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Basic RAG

Retrieval-augmented generation (RAG) is an AI framework that synergizes the capabilities of LLMs and information retrieval systems. It's useful to answer questions or generate content leveraging external knowledge. There are two main steps in RAG: 1) retrieval: retrieve relevant information from a knowledge base with text embeddings stored in a vector store; 2) generation: insert the relevant information to the prompt for the LLM to generate information. In this guide, we will walk through a very basic example of RAG with four implementations:

  • RAG from scratch with Mistral
  • RAG with Mistral and LangChain
  • RAG with Mistral and LlamaIndex
  • RAG with Mistral and Haystack
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RAG from scratch

This section aims to guide you through the process of building a basic RAG from scratch. We have two goals: firstly, to offer users a comprehensive understanding of the internal workings of RAG and demystify the underlying mechanisms; secondly, to empower you with the essential foundations needed to build an RAG using the minimum required dependencies.

Import needed packages

The first step is to install the needed packages mistralai and faiss-cpu and import them:

from mistralai.client import MistralClient
from mistralai.models.chat_completion import ChatMessage
import numpy as np
import os

Get data

In this very simple example, we are getting data from an essay written by Paul Graham:

import requests

response = requests.get('')
text = response.text

We can also save the essay in a local file:

f = open('essay.txt', 'w')

Split document into chunks

In a RAG system, it is crucial to split the document into smaller chunks so that it's more effective to identify and retrieve the most relevant information in the retrieval process later. In this example, we simply split our text by character, combine 2048 characters into each chunk, and we get 37 chunks.

chunk_size = 2048
chunks = [text[i:i + chunk_size] for i in range(0, len(text), chunk_size)]




  • Chunk size: Depending on your specific use case, it may be necessary to customize or experiment with different chunk sizes and chunk overlap to achieve optimal performance in RAG. For example, smaller chunks can be more beneficial in retrieval processes, as larger text chunks often contain filler text that can obscure the semantic representation. As such, using smaller text chunks in the retrieval process can enable the RAG system to identify and extract relevant information more effectively and accurately. However, it's worth considering the trade-offs that come with using smaller chunks, such as increasing processing time and computational resources.
  • How to split: While the simplest method is to split the text by character, there are other options depending on the use case and document structure. For example, to avoid exceeding token limits in API calls, it may be necessary to split the text by tokens. To maintain the cohesiveness of the chunks, it can be useful to split the text by sentences, paragraphs, or HTML headers. If working with code, it's often recommended to split by meaningful code chunks for example using an Abstract Syntax Tree (AST) parser.

Create embeddings for each text chunk

For each text chunk, we then need to create text embeddings, which are numeric representations of the text in the vector space. Words with similar meanings are expected to be in closer proximity or have a shorter distance in the vector space. To create an embedding, use Mistral AI's embeddings API endpoint and the embedding model mistral-embed. We create a get_text_embedding to get the embedding from a single text chunk and then we use list comprehension to get text embeddings for all text chunks.

def get_text_embedding(input):
embeddings_batch_response = client.embeddings(
text_embeddings = np.array([get_text_embedding(chunk) for chunk in chunks])

Load into a vector database

Once we get the text embeddings, a common practice is to store them in a vector database for efficient processing and retrieval. There are several vector database to choose from. In our simple example, we are using an open-source vector database Faiss, which allows for efficient similarity search.

With Faiss, we instantiate an instance of the Index class, which defines the indexing structure of the vector database. We then add the text embeddings to this indexing structure.

import faiss

d = text_embeddings.shape[1]
index = faiss.IndexFlatL2(d)


  • Vector database: When selecting a vector database, there are several factors to consider including speed, scalability, cloud management, advanced filtering, and open-source vs. closed-source.

Create embeddings for a question

Whenever users ask a question, we also need to create embeddings for this question using the same embedding models as before.

question = "What were the two main things the author worked on before college?"
question_embeddings = np.array([get_text_embedding(question)])


  • Hypothetical Document Embeddings (HyDE): In some cases, the user's question might not be the most relevant query to use for identifying the relevant context. Instead, it maybe more effective to generate a hypothetical answer or a hypothetical document based on the user's query and use the embeddings of the generated text to retrieve similar text chunks.

