Semantic Search Setup

Semantic Search Setup Skill

Overview

This skill sets up vector embedding infrastructure for semantic search. Unlike keyword search (FTS5), semantic search finds conceptually similar content even without exact word matches.

Quick Start

from sentence_transformers import SentenceTransformer
import numpy as np

model = SentenceTransformer('all-MiniLM-L6-v2')

# Generate embeddings
texts = ["How to fix a bug", "Debugging software issues"]
embeddings = model.encode(texts, normalize_embeddings=True)

# Compute similarity
similarity = np.dot(embeddings[0], embeddings[1])
print(f"Similarity: {similarity:.3f}")  # ~0.85

When to Use

• Adding AI-powered search to document collections
• Finding conceptually related documents
• Preparing knowledge bases for RAG Q&A systems
• Building recommendation systems
• Enabling "more like this" functionality

How Semantic Search Works

Text Chunk                    Query
    |                           |
    v                           v
+---------+               +---------+
| Embed   |               | Embed   |
| Model   |               | Model   |
+----+----+               +----+----+
     |                         |
     v                         v
[0.12, -0.34, ...]       [0.15, -0.31, ...]
     |                         |
     +------------+------------+
                  |
                  v
           Cosine Similarity
                  |
                  v
             0.847 (similar!)

Model Selection

Recommended: all-MiniLM-L6-v2 for local CPU processing.

Implementation

Step 1: Install Dependencies

pip install sentence-transformers numpy
# or
uv pip install sentence-transformers numpy

Step 2: Database Schema

import sqlite3

def create_embeddings_table(db_path):
    conn = sqlite3.connect(db_path, timeout=30)
    cursor = conn.cursor()

    cursor.execute('''
        CREATE TABLE IF NOT EXISTS embeddings (
            id INTEGER PRIMARY KEY,
            chunk_id INTEGER UNIQUE,
            embedding BLOB NOT NULL,
            model_name TEXT NOT NULL,
            dimension INTEGER NOT NULL,
            created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
            FOREIGN KEY (chunk_id) REFERENCES chunks(id)
        )
    ''')

    # Index for fast lookups
    cursor.execute('''
        CREATE INDEX IF NOT EXISTS idx_embeddings_chunk
        ON embeddings(chunk_id)
    ''')

    conn.commit()
    return conn

Step 3: Embedding Generator

from sentence_transformers import SentenceTransformer
import numpy as np
import os

class EmbeddingGenerator:
    def __init__(self, model_name='all-MiniLM-L6-v2'):
        # Force CPU for stability
        os.environ['CUDA_VISIBLE_DEVICES'] = ''

        self.model_name = model_name
        self.model = SentenceTransformer(model_name)
        self.dimension = self.model.get_sentence_embedding_dimension()

    def embed_text(self, text):
        """Generate normalized embedding for text."""
        embedding = self.model.encode(
            text,
            normalize_embeddings=True,
            show_progress_bar=False
        )
        return embedding.astype(np.float32)

    def embed_batch(self, texts, batch_size=100):
        """Generate embeddings for multiple texts."""
        embeddings = self.model.encode(
            texts,
            batch_size=batch_size,
            normalize_embeddings=True,
            show_progress_bar=True
        )
        return embeddings.astype(np.float32)

    def save_embedding(self, conn, chunk_id, embedding):
        """Save embedding to database."""
        cursor = conn.cursor()
        cursor.execute('''
            INSERT OR REPLACE INTO embeddings
            (chunk_id, embedding, model_name, dimension)
            VALUES (?, ?, ?, ?)
        ''', (
            chunk_id,
            embedding.tobytes(),
            self.model_name,
            self.dimension
        ))
        conn.commit()

Step 4: Batch Processing

def generate_all_embeddings(db_path, batch_size=100):
    """Generate embeddings for all chunks."""
    conn = sqlite3.connect(db_path, timeout=30)
    cursor = conn.cursor()

    generator = EmbeddingGenerator()

    # Get chunks without embeddings
    cursor.execute('''
        SELECT c.id, c.chunk_text
        FROM chunks c
        LEFT JOIN embeddings e ON c.id = e.chunk_id
        WHERE e.id IS NULL
    ''')

    chunks = cursor.fetchall()
    total = len(chunks)
    print(f"Generating embeddings for {total} chunks...")

    for i in range(0, total, batch_size):
        batch = chunks[i:i + batch_size]
        chunk_ids = [c[0] for c in batch]
        texts = [c[1] for c in batch]

        # Generate batch embeddings
        embeddings = generator.embed_batch(texts)

        # Save to database
        for chunk_id, embedding in zip(chunk_ids, embeddings):
            generator.save_embedding(conn, chunk_id, embedding)

        progress = min(i + batch_size, total)
        print(f"Progress: {progress}/{total} ({100*progress/total:.1f}%)")

    conn.close()
    print("Embedding generation complete!")

