# Rerank Service — INDEX

Cross-encoder reranking API for DIDI semantic search precision. Serves the `BAAI/bge-reranker-v2-m3` model behind a Cohere/Jina-compatible HTTP API. Used after kNN retrieval to score `(query, candidate)` pairs and reorder evidence by true relevance, instead of relying solely on pgvector cosine similarity from embeddings.

- **Stack:** Python 3.10+, FastAPI, Uvicorn, Pydantic v2, httpx, vLLM (or llama.cpp) backend
- **URLs:**
  - Dev API:  `http://10.11.10.12:54200`
  - Prod API: `http://10.11.10.12:14200`
  - vLLM internal: `:54201` (Dev) / `:14201` (Prod)
  - llama.cpp internal: `:54210` (Dev) / `:14210` (Prod)
- **Containers:** `didiAI-rerank-api` (FastAPI wrapper) + `didiAI-rerank-vllm` (or `didiAI-rerank-llamacpp`)
- **Model:** `BAAI/bge-reranker-v2-m3` (multilingual cross-encoder, ~8K context window)
- **Port schema:** `x42xx` family (Reranking)

---

## Ce face

Re-scores candidate documents against a query using a cross-encoder model — directly attending over both texts together — for higher precision than embedding cosine similarity alone. The cross-encoder gives a true relevance score in `[0..1]`.

**Pipeline role inside DIDI:**

1. didi-brain `/v1/gather` runs an initial kNN retrieval over pgvector embeddings (dim 1024, separate `embeddings` module).
2. The top-K candidates (typically 50–200) are passed to this rerank service.
3. The reranker scores each `(query, candidate)` pair and returns a relevance-sorted list.
4. didi-brain takes the top-N (e.g. top 10) reranked items as final evidence.

This two-stage retrieval (kNN → cross-encoder rerank) is the standard high-quality semantic search pattern: embeddings handle scale, the cross-encoder handles precision.

---

## API endpoints

Cohere/Jina-compatible. Auth is optional (Bearer token) — enabled if `RERANK_API_TOKENS` is set. Health endpoints stay public.

### `POST /v1/rerank` (alias `POST /v2/rerank`)

Rerank a list of documents against a query.

**Request body:**
```json
{
  "model": "BAAI/bge-reranker-v2-m3",
  "query": "What is machine learning?",
  "documents": ["Machine learning is...", "Cats are pets", "Deep learning..."],
  "top_n": 3,
  "return_documents": false,
  "backend": null
}
```

| Field | Type | Required | Description |
|-------|------|----------|-------------|
| `model` | string | yes | Model identifier (e.g. `BAAI/bge-reranker-v2-m3`) |
| `query` | string | yes | Search query |
| `documents` | string[] | yes | Documents to rerank (1–1000) |
| `top_n` | int | no | Return only top N results (default: all) |
| `return_documents` | bool | no | Echo document text in response |
| `backend` | string | no | Override default: `"vllm"` or `"llamacpp"` |

**Response:**
```json
{
  "id": "rerank-abc123def456",
  "model": "BAAI/bge-reranker-v2-m3",
  "results": [
    {"index": 2, "relevance_score": 0.9523, "document": null},
    {"index": 0, "relevance_score": 0.8876, "document": null}
  ],
  "usage": {"total_tokens": 150},
  "backend": "vllm"
}
```

### `GET /v1/models`

List loaded models. Optional `?backend=vllm|llamacpp` filter.

### `GET /v1/backends`

List enabled backends, e.g. `{"backends": ["vllm", "llamacpp"]}`.

### `GET /health`

Detailed per-backend health. Status is `healthy` / `degraded` / `unhealthy`.

### `GET /ready`

Simple K8s-style readiness probe — returns `{"ready": true}`.

### Error codes

`400` bad request · `401` auth failed · `429` rate-limited (with `Retry-After` header) · `503` backend unavailable · `504` backend timeout. All errors return `{"detail": "..."}`. Every response carries an `X-Request-ID` header for tracing.

Full reference in `API.md`.

---

## Backends

The API is a thin FastAPI router in front of one of two cross-encoder servers. Both backends speak OpenAI-compatible HTTP, so the wrapper unifies them.

| Backend | When | Pros | Cons |
|---------|------|------|------|
| **vLLM** | Production, GPU host | High throughput, batched scoring, lowest latency under load | Requires NVIDIA GPU + CUDA 12.x |
| **llama.cpp** | Dev / CPU fallback | Runs on CPU or modest GPU, GGUF quantized models, low memory | Lower throughput |

The wrapper routes requests via a backend registry. Set `RERANK_DEFAULT_BACKEND` to choose, or override per-request with the `backend` field in the body. Both can be enabled simultaneously (`RERANK_ENABLE_VLLM=true`, `RERANK_ENABLE_LLAMACPP=true`).

vLLM is launched with `--task score` (vLLM 0.8.x cross-encoder mode). llama.cpp is launched with `--reranking`.

