Comparing LangChain and LlamaIndex with 4 tasks

Ming
10 min readJan 11, 2024

LangChain v.s. LlamaIndex — How do they compare?

Generated via MidJourney. Prompt: “A llama and a chain, facing off.” Apparently, MidJourney misunderstood.

In Why RAG is big, I stated my support for Retrieval-Augmented Generation (RAG) as the key technology of private, offline, decentralized LLM applications. When you build something for your own use, you’re fighting alone. You can build from scratch, but it would be more efficient to build upon an existing framework.

AFAIK, two choices exist, aiming at different scopes:

  • LangChain, a generic framework for developing stuff with LLM.
  • LlamaIndex, a framework dedicated for building RAG systems.

Picking a framework is a big investment. You want one that enjoys strong maintainers and vibrant communities. Fortunately, both choices have incorporated last year, so the sizes are quite quantifiable. Here’s how the numbers compare:

I wish Medium can have tables.

Judging from the financials, LlamaIndex is coming strong with a funding amount close to that of LangChain although their target market is much smaller (using GitHub stars as an approximate of community interest). This might indicate better chance of survival for LlamaIndex. That being said, LangChain offers more enterprise-oriented products that can generate revenue (LangServe, LangSmith, …), so the argument may be reversed. It’s a tough call from the monetary perspective.

My Finance 101 could only take me this far. Let’s get to what I’m actually good at and talk in Python. In this article, I’m going to complete some basic tasks with both frameworks in parallel. By presenting the code snippets side-by-side, I hope it helps you make a more informed decision on which to employ in your own RAG chatbot.

Creating a chatbot with a local LLM

For the first task to implement, I chose making a local-only chatbot. This is because I don’t want to pay a cloud service for mock chat messages while learning to use these frameworks.

I chose to keep a LLM running in a standalone inference server, instead of having the frameworks load the multi-gigabyte model into the memory every time I run the scripts. This saves time and avoids wearing down the disk.

While there are multiple API schema for LLM inference, I chose one that is OpenAI-compatible, so that it most closely resembles the official OpenAI endpoint, should you want to.

This is how you would do it with LlamaIndex:

from llama_index.llms import ChatMessage, OpenAILike  

llm = OpenAILike(
api_base="http://localhost:1234/v1",
timeout=600, # secs
api_key="loremIpsum",
is_chat_model=True,
context_window=32768,
)
chat_history = [
ChatMessage(role="system", content="You are a bartender."),
ChatMessage(role="user", content="What do I enjoy drinking?"),
]
output = llm.chat(chat_history)
print(output)

And this is LangChain:

from langchain.schema import HumanMessage, SystemMessage  
from langchain_openai import ChatOpenAI

llm = ChatOpenAI(
openai_api_base="http://localhost:1234/v1",
request_timeout=600, # secs, I guess.
openai_api_key="loremIpsum",
max_tokens=32768,
)
chat_history = [
SystemMessage(content="You are a bartender."),
HumanMessage(content="What do I enjoy drinking?"),
]
print(llm(chat_history))

With both, the API key can be arbitrary, but it must present. I guess it’s a requirement from the OpenAI SDK that is running under the hood in both frameworks.

  • LangChain distinguishes between chat-able LLMs (ChatOpenAI) and completion-only LLMs (OpenAI), while LlamaIndex controls it with a is_chat_model parameter in the constructor.
  • LlamaIndex distinguishes official OpenAI endpoints and OpenAILike endpoints, while LangChain determines where to send requests to via a openai_api_base parameter.
  • While LlamaIndex labels chat messages with the role parameter, LangChain uses separate classes.

So far, things didn’t look very different across the two frameworks. Let’s carry on.

Building a RAG system for local files

With a LLM connected, we can get to business. Let’s now build a simple RAG system that reads from a local folder of text files. Here’s how to achieve that with LlamaIndex, taken largely from this documentation:

from llama_index import ServiceContext, SimpleDirectoryReader, VectorStoreIndex

service_context = ServiceContext.from_defaults(
embed_model="local",
llm=llm, # This should be the LLM initialized in the task above.
)
documents = SimpleDirectoryReader(
input_dir="mock_notebook/",
).load_data()
index = VectorStoreIndex.from_documents(
documents=documents,
service_context=service_context,
)
engine = index.as_query_engine(
service_context=service_context,
)
output = engine.query("What do I like to drink?")
print(output)

With LangChain, number of lines would double, but it would still be manageable:

from langchain_community.document_loaders import DirectoryLoader  

# pip install "unstructured[md]"
loader = DirectoryLoader("mock_notebook/", glob="*.md")
docs = loader.load()

from langchain.text_splitter import RecursiveCharacterTextSplitter

text_splitter = RecursiveCharacterTextSplitter(chunk_size=1000, chunk_overlap=200)
splits = text_splitter.split_documents(docs)

from langchain_community.embeddings.fastembed import FastEmbedEmbeddings
from langchain_community.vectorstores import Chroma

vectorstore = Chroma.from_documents(documents=splits, embedding=FastEmbedEmbeddings())
retriever = vectorstore.as_retriever()

from langchain import hub

# pip install langchainhub
prompt = hub.pull("rlm/rag-prompt")


def format_docs(docs):
return "\n\n".join(doc.page_content for doc in docs)


from langchain_core.runnables import RunnablePassthrough

rag_chain = (
{"context": retriever | format_docs, "question": RunnablePassthrough()}
| prompt
| llm # This should be the LLM initialized in the task above.
)
print(rag_chain.invoke("What do I like to drink?"))