Retrieve similar chunks from the vector database

We can perform a search on the vector database with, which takes two arguments: the first is the vector of the question embeddings, and the second is the number of similar vectors to retrieve. This function returns the distances and the indices of the most similar vectors to the question vector in the vector database. Then based on the returned indices, we can retrieve the actual relevant text chunks that correspond to those indices.

D, I =, k=2) # distance, index
retrieved_chunk = [chunks[i] for i in I.tolist()[0]]


  • Retrieval methods: There are a lot different retrieval strategies. In our example, we are showing a simple similarity search with embeddings. Sometimes when there is metadata available for the data, it's better to filter the data based on the metadata first before performing similarity search. There are also other statistical retrieval methods like TF-IDF and BM25 that use frequency and distribution of terms in the document to identify relevant text chunks.
  • Retrieved document: Do we always retrieve individual text chunk as it is? Not always.
    • Sometime, we would like to include more context around the actual retrieved text chunk. We call the actual retrieved text chunk "child chunk" and our goal is to retrieve a larger "parent chunk" that the "child chunk" belongs to.
    • On occasion, we might also want to provide weights to our retrieve documents. For example, a time-weighted approach would help us retrieve the most recent document.
    • One common issue in the retrieval process is the "lost in the middle" problem where the information in the middle of a long context gets lost. Our models have tried to mitigate this issue. For example, in the passkey task, our models have demonstrated the ability to find a "needle in a haystack" by retrieving a randomly inserted passkey within a long prompt, up to 32k context length. However, it is worth considering experimenting with reordering the document to determine if placing the most relevant chunks at the beginning and end leads to improved results.

Combine context and question in a prompt and generate response

Finally, we can offer the retrieved text chunks as the context information within the prompt. Here is a prompt template where we can include both the retrieved text and user question in the prompt.

prompt = f"""
Context information is below.
Given the context information and not prior knowledge, answer the query.
Query: {question}

Then we can use the Mistral chat completion API to chat with a Mistral model (e.g., mistral-medium-latest) and generate answers based on the user question and the context of the question.

def run_mistral(user_message, model="mistral-medium-latest"):
messages = [
ChatMessage(role="user", content=user_message)
chat_response =
return (chat_response.choices[0].message.content)



'The two main things the author worked on before college were writing and programming. They wrote short stories and tried writing programs on an IBM 1401 in 9th grade.'


  • Prompting techniques: Most of the prompting techniques can be used in developing a RAG system as well. For example, we can use few-shot learning to guide the model's answers by providing a few examples. Additionally, we can explicitly instruct the model to format answers in a certain way.

In the next section, we are going to show you how to do a similar basic RAG with some of the popular RAG frameworks such as LangChain and LlamaIndex.

RAG with LangChain


from langchain_community.document_loaders import TextLoader
from langchain_mistralai.chat_models import ChatMistralAI
from langchain_mistralai.embeddings import MistralAIEmbeddings
from langchain_community.vectorstores import FAISS
from langchain.text_splitter import RecursiveCharacterTextSplitter
from langchain.chains.combine_documents import create_stuff_documents_chain
from langchain_core.prompts import ChatPromptTemplate
from langchain.chains import create_retrieval_chain

# Load data
loader = TextLoader("essay.txt")
docs = loader.load()
# Split text into chunks
text_splitter = RecursiveCharacterTextSplitter()
documents = text_splitter.split_documents(docs)
# Define the embedding model
embeddings = MistralAIEmbeddings(model="mistral-embed", mistral_api_key=api_key)
# Create the vector store
vector = FAISS.from_documents(documents, embeddings)
# Define a retriever interface
retriever = vector.as_retriever()
# Define LLM
model = ChatMistralAI(mistral_api_key=api_key)
# Define prompt template
prompt = ChatPromptTemplate.from_template("""Answer the following question based only on the provided context:


Question: {input}""")

# Create a retrieval chain to answer questions
document_chain = create_stuff_documents_chain(model, prompt)
retrieval_chain = create_retrieval_chain(retriever, document_chain)
response = retrieval_chain.invoke({"input": "What were the two main things the author worked on before college?"})


The two main things the author worked on before college were writing and programming. He wrote short stories and tried programming on an IBM 1401 using Fortran, but he found it difficult to figure out what to do with the machine due to the limited input options. His interest in programming grew with the advent of microcomputers, leading him to write simple games, a program to predict rocket trajectories, and a word processor.

