Retrievo 0.3.0-preview.4

Retrievo

Hybrid search for .NET — BM25 + vectors + RRF fusion, zero infrastructure

Retrievo is an open-source, in-process, in-memory search library for .NET that combines BM25 lexical matching with vector similarity search. Results are merged via Reciprocal Rank Fusion (RRF) into a single ranked list — no external servers, no databases, no infrastructure. Designed for corpora up to ~10k documents: local agent memory, small RAG pipelines, developer tools, and offline/edge scenarios.

Quick Start

using Retrievo;
using Retrievo.Models;

var index = new HybridSearchIndexBuilder()
    .AddDocument(new Document { Id = "1", Body = "Neural networks learn complex patterns." })
    .AddDocument(new Document { Id = "2", Body = "Kubernetes orchestrates container deployments." })
    .Build();

using var _ = index;
var response = index.Search(new HybridQuery { Text = "neural network training", TopK = 5 });

foreach (var r in response.Results)
    Console.WriteLine($"  {r.Id}: {r.Score:F4}");

Benchmarks

Retrieval Quality (NDCG@10)

Validated against BEIR with 245-configuration parameter sweeps per dataset:

Default parameters (LexicalWeight=0.5, VectorWeight=1.0, RrfK=20, TitleBoost=0.5) tuned via cross-dataset harmonic mean optimization.

Query Latency

Measured on BEIR datasets with text-embedding-3-small (1536-dim) embeddings:

Details: benchmarks/

Key Features

Core Search

• Hybrid Retrieval: Combine BM25 and cosine similarity using RRF fusion.
• Standalone Modes: Use lexical-only or vector-only search when needed.
• Explain Mode: Detailed score breakdown for every search result.
• Fielded Search: Title and body fields with independent boost weights.
• Metadata Filters: Exact-match, range, and contains filtering post-fusion.
• Field Definitions: Declare field types (String, StringArray) at index time for automatic filter semantics.
• Finite Vector Validation: Rejects NaN/Infinity embeddings and query vectors with clear exceptions.

Index Management

• Fluent Builder: Clean API for batch construction and folder ingestion.
• Mutable Index: Incremental upserts and deletes with thread-safe commits.
• Unique Document IDs: Builders reject duplicate document IDs within a single build.
• Zero Infrastructure: Runs entirely in-process with no external dependencies.
• Auto-Embedding: Transparently embed documents at index time.

Developer Experience

• SIMD Accelerated: TensorPrimitives-backed vector math with min-heap top-K selection.
• Query Diagnostics: Detailed timing breakdown for every pipeline stage.
• Pluggable Providers: Easy integration with any embedding model or API.
• CLI Tool: Powerful terminal interface for indexing and querying.

Packages

dotnet add package Retrievo --prerelease
dotnet add package Retrievo.AzureOpenAI --prerelease  # optional, for Azure OpenAI embeddings

Current prerelease: 0.3.0-preview.4

Azure OpenAI Embeddings

Plug in an embedding provider and Retrievo handles the rest — documents are embedded at build time, queries at search time.

using Retrievo.AzureOpenAI;

var provider = new AzureOpenAIEmbeddingProvider(
    new Uri("https://your-resource.openai.azure.com/"),
    "your-api-key",
    "text-embedding-3-small");

// Documents are auto-embedded during build
using var index = await new HybridSearchIndexBuilder()
    .AddFolder("./docs")  // loads *.md and *.txt recursively
    .WithEmbeddingProvider(provider)
    .BuildAsync();

// Query text is automatically converted to a vector
var response = await index.SearchAsync(new HybridQuery { Text = "how to deploy", TopK = 5 });

Field Definitions

Declare field types at index time so filters automatically use the right matching strategy:

using var index = new HybridSearchIndexBuilder()
    .DefineField("tags", FieldType.StringArray)         // pipe-delimited by default
    .DefineField("categories", FieldType.StringArray, delimiter: ',')
    .AddDocument(new Document
    {
        Id = "1",
        Body = "Deep learning fundamentals",
        Metadata = new Dictionary<string, string>
        {
            ["tags"] = "ml|deep-learning|neural-nets",
            ["categories"] = "ai,education"
        }
    })
    .Build();

// StringArray fields auto-split and do contains-match; undeclared fields use exact-match
var response = index.Search(new HybridQuery
{
    Text = "deep learning",
    MetadataFilters = new Dictionary<string, string> { ["tags"] = "ml" }
});

Architecture

HybridQuery
    |
    v
+-------------------+
|  HybridSearchIndex |  (orchestrator)
+-------------------+
    |           |
    v           v
+--------+  +----------+
| Lucene |  | Brute-   |
| BM25   |  | Force    |
| Search |  | Cosine   |
+--------+  +----------+
    |           |
    v           v
  ranked      ranked
  list        list
    \         /
     v       v
  +----------+
  | RRF      |
  | Fusion   |
  +----------+
       |
       v
  SearchResponse

Reciprocal Rank Fusion merges multiple ranked lists without score normalization: score(doc) = Σ weight / (k + rank). Documents that rank high on both lexical and vector lists get the biggest boost — surfacing results that are semantically relevant and contain the right keywords.

Roadmap

Build & Test

Requires .NET 8 SDK or later.

dotnet build
dotnet test

238 tests covering retrieval, vector math, fusion, mutable index, filters, field definitions, cancellation, and CLI integration — 0 warnings. CLI integration tests build the CLI project and execute the matching built artifact for the active configuration/TFM, so dotnet test tests/Retrievo.IntegrationTests works from a clean checkout.

Known Limitations

• Lexical (BM25) search is English-only: The lexical retrieval pipeline uses EnglishStemAnalyzer (StandardTokenizer → EnglishPossessiveFilter → LowerCaseFilter → English StopWords → PorterStemmer). Non-English text will not be properly tokenized or stemmed for BM25 matching.
• Vector search is language-agnostic: Semantic search works with any language supported by your embedding model (e.g., multilingual embeddings). Hybrid search inherits the English-only limitation for its lexical component.
• Brute-force vector search is O(n) per query: Designed for corpora up to ~10k documents. For larger corpora, consider ANN-based solutions (planned for Phase 4).
• In-memory only: No persistence or crash recovery. The index must be rebuilt from source documents on each application start.
• No concurrent writers on HybridSearchIndex: The immutable index is built once via the builder. Use MutableHybridSearchIndex for incremental upserts and deletes.
• Single-process: No distributed or shared index support. The index lives in a single process's memory.
• Workaround for non-English corpora: Use vector-only search by omitting lexical configuration, or configure a custom analyzer for your language in a fork.

License

MIT