Document Summarization with Retriever in LangChain

3 min readJan 29, 2024

Objective

To summarize a document using Retrieval Augmented Generation (RAG), you can run both VectorStore Embedding and a Large Language Model (LLM) locally. If available, you can also utilize the GPU, such as the Nvidia 4090, as in my case.

RAG Architecture

The code step by step

Using `WebBaseLoader` to load the document from a web directly

from langchain.document_loaders import WebBaseLoader
loader = WebBaseLoader("https://en.wikisource.org/wiki/Hans_Andersen%27s_Fairy_Tales/The_Emperor%27s_New_Clothes")
# from langchain.document_loaders import TextLoader, DirectoryLoader
# loader = DirectoryLoader('/tmp/', glob="./*.pdf")
documents = loader.load()

I am fond of Hans Andersen’s fairy tales, and this particular story still unfolds around us, visible to all. So let’s summarize it.

I also include the code to load document from PDF as above.

Split the document and embed it with `sentence-transformers` model from HuggingFace

from langchain.text_splitter import RecursiveCharacterTextSplitter
text_splitter = RecursiveCharacterTextSplitter(
  chunk_size=2000,
  chunk_overlap=200
)
texts = text_splitter.split_documents(documents)
from langchain.embeddings import HuggingFaceEmbeddings
embeddings = HuggingFaceEmbeddings(
  model_name="sentence-transformers/paraphrase-multilingual-MiniLM-L12-v2")
  # model_name="bert-base-multilingual-cased")

As we are using a locally installed open-source embedding model, we have the flexibility to set a large value for the token’s `chunk_size` at no cost.

I prefer using the `paraphrase-multilingual-MiniLM-L12-v2 model`, which is 477MB on disk. It is small yet powerful.

Save the embedding into VectorStore

from langchain.vectorstores import Chroma
persist_directory = "/tmp/chromadb"
vectordb = Chroma.from_documents(
  documents=texts, 
  embedding=embeddings,
  persist_directory=persist_directory
)
vectordb.persist()

The database is persisted in `/tmp/chromadb`. Please note that it will be erased if the system reboots. Therefore, you can change the path if you want to keep the database. You can also use other VectorStores, such as FAISS, Qdrant, and Pinecone, which will be explained in other tutorials later.

Specify `PromptTemplate` and `Prompt`

from langchain.prompts import PromptTemplate
prompt_template = """Write a concise summary of the following: "{context}" CONCISE SUMMARY: """
prompt = PromptTemplate(
  template=prompt_template, 
  input_variables=["context", "question"]
)

Load `Mistral` model

from langchain_community.llms import GPT4All
from langchain.callbacks.streaming_stdout import StreamingStdOutCallbackHandler
# Instantiate the model. Callbacks support token-wise streaming
callbacks = [StreamingStdOutCallbackHandler()]
llm = GPT4All(
  model="/home/linux/.cache/huggingface/hub/gpt4all/mistral-7b-openorca.Q4_0.gguf",
  device='gpu',
  n_threads=8)

The `mistral` model is downloaded from `https://gpt4all.io/models/gguf/mistral-7b-openorca.Q4_0.gguf`, at https://gpt4all.io/index.html.

You can also use any other LLM (text-generation), from HuggingFace. For example, gpt2, falcon, etc.

Enable debug and verbose mode (optional)

from langchain.globals import set_verbose, set_debug
set_debug(True)
set_verbose(True)

Enabling debug and verbose mode can print the detailed chain step by step. It’s essential to debug when having issue.

RAG with `RetrievalQA` function

from langchain.chains import RetrievalQA
qa_chain = RetrievalQA.from_chain_type(
  llm=llm,
  retriever=vectordb.as_retriever(), chain_type="stuff",
  chain_type_kwargs={"prompt": prompt},
  return_source_documents=True
)

from langchain.chains import RetrievalQA
response = qa_chain("please summarize this book")
# response = qa_chain("what happened to the emperor?")

Print the results beautifully (optional)

import pprint
pp = pprint.PrettyPrinter(indent=0)
# pp.pprint(response)
pp.pprint(response['result'])

The complete code

from langchain.document_loaders import WebBaseLoader
loader = WebBaseLoader("https://en.wikisource.org/wiki/Hans_Andersen%27s_Fairy_Tales/The_Emperor%27s_New_Clothes")
# from langchain.document_loaders import TextLoader, DirectoryLoader
# loader = DirectoryLoader('/tmp/', glob="./*.pdf")
documents = loader.load()


from langchain.text_splitter import RecursiveCharacterTextSplitter
text_splitter = RecursiveCharacterTextSplitter(
  chunk_size=2000,
  chunk_overlap=200
)
texts = text_splitter.split_documents(documents)


from langchain.embeddings import HuggingFaceEmbeddings
embeddings = HuggingFaceEmbeddings(
model_name="sentence-transformers/paraphrase-multilingual-MiniLM-L12-v2")
# model_name="bert-base-multilingual-cased")


from langchain.vectorstores import Chroma
persist_directory = "/tmp/chromadb"
vectordb = Chroma.from_documents(
  documents=texts, 
  embedding=embeddings,
  persist_directory=persist_directory
)
vectordb.persist()


from langchain.prompts import PromptTemplate
prompt_template = """Write a concise summary of the following: "{context}" CONCISE SUMMARY: """
prompt = PromptTemplate(
  template=prompt_template, 
  input_variables=["context", "question"]
)


from langchain_community.llms import GPT4All
from langchain.callbacks.streaming_stdout import StreamingStdOutCallbackHandler
# Instantiate the model. Callbacks support token-wise streaming
callbacks = [StreamingStdOutCallbackHandler()]
llm = GPT4All(
  model="/home/jeff/.cache/huggingface/hub/gpt4all/mistral-7b-openorca.Q4_0.gguf",
  device='gpu',
  n_threads=8)


from langchain.globals import set_verbose, set_debug
set_debug(True)
set_verbose(True)


from langchain.chains import RetrievalQA
qa_chain = RetrievalQA.from_chain_type(
  llm=llm,
  retriever=vectordb.as_retriever(), chain_type="stuff",
  chain_type_kwargs={"prompt": prompt},
  return_source_documents=True
)


from langchain.chains import RetrievalQA
response = qa_chain("please summarize this book")
# response = qa_chain("what happened to the emperor?")
import pprint
pp = pprint.PrettyPrinter(indent=0)
# pp.pprint(response)
pp.pprint(response['result'])