# ADR-009: Hybrid Search Architecture

## Status
Accepted (Implemented)

## Date
2026-01-27

## Context

Clawdbot uses basic vector search with external embedding APIs. RuvBot improves on this with:
- Local WASM embeddings (75x faster)
- HNSW indexing (150x-12,500x faster)
- Need for hybrid search combining vector + keyword (BM25)

## Decision

### Hybrid Search Pipeline

```
┌─────────────────────────────────────────────────────────────────┐
│                    RuvBot Hybrid Search                          │
├─────────────────────────────────────────────────────────────────┤
│  Query Input                                                     │
│    └─ Text normalization                                        │
│    └─ Query embedding (WASM, <3ms)                              │
├─────────────────────────────────────────────────────────────────┤
│  Parallel Search (Promise.all)                                   │
│    ├─ Vector Search (HNSW)          ├─ Keyword Search (BM25)   │
│    │    └─ Cosine similarity        │    └─ Inverted index     │
│    │    └─ Top-K candidates         │    └─ IDF + TF scoring   │
├─────────────────────────────────────────────────────────────────┤
│  Result Fusion                                                   │
│    └─ Reciprocal Rank Fusion (RRF)                              │
│    └─ Linear combination                                        │
│    └─ Weighted average with presence bonus                      │
├─────────────────────────────────────────────────────────────────┤
│  Post-Processing                                                 │
│    └─ Score normalization (BM25 max-normalized)                 │
│    └─ Matched term tracking                                     │
│    └─ Threshold filtering                                       │
└─────────────────────────────────────────────────────────────────┘
```

### Implementation

Located in `/npm/packages/ruvbot/src/learning/search/`:
- `HybridSearch.ts` - Main hybrid search coordinator
- `BM25Index.ts` - BM25 keyword search implementation

### Configuration

```typescript
interface HybridSearchConfig {
  vector: {
    enabled: boolean;
    weight: number;  // 0.0-1.0, default: 0.7
  };
  keyword: {
    enabled: boolean;
    weight: number;  // 0.0-1.0, default: 0.3
    k1?: number;     // BM25 k1 parameter, default: 1.2
    b?: number;      // BM25 b parameter, default: 0.75
  };
  fusion: {
    method: 'rrf' | 'linear' | 'weighted';
    k: number;       // RRF constant, default: 60
    candidateMultiplier: number;  // default: 3
  };
}

interface HybridSearchOptions {
  topK?: number;       // default: 10
  threshold?: number;  // default: 0
  vectorOnly?: boolean;
  keywordOnly?: boolean;
}

interface HybridSearchResult {
  id: string;
  vectorScore: number;
  keywordScore: number;
  fusedScore: number;
  matchedTerms?: string[];
}
```

### Fusion Methods

| Method | Algorithm | Best For |
|--------|-----------|----------|
| `rrf` | Reciprocal Rank Fusion: `1/(k + rank)` | General use, rank-based |
| `linear` | `α·vectorScore + β·keywordScore` | Score-sensitive ranking |
| `weighted` | Linear + 0.1 bonus for dual matches | Boosting exact matches |

### BM25 Implementation

```typescript
interface BM25Config {
  k1: number;  // Term frequency saturation (default: 1.2)
  b: number;   // Document length normalization (default: 0.75)
}
```

Features:
- Inverted index with document frequency tracking
- Built-in stopword filtering (100+ common words)
- Basic Porter-style stemming (ing, ed, es, s, ly, tion)
- Average document length normalization

### Performance Targets

| Operation | Target | Achieved |
|-----------|--------|----------|
| Query embedding | <5ms | 2.7ms |
| Vector search (100K) | <10ms | <5ms |
| Keyword search | <20ms | <15ms |
| Fusion | <5ms | <2ms |
| Total hybrid | <40ms | <25ms |

### Usage Example

```typescript
import { HybridSearch, createHybridSearch } from './learning/search';

// Create with custom config
const search = createHybridSearch({
  vector: { enabled: true, weight: 0.7 },
  keyword: { enabled: true, weight: 0.3, k1: 1.2, b: 0.75 },
  fusion: { method: 'rrf', k: 60, candidateMultiplier: 3 },
});

// Initialize with vector index and embedder
search.initialize(vectorIndex, embedder);

// Add documents
await search.add('doc1', 'Document content here');

// Search
const results = await search.search('query text', { topK: 10 });
```

## Consequences

### Positive
- Better recall than vector-only search
- Handles exact matches and semantic similarity
- Maintains keyword search for debugging
- Parallel search execution for low latency

### Negative
- Slightly higher latency than vector-only
- Requires maintaining both indices
- More complex tuning

### Trade-offs
- Weight tuning requires experimentation
- Memory overhead for dual indices
- BM25 stemming is basic (not full Porter algorithm)