LeanLucene

A fast, embeddable full-text search engine for .NET. No external processes, no JVM, no Lucene. Write an index, run a query, ship your app.

dotnet add package LeanLucene

Get started  |  API reference

What it does

Area Details
Indexing Memory-mapped segments, BM25 scoring, index-time sorting, schema validation, concurrent multi-thread indexing, CRC-protected commits
Queries Term, boolean, phrase, prefix, wildcard, fuzzy, range, regexp, span, geo bounding box, geo distance, disjunction max
Advanced queries HNSW vector ANN (VectorQuery), filtered vector search, reciprocal rank fusion (RrfQuery), block-join, more-like-this, function score, constant score
Analysis Pluggable tokenisers (standard, n-gram, edge n-gram, CJK bigram), char filters, token filters, stemmers for 10+ languages
Search features Facets, aggregations, highlighting, spell-check suggestions, field collapsing, query caching
Concurrency SearcherManager for near-real-time search, snapshot-based backup, configurable commit retention policies
Observability ActivitySource traces, System.Diagnostics.Metrics via MeterMetricsCollector, structured logs through OpenTelemetry, slow query log, search analytics

Quick start

using Rowles.LeanLucene.Store;
using Rowles.LeanLucene.Index.Indexer;
using Rowles.LeanLucene.Document;
using Rowles.LeanLucene.Document.Fields;
using Rowles.LeanLucene.Search.Searcher;
using Rowles.LeanLucene.Search.Queries;

// Index
using var writer = new IndexWriter(new MMapDirectory("./index"), new IndexWriterConfig());

var doc = new LeanDocument();
doc.Add(new TextField("title", "The quick brown fox"));
doc.Add(new StringField("id", "1"));
writer.AddDocument(doc);
writer.Commit();

// Search
using var searcher = new IndexSearcher(new MMapDirectory("./index"));
var results = searcher.Search(new TermQuery("title", "fox"), topN: 10);

foreach (var hit in results.ScoreDocs)
{
    var fields = searcher.GetStoredFields(hit.Doc);
    Console.WriteLine(fields["id"][0]);
}

See the installation tutorial for a full walkthrough.

Why not just use Lucene?

LeanLucene targets .NET natively. No JNI bridge, no Java runtime dependency, no cross-process communication. The index format is purpose-built for memory-mapped I/O on .NET, and the query engine uses SIMD posting intersection and BlockMax WAND for early termination on large result sets.

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