---
title: "VLLMRanker"
id: vllmranker
slug: "/vllmranker"
description: "This component ranks documents based on their similarity to the query using reranker models served with vLLM."
---

# VLLMRanker

This component ranks documents based on their similarity to the query using reranker models served with [vLLM](https://docs.vllm.ai/).

<div className="key-value-table">

|  |  |
| --- | --- |
| **Most common position in a pipeline** | In a query pipeline, after a component that returns a list of documents such as a [Retriever](../retrievers.mdx) |
| **Mandatory init variables**           | `model`: The name of the reranker model served by vLLM                                    |
| **Mandatory run variables**            | `query`: A query string  <br /> <br />`documents`: A list of document objects             |
| **Output variables**                   | `documents`: A list of document objects                                                   |
| **API reference**                      | [vLLM](/reference/integrations-vllm)                                                      |
| **GitHub link**                        | https://github.com/deepset-ai/haystack-core-integrations/tree/main/integrations/vllm      |
| **Package name** | `vllm-haystack` |

</div>

## Overview

[vLLM](https://docs.vllm.ai/) is a high-throughput and memory-efficient inference and serving engine for LLMs. It exposes an HTTP server, which `VLLMRanker` uses to rerank documents through the `/rerank` endpoint.

`VLLMRanker` expects a vLLM server to be running and accessible at the `api_base_url` parameter (by default, `http://localhost:8000/v1`). Use this component after a Retriever in a query pipeline to reorder the retrieved documents by relevance to the query.

You can also specify the `top_k` parameter to set the maximum number of documents to return, and the `score_threshold` parameter to drop documents with a relevance score below a given value.

If the vLLM server was started with `--api-key`, provide the API key through the `VLLM_API_KEY` environment variable or the `api_key` init parameter using Haystack's [Secret](../../concepts/secret-management.mdx) API.

### Compatible models

vLLM supports a range of reranker models. Check the [vLLM supported models docs](https://docs.vllm.ai/en/stable/models/pooling_models/scoring/#supported-models) for the list of supported architectures and models.

### vLLM-specific parameters

You can pass vLLM-specific parameters through the `extra_parameters` dictionary. These are merged into the request body sent to the `/rerank` endpoint. Use this to pass parameters that are not part of the standard rerank API, such as `truncate_prompt_tokens`. See the [vLLM rerank API docs](https://docs.vllm.ai/en/stable/models/pooling_models/scoring/#rerank-api) for details.

```python
ranker = VLLMRanker(
    model="BAAI/bge-reranker-base",
    extra_parameters={"truncate_prompt_tokens": 256},
)
```

### Embedding meta fields

Some use cases benefit from including meta information (such as a title) alongside the document content when reranking. Pass the names of the meta fields to include through the `meta_fields_to_embed` parameter; they will be concatenated with the document content using `meta_data_separator`.

```python
ranker = VLLMRanker(
    model="BAAI/bge-reranker-base",
    meta_fields_to_embed=["title"],
    meta_data_separator="\n",
)
```

## Usage

Install the `vllm-haystack` package to use the `VLLMRanker`:

```shell
pip install vllm-haystack
```

### Starting the vLLM server

Before using this component, start a vLLM server with a reranker model:

```bash
vllm serve BAAI/bge-reranker-base
```

For details on server options, see the [vLLM CLI docs](https://docs.vllm.ai/en/stable/cli/serve/).

### On its own

```python
from haystack import Document
from haystack_integrations.components.rankers.vllm import VLLMRanker

ranker = VLLMRanker(model="BAAI/bge-reranker-base")

docs = [
    Document(content="The capital of Brazil is Brasilia."),
    Document(content="The capital of France is Paris."),
]
result = ranker.run(query="What is the capital of France?", documents=docs)
print(result["documents"][0].content)

# The capital of France is Paris.
```

### In a pipeline

```python
from haystack import Document, Pipeline
from haystack.components.retrievers.in_memory import InMemoryBM25Retriever
from haystack.document_stores.in_memory import InMemoryDocumentStore
from haystack_integrations.components.rankers.vllm import VLLMRanker

docs = [
    Document(content="Paris is in France"),
    Document(content="Berlin is in Germany"),
    Document(content="Lyon is in France"),
]
document_store = InMemoryDocumentStore()
document_store.write_documents(docs)

retriever = InMemoryBM25Retriever(document_store=document_store)
ranker = VLLMRanker(model="BAAI/bge-reranker-base")

document_ranker_pipeline = Pipeline()
document_ranker_pipeline.add_component(instance=retriever, name="retriever")
document_ranker_pipeline.add_component(instance=ranker, name="ranker")

document_ranker_pipeline.connect("retriever.documents", "ranker.documents")

query = "Cities in France"
result = document_ranker_pipeline.run(
    data={
        "retriever": {"query": query, "top_k": 3},
        "ranker": {"query": query, "top_k": 2},
    },
)

print(result["ranker"]["documents"][0])

# Document(id=..., content: 'Paris is in France', score: ...)
```
