[FEATURE] Add MUVERA ingest and search processors for multi-vector ANN prefetch

[praveenMprasad]

Is your feature request related to a problem?

Multi-vector retrieval models like ColBERT and ColPali produce per-token embeddings that require MaxSim scoring across all token pairs. This is expensive at scale because there's no way to do ANN prefetch on variable-length multi-vector representations — you're forced to either brute-force score every document or rely on external tooling to pre-encode vectors client-side.

Currently, the k-NN plugin supports [lateInteractionScore](https://docs.opensearch.org/latest/query-dsl/specialized/script-score/#late-interaction-score) for MaxSim reranking, but the inner query is typically match_all or a text filter, meaning every matching document gets scored. There's no native way to narrow candidates using the multi-vector embeddings themselves.

What solution would you like?

Add two new processors implementing the MUVERA algorithm (Multi-Vector Retrieval via Fixed Dimensional Encodings, [paper](https://arxiv.org/abs/2405.19504)):

1. muvera ingest processor — Converts variable-length multi-vector embeddings into a single fixed-dimensional encoding (FDE) vector using SimHash clustering and random projections. The FDE is stored in a knn_vector field for ANN indexing. The original multi-vectors remain in _source for reranking.

2. muvera_query search request processor — Intercepts script_score queries containing query_vectors in script params, MUVERA-encodes them into an FDE, and replaces the inner match_all with a knn query on the FDE field. The lateInteractionScore script wrapper stays intact for MaxSim reranking on the prefetched candidates.

User flow

Step 1: Create ingest pipeline

PUT _ingest/pipeline/muvera-ingest
{
  "description": "MUVERA FDE encoding for ColBERT vectors",
  "processors": [
    {
      "muvera": {
        "source_field": "colbert_vectors",
        "target_field": "muvera_fde",
        "dim": 128,
        "fde_dimension": 2560
      }
    }
  ]
}

Defaults: k_sim=4, dim_proj=8, r_reps=20, seed=42. FDE dimension = r_reps * 2^k_sim * dim_proj = 2560. The fde_dimension parameter validates the computed value so the user explicitly acknowledges the output size.

Step 2: Create index

PUT muvera-index
{
  "settings": {
    "index.knn": true,
    "default_pipeline": "muvera-ingest"
  },
  "mappings": {
    "dynamic": false,
    "properties": {
      "muvera_fde": {
        "type": "knn_vector",
        "dimension": 2560,
        "method": {
          "name": "hnsw",
          "space_type": "innerproduct",
          "engine": "faiss"
        }
      },
      "title": { "type": "text" }
    }
  }
}

Note: colbert_vectors is intentionally left unmapped, it stays in _source for reranking but doesn't need its own field mapping.

Step 3: Index documents

POST muvera-index/_doc/1
{
  "title": "example document",
  "colbert_vectors": [
    [0.1, 0.2, ...],
    [0.3, 0.4, ...],
    [0.5, 0.6, ...]
  ]
}

The ingest processor reads colbert_vectors, produces the FDE, and writes it to muvera_fde. Both fields end up in the stored document.

Step 4: Create search pipeline

PUT _search/pipeline/muvera-search
{
  "request_processors": [
    {
      "muvera_query": {
        "target_field": "muvera_fde",
        "dim": 128,
        "fde_dimension": 2560,
        "oversample_factor": 4
      }
    }
  ]
}

Same MUVERA hyperparams as ingest (must match). oversample_factor controls how many candidates the knn prefetch retrieves relative to the requested result size.

Step 5: Search

POST muvera-index/_search?search_pipeline=muvera-search
{
  "size": 10,
  "query": {
    "script_score": {
      "query": { "match_all": {} },
      "script": {
        "source": "lateInteractionScore(params.query_vectors, 'colbert_vectors', params._source, params.space_type)",
        "params": {
          "query_vectors": [[0.1, 0.2, ...], [0.3, 0.4, ...]],
          "space_type": "innerproduct"
        }
      }
    }
  }
}

What happens:

1. Search processor extracts query_vectors from script params
2. MUVERA-encodes them into a query FDE
3. Replaces match_all with knn on muvera_fde (k = size × oversample_factor = 40)
4. lateInteractionScore reranks the 40 candidates using exact MaxSim on original multi-vectors
5. Top 10 returned to user

What alternatives have you considered?

• Client-side MUVERA encoding (works but requires users to maintain encoding logic outside OpenSearch)
• Binary quantization of multi-vectors (lossy, doesn't preserve MaxSim structure)
• Text-based prefetch with BM25 (misses semantic signal from embeddings)

Do you have any additional context?

• MUVERA is already implemented in [fastembed](https://github.com/qdrant/fastembed) (Python) and used in production with Qdrant
• We have a working implementation with unit tests, stable across multiple iterations with random seeds
• The implementation uses only public APIs — no reflection or core OpenSearch modifications required
• Tested end-to-end on a live cluster: ingest pipeline creates FDE vectors, search pipeline rewrites queries, lateInteractionScore reranking produces correct MaxSim scores