Step 5: Semantic Search

def semantic_search(db_path, query, top_k=10):
    """Find most similar chunks to query."""
    conn = sqlite3.connect(db_path, timeout=30)
    cursor = conn.cursor()

    generator = EmbeddingGenerator()
    query_embedding = generator.embed_text(query)

    # Get all embeddings
    cursor.execute('''
        SELECT e.chunk_id, e.embedding, c.chunk_text, d.filename, c.page_num
        FROM embeddings e
        JOIN chunks c ON e.chunk_id = c.id
        JOIN documents d ON c.doc_id = d.id
    ''')

    results = []
    for chunk_id, emb_blob, text, filename, page_num in cursor.fetchall():
        embedding = np.frombuffer(emb_blob, dtype=np.float32)

        # Cosine similarity (embeddings are normalized)
        score = float(np.dot(query_embedding, embedding))

        results.append({
            'chunk_id': chunk_id,
            'score': score,
            'text': text[:500],
            'filename': filename,
            'page': page_num
        })

    # Sort by similarity score
    results.sort(key=lambda x: x['score'], reverse=True)
    conn.close()

    return results[:top_k]

Step 6: Background Service

#!/bin/bash
# embed-service.sh - Background embedding service

DB_PATH="${1:-./knowledge.db}"
BATCH_SIZE="${2:-100}"
LOG_FILE="/tmp/embed.log"
PID_FILE="/tmp/embed.pid"

start() {
    if [ -f "$PID_FILE" ] && kill -0 $(cat "$PID_FILE") 2>/dev/null; then
        echo "Already running (PID: $(cat $PID_FILE))"
        return
    fi

    # Force CPU mode
    export CUDA_VISIBLE_DEVICES=""

    nohup python3 embed.py --db "$DB_PATH" --batch "$BATCH_SIZE" \
        >> "$LOG_FILE" 2>&1 &
    echo $! > "$PID_FILE"
    echo "Started (PID: $!)"
}

stop() {
    if [ -f "$PID_FILE" ]; then
        kill $(cat "$PID_FILE") 2>/dev/null
        rm "$PID_FILE"
        echo "Stopped"
    fi
}

status() {
    if [ -f "$PID_FILE" ] && kill -0 $(cat "$PID_FILE") 2>/dev/null; then
        echo "Running (PID: $(cat $PID_FILE))"
    else
        echo "Not running"
    fi
}

case "$1" in
    start) start ;;
    stop) stop ;;
    status) status ;;
    *) echo "Usage: $0 {start|stop|status}" ;;
esac

Execution Checklist

• Install sentence-transformers and numpy
• Choose appropriate embedding model for use case
• Create embeddings table in database
• Generate embeddings for all text chunks
• Test semantic search with sample queries
• Compare results with keyword search (FTS5)
• Optimize batch size for available memory
• Set up background service for continuous updates

Error Handling

Common Errors

Error: CUDA out of memory

• Cause: GPU memory insufficient for model
• Solution: Set CUDA_VISIBLE_DEVICES="" to force CPU mode

Error: Model download fails

• Cause: Network issues or model not found
• Solution: Check internet connection, verify model name

Error: numpy.frombuffer dimension mismatch

• Cause: Embedding stored with different model
• Solution: Regenerate embeddings with consistent model

Error: sqlite3.OperationalError (database is locked)

• Cause: Concurrent write operations
• Solution: Use timeout=30 and batch commits

Error: Memory issues with large batches

• Cause: Batch size too large for available RAM
• Solution: Reduce batch_size to 50 or lower

Metrics

Performance Tips

1. CPU vs GPU

# Force CPU (more stable, sufficient for most cases)
os.environ['CUDA_VISIBLE_DEVICES'] = ''

# Use GPU if available
# Remove the above line and ensure CUDA is installed

2. Batch Processing

# Larger batches = faster but more memory
batch_size = 100  # Default
batch_size = 500  # If you have 16GB+ RAM
batch_size = 50   # If memory constrained

3. Progress Tracking

from tqdm import tqdm

for i in tqdm(range(0, total, batch_size)):
    # Process batch
    pass

4. Incremental Updates

# Only embed new chunks
cursor.execute('''
    SELECT c.id, c.chunk_text
    FROM chunks c
    LEFT JOIN embeddings e ON c.id = e.chunk_id
    WHERE e.id IS NULL
''')

Best Practices

1. Use normalized embeddings - Set normalize_embeddings=True
2. Force CPU mode - Set CUDA_VISIBLE_DEVICES="" for stability
3. Add SQLite timeout - Use timeout=30 for concurrent access
4. Process in batches - 100-500 chunks per batch
5. Track progress - Save after each batch for resumability
6. Log errors - Capture failures for debugging

Status Monitoring

def get_embedding_status(db_path):
    conn = sqlite3.connect(db_path, timeout=30)
    cursor = conn.cursor()

    cursor.execute('SELECT COUNT(*) FROM chunks')
    total_chunks = cursor.fetchone()[0]

    cursor.execute('SELECT COUNT(*) FROM embeddings')
    embedded = cursor.fetchone()[0]

    conn.close()

    return {
        'total': total_chunks,
        'embedded': embedded,
        'remaining': total_chunks - embedded,
        'progress': f"{100*embedded/total_chunks:.1f}%"
    }

Example Usage

# Generate embeddings
python embed.py --db knowledge.db --batch 100

# Run as background service
./embed-service.sh start

# Check progress
./embed-service.sh status

# Search
python search.py "fatigue analysis requirements"

Related Skills

• knowledge-base-builder - Build the document database first
• rag-system-builder - Add AI Q&A on top of semantic search
• pdf-text-extractor - Extract text from PDFs

Dependencies

pip install sentence-transformers numpy

Optional:

• CUDA toolkit (for GPU acceleration)
• tqdm (for progress bars)

Version History

• 1.1.0 (2026-01-02): Added Quick Start, Execution Checklist, Error Handling, Metrics sections; updated frontmatter with version, category, related_skills
• 1.0.0 (2024-10-15): Initial release with sentence-transformers, cosine similarity search, batch processing