These snippets clearly illustrate the different levels of abstraction across these two frameworks. While LlamaIndex wraps the RAG pipeline with a convenient package called “query engines”, LangChain exposes you to the inner components. They include the concatenator for retrieved documents, the prompt template that says “based on X please answer Y”, and the chain itself (shown in LCEL above).

This lack of abstraction has implications on learners: When building with LangChain, you have to know exactly what you want on the first try. For example, compare where from_documents is invoked. LlamaIndex allows you to play with a Vector Store Index without explicitly choosing a storage backend, whereas LangChain seems to suggest you pick an implementation right away. (Everybody seems to have explicitly picked a backend when they create Vector Indexes from documents with LangChain.) I’m not sure if I’m making an informed decision when choosing a database before I hit a scalability problem.

More interestingly, although both LangChain and LlamaIndex are providing Hugging Face Hub-like cloud services (namely, LangSmith Hub and LlamaHub), it’s LangChain who dialed it to 11. Notice the hub.pull call with LangChain. It downloads nothing but a short, textual template that reads:

You are an assistant for question-answering tasks. Use the following pieces of retrieved context to answer the question. If you don’t know the answer, just say that you don’t know. Use three sentences maximum and keep the answer concise.
Question: {question}
Context: {context}
Answer:

While this indeed encourages sharing eloquent prompts with the community, I feel it’s an overkill. Storing ~1kB of text doesn’t really justify the network call involved with pulling. I hope the downloaded artifacts are cached.

Combining the two: a RAG-enabled chatbot

Up till now, we’ve been building things that aren’t very smart. In the first task, we built something that can maintain a conversation but doesn’t know you very well; in the second, we built something that knows you but doesn’t retain a chat history. Let’s combine these two.

With LlamaIndex, it’s as simple as swapping as_query_engine with as_chat_engine:

# Everything from above, till and including the creation of the index.
engine = index.as_chat_engine()
output = engine.chat("What do I like to drink?")
print(output) # "You enjoy drinking coffee."
output = engine.chat("How do I brew it?")
print(output) # "You brew coffee with a Aeropress."

With LangChain, we need to spell things out quite a bit. Following the official tutorial, let’s define the memory first:

# Everything above this line is the same as that of the last task.
from langchain_core.runnables import RunnablePassthrough, RunnableLambda
from langchain_core.messages import get_buffer_string
from langchain_core.output_parsers import StrOutputParser
from operator import itemgetter
from langchain.memory import ConversationBufferMemory
from langchain.prompts.prompt import PromptTemplate
from langchain.schema import format_document
from langchain_core.prompts import ChatPromptTemplate

memory = ConversationBufferMemory(
return_messages=True, output_key="answer", input_key="question"
)

Here’s the plan:

  1. At the start of LLM’s turn, we load the chat history from the memory.
load_history_from_memory = RunnableLambda(memory.load_memory_variables) | itemgetter(  
"history"
)
load_history_from_memory_and_carry_along = RunnablePassthrough.assign(
chat_history=load_history_from_memory
)

2. We ask the LLM to enrich the question with context: “Taking the chat history into consideration, what should I look for in my notes to answer this question?”

rephrase_the_question = (  
{
"question": itemgetter("question"),
"chat_history": lambda x: get_buffer_string(x["chat_history"]),
}
| PromptTemplate.from_template(
"""You're a personal assistant to the user.
Here's your conversation with the user so far:
{chat_history}
Now the user asked: {question}
To answer this question, you need to look up from their notes about """
)
| llm
| StrOutputParser()
)

(We can’t just concatenate the two, because the topics may have changed during the conversation, making most semantic information in the chat log irrelevant.)
3. We run the RAG pipeline. Notice how we have cheated the LLM by implying “we as the user will be looking up the notes themselves”, but in fact we are asking the LLM to do the heavy lifting now. I feel bad.

retrieve_documents = {  
"docs": itemgetter("standalone_question") | retriever,
"question": itemgetter("standalone_question"),
}

4. We ask the LLM: “Taking the retrieved documents as reference (and — optionally — the conversation so far), what would be your response to the user’s latest question?”

def _combine_documents(docs):  
prompt = PromptTemplate.from_template(template="{page_content}")
doc_strings = [format_document(doc, prompt) for doc in docs]
return "\n\n".join(doc_strings)
compose_the_final_answer = (
{
"context": lambda x: _combine_documents(x["docs"]),
"question": itemgetter("question"),
}
| ChatPromptTemplate.from_template(
"""You're a personal assistant.
With the context below:
{context}
To the question "{question}", you answer:"""
)
| llm
)

