Open In Colab

BM25 Retriever#

In this guide, we define a bm25 retriever that search documents using bm25 method.

This notebook is very similar to the RouterQueryEngine notebook.


If you’re opening this Notebook on colab, you will probably need to install LlamaIndex 🦙.

%pip install llama-index-llms-openai
%pip install llama-index-retrievers-bm25
!pip install llama-index
# NOTE: This is ONLY necessary in jupyter notebook.
# Details: Jupyter runs an event-loop behind the scenes.
#          This results in nested event-loops when we start an event-loop to make async queries.
#          This is normally not allowed, we use nest_asyncio to allow it for convenience.
import nest_asyncio

import os
import openai

os.environ["OPENAI_API_KEY"] = "sk-..."
openai.api_key = os.environ["OPENAI_API_KEY"]
import logging
import sys

logging.basicConfig(stream=sys.stdout, level=logging.INFO)
logging.getLogger().handlers = []

from llama_index.core import (
from llama_index.retrievers.bm25 import BM25Retriever
from llama_index.core.retrievers import VectorIndexRetriever
from llama_index.core.node_parser import SentenceSplitter
from llama_index.llms.openai import OpenAI

Download Data#

!mkdir -p 'data/paul_graham/'
!wget '' -O 'data/paul_graham/paul_graham_essay.txt'

Load Data#

We first show how to convert a Document into a set of Nodes, and insert into a DocumentStore.

# load documents
documents = SimpleDirectoryReader("./data/paul_graham").load_data()
# initialize LLM + node parser
llm = OpenAI(model="gpt-4")
splitter = SentenceSplitter(chunk_size=1024)

nodes = splitter.get_nodes_from_documents(documents)
# initialize storage context (by default it's in-memory)
storage_context = StorageContext.from_defaults()
index = VectorStoreIndex(
HTTP Request: POST "HTTP/1.1 200 OK"

BM25 Retriever#

We will search document with bm25 retriever.

# We can pass in the index, doctore, or list of nodes to create the retriever
retriever = BM25Retriever.from_defaults(nodes=nodes, similarity_top_k=2)
from llama_index.core.response.notebook_utils import display_source_node

# will retrieve context from specific companies
nodes = retriever.retrieve("What happened at Viaweb and Interleaf?")
for node in nodes:

Node ID: 9afb8cb9-42f3-4160-807c-1cd6685fa774
Similarity: 1.6781810094192822
Text: Now that I could write essays again, I wrote a bunch about topics I’d had stacked up. I kept writ…

Node ID: 71de4371-10ff-4d3a-8a31-14b2d4ddec83
Similarity: 1.546534805781164
Text: I couldn’t have put this into words when I was 18. All I knew at the time was that I kept taking …

nodes = retriever.retrieve("What did Paul Graham do after RISD?")
for node in nodes:

Node ID: 42c88008-dd05-4590-8fd1-df85567579ea
Similarity: 5.389397745654172
Text: It was missing a lot of things you’d want in a programming language. So these had to be added, an…

Node ID: 4d98c2ad-60cc-4cc0-abe2-b95c0c4be87b
Similarity: 1.141523170922594
Text: Painting students were supposed to express themselves, which to the more worldly ones meant to tr…

Router Retriever with bm25 method#

Now we will combine bm25 retriever with vector index retriever.

from import RetrieverTool

vector_retriever = VectorIndexRetriever(index)
bm25_retriever = BM25Retriever.from_defaults(nodes=nodes, similarity_top_k=2)

retriever_tools = [
        description="Useful in most cases",
        description="Useful if searching about specific information",
from llama_index.core.retrievers import RouterRetriever

retriever = RouterRetriever.from_defaults(
# will retrieve all context from the author's life
nodes = retriever.retrieve(
    "Can you give me all the context regarding the author's life?"
for node in nodes:
HTTP Request: POST "HTTP/1.1 200 OK"
Selecting retriever 0: The author's life context is a broad topic and can be useful in most cases..
HTTP Request: POST "HTTP/1.1 200 OK"

Node ID: 9afb8cb9-42f3-4160-807c-1cd6685fa774
Similarity: 0.7961776744788842
Text: Now that I could write essays again, I wrote a bunch about topics I’d had stacked up. I kept writ…

Node ID: b1ffb78d-dc4c-439b-906a-cca73b713940
Similarity: 0.7924813308773564
Text: We actually had one of those little stoves, fed with kindling, that you see in 19th century studi…

Advanced - Hybrid Retriever + Re-Ranking#

Here we extend the base retriever class and create a custom retriever that always uses the vector retriever and BM25 retreiver.

Then, nodes can be re-ranked and filtered. This lets us keep intermediate top-k values large and letting the re-ranking filter out un-needed nodes.