Visit our community cookbook example to discover how to use LangChain's LangGraph with the Mistral API to perform Corrective RAG, which enables correction of poor quality retrieval or generations.

RAG with LlamaIndex


from llama_index import VectorStoreIndex, SimpleDirectoryReader
from llama_index.llms import MistralAI
from llama_index.embeddings import MistralAIEmbedding
from llama_index import ServiceContext
from llama_index.query_engine import RetrieverQueryEngine

# Load data
reader = SimpleDirectoryReader(input_files=["essay.txt"])
documents = reader.load_data()

# Define LLM and embedding model
llm = MistralAI(api_key=api_key, model="mistral-medium")
embed_model = MistralAIEmbedding(model_name="mistral-embed", api_key=api_key)
service_context = ServiceContext.from_defaults(llm=llm, embed_model=embed_model)

# Create vector store index
index = VectorStoreIndex.from_documents(documents, service_context=service_context)

# Create query engine
query_engine = index.as_query_engine(similarity_top_k=2)
response = query_engine.query(
"What were the two main things the author worked on before college?"


The two main things the author worked on before college, outside of school, were writing and programming. They wrote short stories and attempted to write programs using an early version of Fortran on an IBM 1401.

Visit out our community cookbook example to learn how to use LlamaIndex with the Mistral API to perform complex queries over multiple documents using a ReAct agent, an autonomous LLM-powered agent capable of using tools.

RAG with Haystack


from haystack import Pipeline
from haystack.document_stores.in_memory import InMemoryDocumentStore
from haystack.dataclasses import ChatMessage
from haystack.utils.auth import Secret

from import DynamicChatPromptBuilder
from haystack.components.converters import TextFileToDocument
from haystack.components.preprocessors import DocumentSplitter
from haystack.components.retrievers.in_memory import InMemoryEmbeddingRetriever
from haystack.components.writers import DocumentWriter
from haystack_integrations.components.embedders.mistral import MistralDocumentEmbedder, MistralTextEmbedder
from haystack_integrations.components.generators.mistral import MistralChatGenerator

document_store = InMemoryDocumentStore()

docs = TextFileToDocument().run(sources=["essay.txt"])
split_docs = DocumentSplitter(split_by="passage", split_length=2).run(documents=docs["documents"])
embeddings = MistralDocumentEmbedder(api_key=Secret.from_token(api_key)).run(documents=split_docs["documents"])

text_embedder = MistralTextEmbedder(api_key=Secret.from_token(api_key))
retriever = InMemoryEmbeddingRetriever(document_store=document_store)
prompt_builder = DynamicChatPromptBuilder(runtime_variables=["documents"])
llm = MistralChatGenerator(api_key=Secret.from_token(api_key),

chat_template = """Answer the following question based on the contents of the documents.\n
Question: {{query}}\n
{% for document in documents %}
{% endfor%}
messages = [ChatMessage.from_user(chat_template)]

rag_pipeline = Pipeline()
rag_pipeline.add_component("text_embedder", text_embedder)
rag_pipeline.add_component("retriever", retriever)
rag_pipeline.add_component("prompt_builder", prompt_builder)
rag_pipeline.add_component("llm", llm)

rag_pipeline.connect("text_embedder.embedding", "retriever.query_embedding")
rag_pipeline.connect("retriever.documents", "prompt_builder.documents")
rag_pipeline.connect("prompt_builder.prompt", "llm.messages")

question = "What were the two main things the author worked on before college?"

result =
"text_embedder": {"text": question},
"prompt_builder": {"template_variables": {"query": question}, "prompt_source": messages},
"llm": {"generation_kwargs": {"max_tokens": 225}},



The two main things the author worked on before college were writing and programming. He wrote short stories, which he admitted were awful, and essays about various topics. He also worked on spam filters and painted. Additionally, he started having dinners for a group of friends every Thursday night, which taught him how to cook for groups. He also bought a building in Cambridge to use as an office. The author was drawn to writing essays, which he started publishing online, and this helped him figure out what to work on. He also experimented with painting and studied AI in college.