5. We append the final response to the chat history.

# Putting all 4 stages together...
final_chain = (
load_history_from_memory_and_carry_along
| {"standalone_question": rephrase_the_question}
| retrieve_documents
| compose_the_final_answer
)
# Demo.
inputs = {"question": "What do I like to drink?"}
output = final_chain.invoke(inputs)
memory.save_context(inputs, {"answer": output.content})
print(output) # "You enjoy drinking coffee."
inputs = {"question": "How do I brew it?"}
output = final_chain.invoke(inputs)
memory.save_context(inputs, {"answer": output.content})
print(output) # "You brew coffee with a Aeropress."

That’s quite a journey! We learned a lot about how a LLM-powered application is usually built. Especially, we exploited the LLM a couple of times by having it assume different persona: a query generator, someone who summarizes retrieved documents, and finally the participant of our conversation. I also hope you got adequately comfortable with the LCEL by now.

Upgrading to agents

If you treat the LLM persona that talks to you as a person, the RAG pipeline can be thought of a tool that the person uses. A person can have access to more than one tool, and so can a LLM. You can give it tools for searching Google, looking up Wikipedia, checking weather forecasts, etc. In this way, your chatbot can answer questions about things outside of its immediate knowledge.

It doesn’t have to be informational tools. By giving our LLM tools like searching the web, placing some shopping orders, replying to your emails, etc., you can make it capable of affecting the reality and making a difference to the world.

With many tools comes the need to decide which ones to use, and in what order. This ability is referred to as “agency”. The persona of your LLM who has agency is thus called an “Agent”.

There are multiple ways to give agency to a LLM application. The most model-generic (and thus self-host-friendly) way is perhaps the ReAct paradigm, which I wrote a bit more about in the previous post.

To do that in LlamaIndex,

# Everything above this line is the same as in the above two tasks,  
# till and including where `notes_query_engine` is defined.
# Let's convert the query engine into a tool.
from llama_index.tools import ToolMetadata
from llama_index.tools.query_engine import QueryEngineTool

notes_query_engine_tool = QueryEngineTool(
query_engine=notes_query_engine,
metadata=ToolMetadata(
name="look_up_notes",
description="Gives information about the user.",
),
)
from llama_index.agent import ReActAgent

agent = ReActAgent.from_tools(
tools=[notes_query_engine_tool],
llm=llm,
service_context=service_context,
)
output = agent.chat("What do I like to drink?")
print(output) # "You enjoy drinking coffee."
output = agent.chat("How do I brew it?")
print(output) # "You can use a drip coffee maker, French press, pour-over, or espresso machine."

Note that, for our follow-up question “how do I brew coffee”, the agent answered differently from when it was merely a query engine. This is because agents can make their own decision about whether to look up from our notes. If they feel confident enough to answer the question, the agent may choose to not use any tool at all. Our question of “how do I …” can be interpreted both ways: either about generic options, or factual recollections. Apparently, the agent chose to understood it the former way, whereas our query engine (which has a duty of looking up documents from the index) had to pick the latter.

Interestingly, agents are a use case that LangChain decided to provide a high-level abstraction for:

# Everything above is the same as in the 2nd task, till and including where we defined `rag_chain`.  
# Let's convert the chain into a tool.
from langchain.agents import AgentExecutor, Tool, create_react_agent

tools = [
Tool(
name="look_up_notes",
func=rag_chain.invoke,
description="Gives information about the user.",
),
]
react_prompt = hub.pull("hwchase17/react-chat")
agent = create_react_agent(llm, tools, react_prompt)
agent_executor = AgentExecutor.from_agent_and_tools(agent=agent, tools=tools)

result = agent_executor.invoke(
{"input": "What do I like to drink?", "chat_history": ""}
)
print(result) # "You enjoy drinking coffee."
result = agent_executor.invoke(
{
"input": "How do I brew it?",
"chat_history": "Human: What do I like to drink?\nAI: You enjoy drinking coffee.",
}
)
print(result) # "You can use a drip coffee maker, French press, pour-over, or espresso machine."

Although we still had to manually manage the chat history, it’s much easier to make an agent compared to making a RAG chain. create_react_agent and AgentExecutor cover most of the wiring work under the hood.

Summary

LlamaIndex and LangChain are two frameworks for building LLM applications. While LlamaIndex focuses on RAG use cases, LangChain seems more widely adopted. But how do they differ in practice? In this post, I compared the two frameworks in completing four common tasks:

  1. Connecting to a local LLM instance and build a chatbot.
  2. Indexing local files and building a RAG system.
  3. Combining the two above and making a chatbot with RAG capabilities.
  4. Converting the chatbot into an agent, so that it may use more tools and do simple reasoning.

I hope they help you make an informed choice for your LLM application. Also, good luck with your journey building your own chatbot!

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