantizer, posting blocks, + read only for final re-ranking cluster manifests, generation manifest + read for candidate generation +``` + +Each vector index is one MDT partition. Creating an index: + +```sql +CREATE INDEX embedding_idx +ON products +USING VECTOR (embedding) +OPTIONS ( + 'vector.dimension' = '768', + 'vector.metric' = 'cosine', + 'vector.quantizer' = 'IVF_RABITQ', + 'vector.num_clusters'= '4096' +); +``` + +creates: + +```text +.hoodie/metadata/vector_index_embedding_idx/ +``` + +and does not change the base-table schema: + +```text +products/category=electronics/<file-group>.parquet +├── _hoodie_record_key = p001 +├── _hoodie_partition_path = category=electronics +├── user columns = ... +└── embedding ARRAY<FLOAT> = [0.12, -0.08, ...] +``` + +The query path uses the MDT first to discover candidates, then reads base-table vectors for +exact re-ranking: + +```text +query vector + → compare to centroids, pick nprobe clusters (in-memory, ms) + → MDT prefix-scan those clusters' posting blocks (targeted range reads) + → two-pass RaBitQ scoring, keep refineFactor·K best (bit math + error bounds) + → validate candidate freshness via Record Level Index (batched point lookups) + → fetch ONLY those rows from the base table by position (page-level reads) + → exact distance on real vectors → final top-K +``` + +--- + +## 3. IVF + RaBitQ Index Algorithm + +Every practical ANN index answers two questions: **where to look** (avoid scanning +everything) and **how to compare cheaply** (avoid full-precision math on what is scanned). + +### 3.1 Where to look: IVF routing + +Inverted File (IVF) indexing clusters vectors with KMeans into `numClusters` groups (e.g. +~4K–64K). Each vector belongs to its nearest centroid. A query compares against the centroids +only (thousands, not billions), selects the `nprobe` nearest clusters, and scans only those +clusters' entries. `nprobe` is the recall dial. + +IVF is the right fit for a lakehouse-resident index because a cluster's entries can be stored +**contiguously**, which maps directly onto sorted key ranges in Hudi's MDT (§4). Graph +indexes (HNSW) give excellent in-memory recall but require random traversal of the whole +graph, which fights columnar, immutable, object-store storage. + +### 3.2 How to compare cheaply: RaBitQ quantization + +Inside a probed cluster there are still thousands of full vectors. Quantization stores a +small *code* per vector plus a few correction scalars, so most comparisons run on compressed +codes and only the best few hundred candidates are re-checked against real vectors. + +RaBitQ is chosen over scalar (SQ), product (PQ), and plain binary quantization for four +reasons: + +1. **Unbiased estimator with a provable per-vector error bound.** Each code carries scalars + that turn a cheap bit-level dot product into an *unbiased* estimate of the true distance, + plus a bound on how wrong it can be. The bound enables **safe pruning**: skip a vector + only when even its best plausible distance cannot make top-K. +2. **No codebooks.** RaBitQ needs only a random rotation (a seed) and the centroids — both + tiny, both versioned in the index metadata. Nothing to retrain when data drifts. +3. **Tunable precision.** B = 1 bit/dim is a fast coarse filter; B = 4 bits/dim gives + near-SQ quality at ~8× less space. Both are used in a two-pass scan (§6.2). +4. **Metric-flexible.** One stored code serves L2, cosine, and dot-product; the metric is + applied at query time. + +The four ideas, precisely: + +- **Residual.** Store each vector as its difference from its centroid, `r = v − c`. + Residuals are small and centered, which is what lets few bits go far. +- **Rotation.** Apply one fixed random orthonormal rotation `R` (derived from a per-generation + seed) to everything first: `x = R·r`. This spreads information evenly across dimensions, + which is what makes the error bound hold for any data distribution. Only the seed is stored. +- **Code.** Quantize `x` to B bits per dimension, stored as **bit planes** — plane 0 holds + bit 0 of every dimension, plane 1 bit 1, etc. The top plane alone is a 1-bit sign sketch. + Scoring a plane against the (transformed) query is `AND`/`XOR` + `popcount` — a few CPU + instructions per 64 dimensions. +- **Factors.