---

## How didi-brain uses it

didi-brain's gather/retrieval flow:

1. `services/gather.py` builds an initial candidate set via pgvector kNN over the `embeddings` module (1024-dim vectors).
2. Calls the reranker via `shared/reranker_client.py` — typically wrapping the `/v1/rerank` endpoint with the brain's HTTPS/auth config.
3. Picks the top-N reranked candidates as final evidence chunks.
4. Passes them to the LLM as grounded context.

The reranker is therefore in the critical path of every brain `/v1/gather` call. Its latency budget is small (a few hundred ms), so vLLM batching matters in production.

---

## Structura fisiere

```
rerank/
├── README.md                # Quick start, install, env vars table
├── API.md                   # Full HTTP API reference + curl/Python examples
├── INDEX.md                 # This file
├── pyproject.toml           # Package metadata, deps (fastapi, httpx, openai SDK)
├── .env.example             # All RERANK_* env vars documented
├── deploy/
│   ├── Dockerfile           # API wrapper image
│   ├── docker-compose.yml   # api / vllm / llamacpp profiles
│   └── deploy.sh            # Helper: ./deploy.sh --profile vllm -d
├── src/rerank/
│   ├── __init__.py          # Public exports (RerankClient, ...)
│   ├── cli.py               # `rerank` entrypoint — `python -m rerank.cli`
│   ├── client.py            # RerankClient (Python SDK to call this API)
│   ├── config.py            # Pydantic Settings, RERANK_ env prefix
│   ├── schemas.py           # Pydantic request/response models
│   ├── exceptions.py        # Custom error types
│   ├── logging.py           # Structured logging setup
│   ├── types.py             # Backend literal types
│   ├── api/
│   │   ├── app.py           # FastAPI app factory
│   │   ├── dependencies.py  # Auth + rate-limit DI
│   │   ├── middleware.py    # X-Request-ID, rate limit, error handlers
│   │   └── routes/
│   │       ├── rerank.py    # POST /v1/rerank, /v2/rerank
│   │       ├── models.py    # GET /v1/models, /v1/backends
│   │       └── health.py    # GET /health, /ready
│   └── backends/
│       ├── base.py          # Abstract BackendBase (rerank, health, list_models)
│       ├── vllm_backend.py  # vLLM via OpenAI SDK (--task score)
│       ├── llamacpp_backend.py # llama.cpp --reranking endpoint
│       └── registry.py      # Backend registry + selector
└── tests/
    ├── conftest.py
    ├── test_config.py
    └── test_schemas.py
```

---

## Configuration

All env vars use the `RERANK_` prefix and are read via Pydantic Settings (`config.py`).

**Required (no defaults):**

| Var | Example |
|-----|---------|
| `RERANK_DEFAULT_BACKEND` | `vllm` or `llamacpp` |
| `RERANK_ENABLE_VLLM` | `true` / `false` |
| `RERANK_ENABLE_LLAMACPP` | `true` / `false` |
| `RERANK_EXTERNAL_URL` | `http://localhost:14200` (used in OpenAPI spec) |

**API server:**

| Var | Default | Notes |
|-----|---------|-------|
| `RERANK_HOST` | `0.0.0.0` | |
| `RERANK_PORT` | `14200` | Prod 14200 / Dev 54200 |
| `RERANK_API_TOKENS` | (empty) | Comma-separated; if empty, auth is disabled |
| `RERANK_RATE_LIMIT_RPS` | `20.0` | |
| `RERANK_RATE_LIMIT_BURST` | `50` | |
| `RERANK_MAX_CONCURRENT_RERANKS` | `20` | |
| `RERANK_REQUEST_TIMEOUT` | `120.0` s | |
| `RERANK_CONNECT_TIMEOUT` | `10.0` s | |
| `RERANK_LOG_LEVEL` | `INFO` | |
| `RERANK_LOG_JSON` | `false` | |

**Backend base URLs (app settings — fields on `RerankSettings`):**

| Var | Default | Notes |
|-----|---------|-------|
| `RERANK_VLLM_BASE_URL` | `http://localhost:54201` | Where the API wrapper reaches vLLM |
| `RERANK_VLLM_API_KEY` | (empty) | Optional API key for the vLLM server |
| `RERANK_LLAMACPP_BASE_URL` | `http://localhost:54210` | Where the API wrapper reaches llama.cpp |

> Only `*_BASE_URL` / `*_API_KEY` are read by the API app (`config.py`). The model id, port, GPU and context knobs below are **not** `RerankSettings` fields.

