Vector Similarity Search Service

An educational HTTP service for vector similarity search using BGE-M3 embeddings with configurable ANNOY or HNSW indexing backends.

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命令行工具：稠密检索与 ANN 对比（cli.py，实验 3-4）

除了上面的 HTTP 服务，本项目还提供一个开箱即用、可离线复现的命令行工具 cli.py， 把书中实验 3-4 的两个观察点直接跑成可量化的数字，无需先启动服务：

1. 稠密嵌入检索的语义能力——在带标注的小型语料上计算 recall@k / precision@k / MRR；
2. ANN 索引后端对比（实验 3-4 的重点）——复用服务端 indexing.py 里的 ANNOY / HNSW 实现，测量二者相对精确暴力检索的召回率、建索引耗时与查询延迟。

用法

# 1) 单条稠密查询（默认查询 "a cat playing"，需要嵌入模型）
python cli.py -q "model distillation" -k 3

# 2) 检索质量评测：recall@k / precision@k / MRR
python cli.py --eval

# 2') 离线复现：用已缓存的小模型（无需下载 2.3GB 的 BGE-M3）
python cli.py --embedding-model sentence-transformers/all-MiniLM-L6-v2 --eval

# 3) ANN 后端对比（合成向量，完全离线、无需任何模型）
python cli.py --compare-ann -k 10
python cli.py --compare-ann --backend hnsw --hnsw-ef-search 200 -k 10   # 调 ef_search 看召回随之上升

# 自定义语料 / 标注 / 输出
python cli.py --corpus my.json --labels my_labels.json --eval -o result.json

python cli.py --help 提供完整的中文参数说明（--corpus / --query / --embedding-model / --top-k / --output，以及 ANN 对比的各项索引超参）。

常用参数

参数	说明
-q, --query	查询字符串（默认 a cat playing）
-c, --corpus	语料文件（.json 数组 或 .jsonl 每行一篇）；缺省用内置示例语料
-k, --top-k	返回前 k 条结果（默认 5）
-o, --output	把结果 / 评测指标写入 JSON 文件
--embedding-model	嵌入模型名（默认 BAAI/bge-m3；离线可用 sentence-transformers/all-MiniLM-L6-v2）
--pooling	池化方式 auto（bge* 用 cls，其余 mean）/ mean / cls
--eval	在标注集上评测 recall@k / precision@k / MRR
--compare-ann	对比 ANNOY / HNSW（合成向量，无需模型）
--ann-base / --ann-dim / --ann-queries	合成底库规模 / 维度 / 查询数（默认 3000 / 128 / 100）
--annoy-n-trees / --hnsw-M / --hnsw-ef-search	两类 ANN 的关键索引超参

实测结果（真实运行，非杜撰）

稠密检索质量（内置 12 篇语料，all-MiniLM-L6-v2，离线）：

宏平均  recall@5=1.000  precision@5=0.320  MRR=1.000

其中查询 a cat playing 的相关文档只用 kitten / feline 表达、不含字面 "cat"， 稠密检索仍把它们排到第 1、2 名——这正是稠密相对稀疏 BM25（实验 3-5 会漏召回）的语义优势。

ANN 后端对比（3000 条 128 维随机单位向量，100 条查询，top-10）：HNSW 的召回率随 ef_search 单调上升，体现"精度 / 速度"取舍：

配置	recall@10	平均查询延迟
HNSW ef_search=20	0.562	0.05 ms
HNSW ef_search=200	0.991	0.25 ms

环境提示：本工具对每个后端会先做"自查询自身向量"的健康检查。在部分 macOS/arm64 环境下， annoy==1.17.3 的预编译轮子存在缺陷（连查询库中已有向量都只返回它自己），此时工具会打印 [警告] ...疑似当前环境下损坏 并把该后端的数字标记为不可信。HNSW 不受影响。若要复现完整的 ANNOY vs HNSW 对比，请在 annoy 正常工作的环境（如 Linux x86_64）中运行。