To best demonstrate this, we will use a larger set of source documents – Chapter 3 from the 2022 IPCC Climate Report.

Setup data#

!curl --output IPCC_AR6_WGII_Chapter03.pdf
# !pip install pypdf
from llama_index.core import (
from llama_index.core.node_parser import SentenceSplitter
from llama_index.llms.openai import OpenAI

# load documents
documents = SimpleDirectoryReader(
# initialize llm + node parser
# -- here, we set a smaller chunk size, to allow for more effective re-ranking
llm = OpenAI(model="gpt-3.5-turbo")
splitter = SentenceSplitter(chunk_size=256)
# limit to a smaller section
nodes = splitter.get_nodes_from_documents(
# initialize storage context (by default it's in-memory)
storage_context = StorageContext.from_defaults()
index = VectorStoreIndex(nodes, storage_context=storage_context)
from llama_index.retrievers.bm25 import BM25Retriever

# retireve the top 10 most similar nodes using embeddings
vector_retriever = index.as_retriever(similarity_top_k=10)

# retireve the top 10 most similar nodes using bm25
bm25_retriever = BM25Retriever.from_defaults(nodes=nodes, similarity_top_k=10)

Custom Retriever Implementation#

from llama_index.core.retrievers import BaseRetriever

class HybridRetriever(BaseRetriever):
    def __init__(self, vector_retriever, bm25_retriever):
        self.vector_retriever = vector_retriever
        self.bm25_retriever = bm25_retriever

    def _retrieve(self, query, **kwargs):
        bm25_nodes = self.bm25_retriever.retrieve(query, **kwargs)
        vector_nodes = self.vector_retriever.retrieve(query, **kwargs)

        # combine the two lists of nodes
        all_nodes = []
        node_ids = set()
        for n in bm25_nodes + vector_nodes:
            if n.node.node_id not in node_ids:
        return all_nodes

hybrid_retriever = HybridRetriever(vector_retriever, bm25_retriever)

Re-Ranker Setup#

!pip install sentence-transformers
from llama_index.core.postprocessor import SentenceTransformerRerank

reranker = SentenceTransformerRerank(top_n=4, model="BAAI/bge-reranker-base")


from llama_index.core import QueryBundle

retrieved_nodes = hybrid_retriever.retrieve(
    "What is the impact of climate change on the ocean?"
reranked_nodes = reranker.postprocess_nodes(
        "What is the impact of climate change on the ocean?"

print("Initial retrieval: ", len(retrieved_nodes), " nodes")
print("Re-ranked retrieval: ", len(reranked_nodes), " nodes")
from llama_index.core.response.notebook_utils import display_source_node

for node in reranked_nodes:

Node ID: 735c563d-e68c-42e7-bbeb-d360a2918d22
Similarity: 0.6910567283630371
Text: lb88\8a|hiac2:,sgHfvyȔskz4 4+)t$EAs9<"L䴺-iai]}?R)Jv8Q0V#9>f=3߼ב&WTFKNӅ9GN8v`4׏g tz…

Node ID: 07a796db-8307-47a0-83a0-9b5f953b86b0
Similarity: 0.609274685382843
Text: {_d3<,fCȀ0K0~n(A ̙$saxt :xY3z/ߕXAWwpTeHY0HZHe̚kÇ>.82%ϖ^.rCS.2^12C?

Node ID: 0ee98fbd-d015-4cd4-aba0-812423880915
Similarity: 0.5765283107757568
Text: X+V/Z 6;O8/%Z3EC\asU1f(xLͼ]X\q”1}.66OKGSi9ǧ’(?Iʲi=4ޮ^m,Mp1~lBxP3]S<?C)3WM;WGQt@”l…

Node ID: 2562bbfb-fda2-432d-9674-5ba77ff4b767
Similarity: 0.3882860541343689
Text: PxS0s.MAK|+MOUÊ^;tc0 $XHmL<ҁ;Q@nUd&ի 2`B01i^yOw:rsM冫S*wu4a{-.Խ=j鷩nQD_A޾A%~T>~’OxU gk…

Full Query Engine#

from llama_index.core.query_engine import RetrieverQueryEngine

query_engine = RetrieverQueryEngine.from_args(

response = query_engine.query(
    "What is the impact of climate change on the ocean?"
from llama_index.core.response.notebook_utils import display_response


Final Response: Climate change can have a significant impact on the ocean. Rising temperatures can lead to the melting of polar ice caps, resulting in sea-level rise and coastal flooding. It can also disrupt ocean currents and affect marine ecosystems, including coral reefs and fish populations. Additionally, climate change can lead to ocean acidification, which can harm marine life and impact the overall health of the ocean.