**Backend service variables (docker-compose only):**

These are consumed by `deploy/docker-compose.yml` to launch the vLLM / llama.cpp containers — they configure the backend server, not the API app.

| Var | Default | Description |
|-----|---------|-------------|
| `RERANK_VLLM_MODEL` | `BAAI/bge-reranker-v2-m3` | HF model id loaded by the vLLM container |
| `RERANK_VLLM_PORT` | `14201` | Host port mapped to the vLLM container |
| `RERANK_VLLM_GPU` | `0` | `CUDA_VISIBLE_DEVICES` for the vLLM container |
| `RERANK_VLLM_GPU_UTIL` | `0.50` | vLLM `--gpu-memory-utilization` |
| `RERANK_VLLM_MAX_LEN` | `8192` | vLLM `--max-model-len` |
| `RERANK_LLAMACPP_MODEL` | `bge-reranker-v2-m3-q4_k_m.gguf` | GGUF filename inside `MODELS_DIR` |
| `RERANK_LLAMACPP_PORT` | `14210` | Host port mapped to the llama.cpp container |
| `RERANK_LLAMACPP_CTX` | `8192` | llama.cpp context size |
| `RERANK_LLAMACPP_THREADS` | `4` | llama.cpp threads |
| `RERANK_LLAMACPP_PARALLEL` | `4` | llama.cpp parallel slots |
| `MODELS_DIR` | `/cai2_ds_storage/models` | GGUF model directory for llama.cpp |
| `HF_CACHE_DIR` | `/cai2_ds_storage/hf_cache` | Mounted into the vLLM container |
| `HF_TOKEN` | (unset) | Forwarded as `HUGGING_FACE_HUB_TOKEN` |

Full list with comments in `.env.example`.

---

## Performance / model

- **Model:** `BAAI/bge-reranker-v2-m3` — multilingual cross-encoder, ~8K context window, ~568M params.
- **Output:** single relevance score per `(query, document)` pair, normalized to `[0..1]`.
- **Expected latency:** roughly 100–300 ms for ~20 candidates per query on a single mid-range GPU with vLLM batching; scales sublinearly with batch size.
- **Throughput:** dominated by GPU memory and `RERANK_VLLM_GPU_UTIL` / `RERANK_MAX_CONCURRENT_RERANKS`. vLLM continuous batching helps a lot under concurrent load.
- **Memory:** vLLM defaults to 50% GPU memory util (`RERANK_VLLM_GPU_UTIL=0.50`), so it can co-host on a GPU shared with other modules. llama.cpp Q4_K_M quantization fits comfortably on CPU.

---

## Deployment

Docker Compose with three profiles in `deploy/docker-compose.yml`:

```bash
cd /home/admin365/didi_mono/ai_platform/modules/rerank/deploy

# Production (GPU + vLLM)
./deploy.sh --profile vllm -d

# Lightweight / no-GPU
./deploy.sh --profile llamacpp -d

# API only (external rerank servers running elsewhere)
./deploy.sh --profile api -d

# Logs / stop
./deploy.sh --profile vllm --logs
./deploy.sh --profile vllm --down
```

Containers:
- `didiAI-rerank-api` — FastAPI wrapper, port `RERANK_PORT` (14200/54200)
- `didiAI-rerank-vllm` — `vllm/vllm-openai:v0.8.5`, GPU device pinned via `RERANK_VLLM_GPU`, internal port 14201
- `didiAI-rerank-llamacpp` — `ghcr.io/ggml-org/llama.cpp:server-b4769`, internal port 8080 → host 14210/54210

All services share the external `didi-network` Docker network so other DIDI modules (didi-brain, embeddings) can reach them by container name.

Health gates: API has a 30-s `/health` healthcheck; vLLM has a 5-min start period to allow model load on first boot.

---

## Related

- **Embeddings module** (`ai_platform/modules/embeddings`) — separate service, dim 1024, used for the initial pgvector kNN stage that feeds this reranker.
- **didi-brain** (`ai_platform/modules/didi_brain/brain_api`) — main consumer; `services/gather.py` calls rerank after kNN; `shared/reranker_client.py` is the helper.
- **GPU host:** `10.11.10.17` / `10.11.10.12` (see project network architecture doc).