Iciclecreek.Lucene.Net.Vector

Iciclecreek.Lucene.Net.Vector

Adds vector similarity search to Lucene.Net 4.8.

Embeddings are stored as BinaryDocValuesField and queried through KnnVectorQuery, a native Lucene Query that composes with BooleanQuery for filtered and hybrid search. An HNSW index is built and cached automatically per IndexReader — no manual index management required.

Installation

dotnet add package Iciclecreek.Lucene.Net.Vector

Dependencies

• Lucene.Net 4.8.0-beta00017
• HNSW (Microsoft HNSW.Net — SIMD cosine distance)

Usage

Indexing vectors

Store embeddings alongside your documents using BinaryDocValuesField:

using Iciclecreek.Lucene.Net.Vector;
using Lucene.Net.Documents;
using Lucene.Net.Index;

var doc = new Document
{
    new StringField("id", "1", Field.Store.YES),
    new TextField("text", "The cat sat on the mat", Field.Store.YES),
    new StringField("category", "animals", Field.Store.YES),
    new BinaryDocValuesField("embedding", VectorSerializer.ToBytesRef(vector)),
};
writer.AddDocument(doc);
writer.Commit();

Vector search

KnnVectorQuery is a standard Lucene Query — use it with IndexSearcher like any other query:

using var reader = DirectoryReader.Open(directory);
var searcher = new IndexSearcher(reader);
var queryVector = GetVector(userQuery) 
var query = new KnnVectorQuery("embedding", queryVector, k: 10, reader);
var topDocs = searcher.Search(query, 10);

foreach (var scoreDoc in topDocs.ScoreDocs)
{
    var doc = searcher.Doc(scoreDoc.Doc);
    Console.WriteLine($"{doc.Get("id")}: {scoreDoc.Score}");
}

The HNSW index is built automatically on the first query and cached for the lifetime of the IndexReader.

Filtered search

Compose KnnVectorQuery with BooleanQuery to combine vector similarity with field filters:

var boolQuery = new BooleanQuery
{
    { new TermQuery(new Term("category", "animals")), Occur.MUST },
    { new KnnVectorQuery("embedding", queryVector, k: 10, reader), Occur.MUST },
};
var topDocs = searcher.Search(boolQuery, 10);

After updates

When you add, update, or delete documents, open a new reader — the HNSW index rebuilds automatically:

writer.DeleteDocuments(new Term("id", "old-doc"));
writer.Commit();

using var newReader = DirectoryReader.Open(directory);
var searcher = new IndexSearcher(newReader);
// KnnVectorQuery will build a fresh HNSW index from the new reader's DocValues
var query = new KnnVectorQuery("embedding", queryVector, k: 10, newReader);

HNSW options

Pass VectorIndexOptions to tune the HNSW graph:

var options = new VectorIndexOptions
{
    M = 16,                                        // Max edges per node (default: 16)
    ConstructionPruning = 200,                     // Build quality (default: 200)
    EfSearch = 50,                                 // Search quality (default: 50)
    Distance = VectorDistanceFunction.Cosine,      // Distance function (default: Cosine)
};
var query = new KnnVectorQuery("embedding", queryVector, k: 10, reader, options);

RAG (Retrieval-Augmented Generation)

A typical RAG pipeline: embed your documents at index time, then at query time embed the user's question, retrieve relevant context via vector search, and pass it to an LLM.

// --- Setup: any IEmbeddingGenerator (OpenAI, Ollama, local ONNX, etc.) ---
IEmbeddingGenerator<string, Embedding<float>> embedder = ...;
IChatClient chatClient = ...;

// --- Index time: embed and store documents ---
using var directory = FSDirectory.Open("my-index");
using var analyzer = new StandardAnalyzer(LuceneVersion.LUCENE_48);
var config = new IndexWriterConfig(LuceneVersion.LUCENE_48, analyzer);
using var writer = new IndexWriter(directory, config);

foreach (var chunk in documentChunks)
{
    var embedding = await embedder.GenerateAsync([chunk.Text]);
    writer.AddDocument(new Document
    {
        new StringField("id", chunk.Id, Field.Store.YES),
        new StoredField("text", chunk.Text),
        new BinaryDocValuesField("embedding",
            VectorSerializer.ToBytesRef(embedding[0].Vector.ToArray())),
    });
}
writer.Commit();

// --- Query time: embed question, retrieve context, generate answer ---
var question = "How does photosynthesis work?";
var questionEmbedding = await embedder.GenerateAsync([question]);
var queryVector = questionEmbedding[0].Vector.ToArray();

using var reader = DirectoryReader.Open(directory);
var searcher = new IndexSearcher(reader);
var query = new KnnVectorQuery("embedding", queryVector, k: 5, reader);
var topDocs = searcher.Search(query, 5);

// Gather context from top matches
var context = string.Join("\n\n", topDocs.ScoreDocs
    .Select(sd => searcher.Doc(sd.Doc).Get("text")));

// Pass to LLM
var response = await chatClient.GetResponseAsync($"""
    Answer the question based on the following context:

    {context}

    Question: {question}
    """);

Console.WriteLine(response);

This composes with Lucene's full query model — you can add metadata filters, full-text search, or boost certain fields alongside the vector search:

// RAG with metadata filter: only search recent documents
var filteredRag = new BooleanQuery
{
    { NumericRangeQuery.NewInt64Range("timestamp", recentCutoff, null, true, false), Occur.MUST },
    { new KnnVectorQuery("embedding", queryVector, k: 10, reader), Occur.MUST },
};

Notes

• Vectors are stored in Lucene as BinaryDocValuesField (4 bytes per float, little-endian). The HNSW graph is an in-memory acceleration structure built automatically from DocValues.
• HNSW caching — the graph is cached per (IndexReader, fieldName). Since an IndexReader is an immutable snapshot, the cached graph is valid for the reader's entire lifetime. Opening a new reader after commits triggers a fresh build.
• Deleted documents are automatically excluded — Lucene's LiveDocs filtering ensures deleted docs are not loaded into the HNSW graph.
• Distance functions: Cosine (general purpose, handles unnormalized vectors) and CosineForUnits (faster, requires pre-normalized unit vectors).

License

MIT