Features

• BGE-M3 Model: State-of-the-art multilingual embedding model supporting:
• Dense embeddings for semantic search
• Multi-language support (100+ languages)

• Long context (up to 8192 tokens)

• Dual Indexing Backends:

• ANNOY (Approximate Nearest Neighbors Oh Yeah): Fast, memory-efficient tree-based index

• HNSW (Hierarchical Navigable Small World): High-precision graph-based index

• Educational Logging: Extensive debug logs showing:

• Embedding generation process
• Index operations (insert/delete/search)
• Performance metrics

• Vector statistics

• RESTful API: Clean HTTP endpoints for:

• Document indexing (insert/update)
• Document deletion
• Similarity search

• Statistics and monitoring

• In-Memory Storage: Pure in-memory operation for simplicity (no persistence)

Architecture

┌──────────────────┐
│   HTTP Client    │
└────────┬─────────┘
         │
         ▼
┌──────────────────┐
│  FastAPI Server  │
└────────┬─────────┘
         │
         ▼
    ┌────┴────┐
    │         │
    ▼         ▼
┌──────────┐ ┌──────────────┐
│ Document │ │  Embedding   │
│  Store   │ │   Service    │
└──────────┘ │  (BGE-M3)    │
             └──────┬─────────┘
                    │
                    ▼
          ┌─────────┴──────────┐
          │                    │
          ▼                    ▼
    ┌──────────┐        ┌──────────┐
    │  ANNOY   │        │   HNSW   │
    │  Index   │        │  Index   │
    └──────────┘        └──────────┘

Installation

Prerequisites

• Python 3.8 or higher
• macOS (optimized for M1/M2 chips) or Linux
• At least 4GB RAM (8GB recommended)
• CUDA-compatible GPU (optional, for faster embedding generation)

Setup

1. Install dependencies:

cd projects/week3/dense-embedding
pip install -r requirements.txt

1. Download BGE-M3 model (will be downloaded automatically on first run):
2. Model size: ~2.3GB
3. Will be cached in HuggingFace cache directory

Usage

Starting the Service

With HNSW Index (Default)

python main.py

With ANNOY Index

python main.py --index-type annoy

With Custom Configuration

python main.py --index-type hnsw --host 0.0.0.0 --port 4242 --debug --show-embeddings

Available Options

• --index-type: Choose index backend (annoy or hnsw, default: hnsw)
• --host: Server host (default: 0.0.0.0)
• --port: Server port (default: 4240)
• --debug: Enable debug mode with verbose logging
• --show-embeddings: Show embedding vectors in logs (educational)

API Documentation

Once the service is running, visit: - Interactive docs: http://localhost:4240/docs - OpenAPI schema: http://localhost:4240/openapi.json

API Endpoints

1. Index Document

POST /index

Index a new document or update existing one.

Request:

{
  "text": "Machine learning is a subset of artificial intelligence.",
  "doc_id": "doc_001",  // Optional, auto-generated if not provided
  "metadata": {         // Optional metadata
    "category": "AI",
    "author": "John Doe"
  }
}

Response:

{
  "success": true,
  "doc_id": "doc_001",
  "message": "Document indexed successfully using hnsw",
  "index_size": 1
}

2. Search Documents

POST /search

Search for similar documents.

Request:

{
  "query": "What is deep learning?",
  "top_k": 5,
  "return_documents": true
}

Response:

{
  "success": true,
  "query": "What is deep learning?",
  "results": [
    {
      "doc_id": "doc_001",
      "score": 0.8543,
      "text": "Machine learning is a subset...",
      "metadata": {"category": "AI"},
      "rank": 1
    }
  ],
  "total_results": 5,
  "search_time_ms": 12.5
}

3. Delete Document

DELETE /index

Delete a document from the index.

Request:

{
  "doc_id": "doc_001"
}

Response:

{
  "success": true,
  "message": "Document doc_001 deleted successfully",
  "index_size": 0
}

4. Get Statistics

GET /stats

Get service statistics.