** A handful of small scalars per vector (residual norm, two rescale factors, an + error term, a centroid correction) that convert plane math into an unbiased distance + estimate plus its confidence interval. + +At query time the query vector is transformed the same way (`R·(q − c)` per probed cluster), +and the per-plane popcounts combine with the stored factors into the estimate and its bound. +The estimate builds the shortlist; exact base-table distances produce the final ranking (§6). + +### 3.3 Why this fits Hudi + +- Centroids are small enough to load at planning time: `K × D` floats (~12 MB for K=4096, + D=768). +- Codes are compact: a 1B × 128-dim float table's raw vectors are ~512 GB; the RaBitQ index + including keys and locators is ~136 GB, and the scanned portion per query is tens of MB. +- Posting keys are prefix-scannable by generation, cluster, and shard (§4). +- Quantizer state is stable: a seed and centroids, no per-generation learned codebook. +- Exact re-ranking preserves correctness for returned candidates. + +--- + +## 4. Metadata Table Storage Model: the Posting Block + +This section describes the core storage contribution of this RFC. + +### 4.1 The posting block + +A naive index would write one MDT record per indexed vector. At a billion rows that is a +billion MDT records per generation — prohibitive to write, compact, scan, and clean. + +Instead, the index sorts entries by `(cluster, fileGroup, rowPosition)` and packs +~1–4K vectors' worth of codes and metadata into a single MDT record — a **posting block** — +laid out column-wise (structure-of-arrays) so each scan pass touches only the columns it +needs: + +```text +POSTING BLOCK (~512 KB target, one MDT record) +┌───────────────────────────────────────────────┐ +│ S1 sign planes ← pass 1 touches this │ +│ S2 extra bit planes ← pass 2, survivors │ +│ S3 factor arrays ← both passes │ +│ S4 row locators ← finalists only │ +│ S5 dictionaries (file groups, partitions) │ +│ S6 record keys ← finalists only │ +└───────────────────────────────────────────────┘ +key: 0x10 | generation | clusterId | shardId | blockId +``` + +Consequences: + +- **~1000× fewer MDT records.** One block record replaces ~1–4K per-vector records, cutting + write amplification, compaction cost, and cleaner load by three orders of magnitude. +- **Contiguous cluster scans.** The binary key scheme makes "scan cluster 12345" a single + contiguous HFile range read rather than thousands of point lookups. +- **Pay only for promise.** Column-wise layout means pass 1 reads only sign planes + factors; + only survivors touch extra planes; only finalists touch locators and keys (§6.2). + +### 4.2 Row families + +The `vector_index_<name>` partition holds several record families under one binary-sorted key +scheme, so one prefix scan of a cluster returns its blocks and any fresh deltas together: + +| Key family | Cardinality | Purpose | +|---|---:|---| +| `__manifest__` | 1 | Active generation pointer used by readers. | +| `__centroids__` | 1 | Serialized `K × D` centroid matrix. | +| `__quantizer__` | 1 | RaBitQ type, bit width `B`, rotation seed, normalization flag. | +| `M\|<generation>` | generations | Immutable generation-level metadata. | +| `C\|<generation>\|<cluster>` | K per generation | Cluster manifest: shard count, vector count, candidate file groups, delta/tombstone counters. | +| `P\|<generation>\|<cluster>\|<shard>\|<blockId>` | blocks per generation | **Posting block** (packed codes, factors, locators, keys). | +| `P\|...\|<DELTA>` | deltas | Small per-record delta records appended to a cluster range between compactions. | + +### 4.3 Posting shards + +Large clusters are split into posting shards so one hot cluster does not become one oversized +prefix range. The cluster manifest stores `shardCount`; writers compute +`shardId = hash(record_key) % shardCount`. Within a shard, entries are packed into blocks by +`blockId`. Sharding changes physical layout only, never vector semantics. + +### 4.4 Delta records + +Between compactions, per-commit vector writes append small **delta records** (`blockId` +marked `DELTA`) at the end of the same cluster key