deepset-ai--haystack/docs-website/versioned_docs/version-2.21/pipeline-components/retrievers/elasticsearchbm25retriever.mdx

---
title: "ElasticsearchBM25Retriever"
id: elasticsearchbm25retriever
slug: "/elasticsearchbm25retriever"
description: "A keyword-based Retriever that fetches Documents matching a query from the Elasticsearch Document Store."
---

# ElasticsearchBM25Retriever

A keyword-based Retriever that fetches Documents matching a query from the Elasticsearch Document Store.

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

|  |  |
| --- | --- |
| **Most common position in a pipeline** | 1. Before a [`PromptBuilder`](../builders/promptbuilder.mdx)   in a RAG pipeline  2. The last component in the semantic search pipeline  3. Before an [`ExtractiveReader`](../readers/extractivereader.mdx)   in an extractive QA pipeline |
| **Mandatory init variables**           | `document_store`: An instance of [ElasticsearchDocumentStore](../../document-stores/elasticsearch-document-store.mdx)                                                                                                                       |
| **Mandatory run variables**            | `query`: A string                                                                                                                                                                                                       |
| **Output variables**                   | `documents`: A list of documents (matching the query)                                                                                                                                                                   |
| **API reference**                      | [Elasticsearch](/reference/integrations-elasticsearch)                                                                                                                                                                         |
| **GitHub link**                        | https://github.com/deepset-ai/haystack-core-integrations/tree/main/integrations/elasticsearch                                                                                                                         |

</div>

## Overview

`ElasticsearchBM25Retriever` is a keyword-based Retriever that fetches Documents matching a query from an `ElasticsearchDocumentStore`. It determines the similarity between Documents and the query based on the BM25 algorithm, which computes a weighted word overlap between the two strings.

Since the `ElasticsearchBM25Retriever` matches strings based on word overlap, it’s often used to find exact matches to names of persons or products, IDs, or well-defined error messages. The BM25 algorithm is very lightweight and simple. Nevertheless, it can be hard to beat with more complex embedding-based approaches on out-of-domain data.

In addition to the `query`, the `ElasticsearchBM25Retriever` accepts other optional parameters, including `top_k` (the maximum number of Documents to retrieve) and `filters` to narrow down the search space.
When initializing Retriever, you can also adjust how [inexact fuzzy matching](https://www.elastic.co/guide/en/elasticsearch/reference/current/common-options.html#fuzziness) is performed, using the `fuzziness` parameter.

If you want a semantic match between a query and documents, you can use `ElasticsearchEmbeddingRetriever`, which uses vectors created by embedding models to retrieve relevant information.

## Installation

[Install](https://www.elastic.co/guide/en/elasticsearch/reference/current/install-elasticsearch.html) Elasticsearch and then [start](https://www.elastic.co/guide/en/elasticsearch/reference/current/starting-elasticsearch.html) an instance. Haystack supports Elasticsearch 8.

If you have Docker set up, we recommend pulling the Docker image and running it.

```shell
docker pull docker.elastic.co/elasticsearch/elasticsearch:8.11.1
docker run -p 9200:9200 -e "discovery.type=single-node" -e "ES_JAVA_OPTS=-Xms1024m -Xmx1024m" -e "xpack.security.enabled=false" elasticsearch:8.11.1
```

As an alternative, you can go to [Elasticsearch integration GitHub](https://github.com/deepset-ai/haystack-core-integrations/tree/main/integrations/elasticsearch) and start a Docker container running Elasticsearch using the provided `docker-compose.yml`:

```shell
docker compose up
```

Once you have a running Elasticsearch instance, install the `elasticsearch-haystack` integration:

```shell
pip install elasticsearch-haystack
```

## Usage

### On its own

```python
from haystack import Document
from haystack_integrations.components.retrievers.elasticsearch import (
    ElasticsearchBM25Retriever,
)
from haystack_integrations.document_stores.elasticsearch import (
    ElasticsearchDocumentStore,
)
from elasticsearch import Elasticsearch

document_store = ElasticsearchDocumentStore(hosts="http://localhost:9200/")
documents = [
    Document(content="There are over 7,000 languages spoken around the world today."),
    Document(
        content="Elephants have been observed to behave in a way that indicates a high level of self-awareness, such as recognizing themselves in mirrors.",
    ),
    Document(
        content="In certain parts of the world, like the Maldives, Puerto Rico, and San Diego, you can witness the phenomenon of bioluminescent waves.",
    ),
]
document_store.write_documents(documents=documents)

retriever = ElasticsearchBM25Retriever(document_store=document_store)
retriever.run(query="How many languages are spoken around the world today?")
```

### In a RAG pipeline

Set your `OPENAI_API_KEY` as an environment variable and then run the following code:

```python

from haystack_integrations.components.retrievers.elasticsearch import (
    ElasticsearchBM25Retriever,
)
from haystack_integrations.document_stores.elasticsearch import (
    ElasticsearchDocumentStore,
)

from elasticsearch import Elasticsearch

from haystack import Document
from haystack import Pipeline
from haystack.components.builders.answer_builder import AnswerBuilder
from haystack.components.builders.prompt_builder import PromptBuilder
from haystack.components.generators import OpenAIGenerator
from haystack.document_stores.types import DuplicatePolicy

import os

api_key = os.environ["OPENAI_API_KEY"]

## Create a RAG query pipeline
prompt_template = """
    Given these documents, answer the question.\nDocuments:
    {% for doc in documents %}
        {{ doc.content }}
    {% endfor %}

    \nQuestion: {{question}}
    \nAnswer:
    """

document_store = ElasticsearchDocumentStore(hosts="http://localhost:9200/")

## Add Documents

documents = [
    Document(content="There are over 7,000 languages spoken around the world today."),
    Document(
        content="Elephants have been observed to behave in a way that indicates a high level of self-awareness, such as recognizing themselves in mirrors.",
    ),
    Document(
        content="In certain parts of the world, like the Maldives, Puerto Rico, and San Diego, you can witness the phenomenon of bioluminescent waves.",
    ),
]

## DuplicatePolicy.SKIP param is optional, but useful to run the script multiple times without throwing errors
document_store.write_documents(documents=documents, policy=DuplicatePolicy.SKIP)

retriever = ElasticsearchBM25Retriever(document_store=document_store)
rag_pipeline = Pipeline()
rag_pipeline.add_component(name="retriever", instance=retriever)
rag_pipeline.add_component(
    instance=PromptBuilder(template=prompt_template),
    name="prompt_builder",
)
rag_pipeline.add_component(instance=OpenAIGenerator(api_key=api_key), name="llm")
rag_pipeline.add_component(instance=AnswerBuilder(), name="answer_builder")
rag_pipeline.connect("retriever", "prompt_builder.documents")
rag_pipeline.connect("prompt_builder", "llm")
rag_pipeline.connect("llm.replies", "answer_builder.replies")
rag_pipeline.connect("llm.meta", "answer_builder.meta")
rag_pipeline.connect("retriever", "answer_builder.documents")

question = "How many languages are spoken around the world today?"
result = rag_pipeline.run(
    {
        "retriever": {"query": question},
        "prompt_builder": {"question": question},
        "answer_builder": {"query": question},
    },
)
print(result["answer_builder"]["answers"][0].data)
```

Here’s an example output you might get:

```python
"Over 7,000 languages are spoken around the world today"
```