Response:

{
  "index_type": "hnsw",
  "index_size": 100,
  "document_count": 100,
  "embedding_dimension": 1024,
  "model_name": "BAAI/bge-m3"
}

5. List Documents

GET /documents?limit=10

List documents in the store.

Testing

Run Demo Client

The demo client showcases all features with sample documents:

python test_client.py

Run Performance Test

Test indexing and search performance with synthetic data:

python test_client.py --performance

Manual Testing with curl

Index a document:

curl -X POST http://localhost:4240/index \
  -H "Content-Type: application/json" \
  -d '{"text": "This is a test document about machine learning."}'

Search for similar documents:

curl -X POST http://localhost:4240/search \
  -H "Content-Type: application/json" \
  -d '{"query": "artificial intelligence", "top_k": 5}'

Index Comparison

ANNOY (Approximate Nearest Neighbors Oh Yeah)

Pros: - Very fast indexing - Low memory footprint - Good for static datasets - Supports multiple distance metrics

Cons: - Requires rebuild for deletion - No incremental updates after build - Trade-off between speed and accuracy (controlled by n_trees)

Best for: - Large-scale similarity search - Read-heavy workloads - Memory-constrained environments

HNSW (Hierarchical Navigable Small World)

Pros: - High recall accuracy - Supports incremental updates - Fast search with good precision - Supports soft deletion

Cons: - Higher memory usage - Slower indexing than ANNOY - More complex parameter tuning

Best for: - Dynamic datasets - High-precision requirements - Balanced read/write workloads

Configuration

Environment Variables

You can configure the service using environment variables with the VEC_ prefix:

export VEC_INDEX_TYPE=hnsw
export VEC_MODEL_NAME=BAAI/bge-m3
export VEC_USE_FP16=true
export VEC_MAX_SEQ_LENGTH=512
export VEC_MAX_DOCUMENTS=100000
export VEC_LOG_LEVEL=DEBUG

# ANNOY specific
export VEC_ANNOY_N_TREES=50
export VEC_ANNOY_METRIC=angular

# HNSW specific
export VEC_HNSW_EF_CONSTRUCTION=200
export VEC_HNSW_M=32
export VEC_HNSW_EF_SEARCH=100
export VEC_HNSW_SPACE=cosine

Educational Features

This service includes extensive logging for educational purposes:

1. Embedding Generation Logs: Shows the process of converting text to vectors
2. Index Operation Logs: Detailed information about index updates
3. Search Process Logs: Step-by-step search execution
4. Performance Metrics: Timing information for all operations
5. Vector Statistics: Min/max/mean values of embeddings (when enabled)

Enable full educational logging:

python main.py --debug --show-embeddings

Performance Considerations

Memory Usage

• BGE-M3 model: ~2.3GB
• Per document overhead: ~4KB (1024-dim float32 embedding)
• ANNOY index: ~(4 * dimension * n_items * n_trees / 2) bytes
• HNSW index: ~(4 * dimension * n_items * M * 2) bytes

Optimization Tips

1. For ANNOY:
2. Increase n_trees for better accuracy (slower build)
3. Use angular metric for normalized vectors

4. Build index after batch insertions

5. For HNSW:

6. Increase M for better recall (more memory)
7. Increase ef_construction for better index quality (slower build)

8. Adjust ef_search for speed/accuracy trade-off

9. General:

10. Use FP16 for faster inference (slight accuracy loss)
11. Batch document insertions when possible
12. Limit max_seq_length based on your documents

Troubleshooting

Common Issues

1. Out of Memory:
2. Reduce batch size
3. Use FP16 mode
4. Lower max_seq_length

5. Use ANNOY instead of HNSW

6. Slow Indexing:

7. Reduce HNSW ef_construction
8. Reduce ANNOY n_trees

9. Use GPU if available

10. Poor Search Quality:

11. Increase ANNOY n_trees
12. Increase HNSW M and ef_search
13. Check if documents are too short/long

References

• BGE-M3 Paper
• BGE-M3 Model
• ANNOY Library
• HNSWlib
• FastAPI Documentation

License

This is an educational project for learning purposes.