range. Because they share the prefix, a +single cluster prefix scan sees packed blocks and fresh deltas in one pass (§6, §7). + +### 4.5 Generation model + +A generation is a consistent set of centroid, quantizer, cluster, and posting-block metadata: + +```text +__manifest__ -> active generation id +M|<gen> -> immutable generation metadata +__centroids__ -> centroid matrix for the active generation +__quantizer__ -> RaBitQ seed / width / normalization +C|<gen>|... -> cluster manifests +P|<gen>|... -> posting blocks + deltas +``` + +A query uses one generation consistently. Publishing a generation is atomic from a reader's +perspective: write the generation's rows, commit them to MDT, then flip `__manifest__`. Old +generations are retained until no retained table snapshot needs them. This enables +blue/green rebuilds with clean rollback. + +--- + +## 5. Bootstrap and Write Path + + + +### 5.1 Spark bootstrap + +Bootstrap builds a complete generation from a table snapshot in two distributed phases: + +```text +1. Read latest base-table file slices; extract record key, partition path, file group, + base instant, and vector bytes. +2. Train IVF centroids with Spark ML KMeans over a bounded sample. +3. Broadcast centroids; in mapPartitions, assign each vector to its nearest cluster, + compute shard id, and encode the RaBitQ code + factors. +4. Sort entries by (cluster, fileGroup, rowPosition) and pack them into posting blocks. +5. Emit __centroids__, __quantizer__, __manifest__, M|, C| (with shard/vector counts and + candidate file groups), and P| posting-block records. +``` + +The training sample satisfies percentage and per-cluster floors, e.g.: + +```text +targetSample = min(N, max(1M, 256 * K, min(10M, 0.5%–1% of N))) +``` + +### 5.2 Incremental inserts and vector updates + +For an inserted row or a vector-changing update, the metadata writer's vector hook computes +the nearest centroid, cluster id, shard id, RaBitQ code, optional scalar, and base-table +location, and appends a posting **delta** to the active generation's cluster range. If an +update moves a record to a different cluster/shard, the stale entry is made invisible through +delta supersession and query-time arbitration (§6.3, §9.3). + +### 5.3 Non-vector updates and deletes + +If an update does not change the vector, the code and cluster placement stay valid; if the +record moved to a different base-table file group, its locator is refreshed at the next +compaction (before old file slices are cleaned). Deletes require **no index write**: the +Record Level Index already knows a key is gone, and query-time arbitration drops +stale/deleted candidates. + +--- + +## 6. Read Path + + + +### 6.1 Query planning + +```text +1. Validate the query dimension against the indexed vector schema. +2. Load the active generation: __manifest__, __centroids__, __quantizer__, and cluster + manifests (cacheable on the driver until the generation changes). +3. Probe centroids and select the top-nprobe clusters. +4. From cluster manifests, resolve shard prefixes and candidate file groups. +``` + +### 6.2 Two-pass scan over posting blocks + +For each selected `(cluster, shard)`, prefix-scan `P|<gen>|<cluster>|<shard>|*` (blocks + +deltas) and score column-wise: + +```text +Pass 1 (bound + prune): read S1 sign planes + S3 factors. For every vector compute an + optimistic bound — the best distance it could possibly achieve. Skip vectors whose + best case cannot beat the current K-th candidate (typically 85–95% pruned). +Pass 2 (refine): for survivors, read S2 extra bit planes and compute the full + multibit unbiased estimate; keep the refineFactor·K best. +Finalize: only the surviving finalists touch S4 locators and S6 keys to learn + where they live and who they are. +``` + +Cost is proportional to promise, not to table size. Delta records encountered in the same +scan are scored identically and supersede matching packed entries. + +### 6.3 Freshness arbitration and exact re-rank + +Posting locators are treated as *hints*. The finalists' record keys are validated against the +Record Level Index in batched point lookups on the same pinned snapshot: + +- moved file group → re-resolve location by key; +- missing → deleted → drop the candidate. + +Surviving finalists' exact vectors are then read from the base table using row-position / +page-level positional reads (the ~500 candidate rows cost megabytes, not a full slice scan), +exact distances are computed, and the final top-K is returned. + +### 6.4 Batched queries + +For a relation of query vectors, the plan shares MDT work: load generation metadata once, +encode all queries, probe per query, group by selected `(cluster, shard)`, scan each range +once while maintaining a per-query top-R heap, then read the union of candidate rows and +exact re-rank per query. + +### 6.5 Fallback + +If required vector metadata is missing, incompatible with the query dimension, or unavailable +for the requested snapshot, Hudi must not return incorrect ANN results — it falls back to an +exact table scan or fails per the configured failure mode. + +--- + +## 7. Maintenance, Rebalancing, and Cleaner + +Vectors change. Index health is kept without global rebuilds through three tiers of +increasing rarity (LIRE — Local Incremental Rebalancing). + +### 7.1 Per commit (continuous) + +The vector hook appends posting deltas for written rows and increments per-cluster counters +(live / delta / tombstone) additively. Deletes need no index write. + +### 7.2 Compaction (periodic, counter-triggered) + +When a cluster's delta count crosses a threshold, MDT compaction rewrites that cluster's +range: fold deltas into packed blocks, drop dead entries, refresh locators for rewritten file +groups, and reset counters. This bounds read cost so deltas never accumulate past a few +percent of a cluster. + +### 7.3 Rebalancing (rare, statistics-triggered) + +Sustained skew triggers a **local** split/merge: fetch the affected cluster's raw vectors +(the same positional reader the query path uses), run a local 2-means, re-encode into two +clusters, and reassign a bounded set of boundary vectors from neighbors — committed under a +new centroid epoch so in-flight queries stay consistent. Each operation touches one cluster +and its neighbors, never the whole index. At realistic churn (~1%/day) splits trigger weeks +apart. + +### 7.4 Triggers and cleaner coordination + +| Trigger | Condition | Action | +|---|---|---| +| Delta accumulation | delta count over threshold | Compact the cluster range. | +| Cluster imbalance | `max_cluster / avg_cluster > threshold` | Local split / merge. | +| Centroid drift | cluster mean moved beyond threshold | Recompute affected centroids / new epoch. | +| Large replace commit | many file groups replaced | Refresh locators or rebuild a generation. | +| Periodic validation | scheduled health check | Compare postings against base-table records. | + +The metadata cleaner retains vector generations as long as any retained table snapshot may +need them; visible entries must point to live base-table records for every snapshot MDT +serves. + +--- + +## 8. Spark API Surface + +Table-valued function (single-query): + +```sql +SELECT * FROM hudi_vector_search( + table => 'products', + column => 'embedding', + query => ARRAY(0.1, 0.2, ...), + k => 10, + metric => 'cosine', -- l2 | cosine | dot_product + mode => 'approximate' -- or 'exact_rerank' +); +``` + +Datasource options (integration/testing): + +```properties +hoodie.datasource.read.vector.index.name=embedding_idx +hoodie.datasource.read.vector.query.vector=[0.1,0.2,...] +hoodie.datasource.read.vector.query.nprobes=8 +hoodie.datasource.read.vector.query.topk=100 +``` + +Index DDL: + +```sql +CREATE INDEX embedding_idx +ON products +USING VECTOR (embedding) +OPTIONS ( + 'vector.dimension' = '768', + 'vector.metric' = 'cosine', + 'vector.quantizer' = 'IVF_RABITQ', + 'vector.num_clusters' = '4096', + 'vector.rabitq.bits' = '4', + 'vector.rabitq.seed' = '42', + 'vector.rabitq.assume_normalized' = 'false', + 'vector.query.nprobes' = '8' Review Comment: Removed the misleading headline comparison and added Appendix A with configuration-specific BIGANN results. §8 keeps defaults distinct from measured operating points and warns that recall belongs to a built generation/configuration. -- This is an automated message from the Apache Git Service. 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