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SearXNG BM25 Reranker

An external SearXNG plugin that reranks search results using BM25 text relevance scoring to improve search quality.

Features

• BM25F Multi-Field Weighting — Title weight 2.0, content weight 1.0, prioritizing title-matching results
• RRF Fusion Ranking — Combines BM25 scores with original engine rankings via Reciprocal Rank Fusion, rather than replacing them
• CJK Tokenization — Built-in zero-dependency CJK tokenizer (unigram + bigram), no jieba required
• Zero External Dependencies — Core BM25 engine from zerodep/sparse_search, pure standard library
• Plug and Play — Standard SearXNG external plugin, deployable via volume mount or pip install

How It Works

Search request → Engines return results → [post_search hook]
                                                ↓
                                   Build temporary BM25F index (title + content)
                                                ↓
                                   BM25 retrieval with original query
                                                ↓
                                   RRF fusion (engine ranking + BM25 ranking, k=60)
                                                ↓
                                   Rewrite positions to influence scoring → Reranked results

The plugin hooks into the post_search phase, after all engine results are collected but before final scores are calculated. By rewriting each result's positions list, it influences SearXNG's built-in calculate_score() formula (weight / position), achieving non-invasive reranking.

Installation

Option 1: Volume Mount (Recommended for Quick Deployment)

1. Clone and copy the plugin code to your server:

git clone https://github.com/Oaklight/searxng-bm25-reranker.git
cp -r searxng-bm25-reranker/src/searxng_bm25_reranker /path/to/plugins/

2. Update compose.yaml:

services:
  searxng:
    volumes:
      - /path/to/plugins:/usr/local/searxng/plugins:ro
    environment:
      - PYTHONPATH=/usr/local/searxng/plugins

3. Register the plugin in settings.yml:

plugins:
  searxng_bm25_reranker.SXNGPlugin:
    active: true

4. Restart the container:

docker compose restart searxng

Option 2: pip Install (For Custom Images)

FROM searxng/searxng:latest
RUN pip install --no-cache-dir searxng-bm25-reranker

You still need to register the plugin in settings.yml.

Configuration

The plugin works out of the box with no additional configuration. Default parameters:

Project Structure

src/searxng_bm25_reranker/
├── __init__.py          # SXNGPlugin class, post_search reranking logic
├── _tokenizer.py        # CJK-aware tokenizer (unigram + bigram)
└── _vendor/
    └── sparse_search.py # BM25 engine from zerodep (vendored)

Acknowledgements

• SearXNG — Privacy-respecting metasearch engine
• zerodep/sparse_search — Zero-dependency BM25 full-text search engine

License

AGPL-3.0