llama-index

name	llama-index
description	Comprehensive guide for building LLM applications with LlamaIndex, including data loaders, indexes, query engines, chat engines, vector stores, retrievers, agents, evaluation, streaming, and observability.
metadata	{"author":"mte90","version":"1.0.0","tags":["llama-index","llm","rag","ai","python","vector-database","openai","agents"]}

LlamaIndex Development

Complete guide for building LLM applications with LlamaIndex framework.

Overview

LlamaIndex is a data framework for LLM applications, providing tools for data ingestion, indexing, and retrieval.

Key Characteristics:

• RAG (Retrieval Augmented Generation) support
• Multiple data connectors (300+)
• Various index types
• Query and chat engines
• Vector store integrations
• Agent framework

Installation

Setup

# Basic installation
pip install llama-index

# With OpenAI
pip install llama-index-llms-openai
pip install llama-index-embeddings-openai

# With vector stores
pip install llama-index-vector-stores-chroma
pip install llama-index-vector-stores-pinecone
pip install llama-index-vector-stores-qdrant

# With evaluation
pip install llama-index-llms-openai ragas

Basic Configuration

import os
from llama_index.core import Settings
from llama_index.llms.openai import OpenAI
from llama_index.embeddings.openai import OpenAIEmbedding

# Set API key
os.environ["OPENAI_API_KEY"] = "your-api-key"

# Configure global settings
Settings.llm = OpenAI(model="gpt-4o", temperature=0.0)
Settings.embed_model = OpenAIEmbedding(model="text-embedding-3-small")
Settings.chunk_size = 512
Settings.chunk_overlap = 50

Quick Start

Basic RAG Pipeline

from llama_index.core import VectorStoreIndex, SimpleDirectoryReader

# Load documents
documents = SimpleDirectoryReader("./data").load_data()

# Create index
index = VectorStoreIndex.from_documents(documents)

# Create query engine
query_engine = index.as_query_engine()

# Query
response = query_engine.query("What is the main topic?")
print(response)

With Storage

from llama_index.core import VectorStoreIndex, SimpleDirectoryReader, StorageContext
from llama_index.embeddings.openai import OpenAIEmbedding
import chromadb
from llama_index.vector_stores.chroma import ChromaVectorStore

# Setup ChromaDB
db = chromadb.PersistentClient(path="./chroma_db")
chroma_collection = db.get_or_create_collection("my_collection")
vector_store = ChromaVectorStore(chroma_collection=chroma_collection)
storage_context = StorageContext.from_defaults(vector_store=vector_store)

# Load and index
documents = SimpleDirectoryReader("./data").load_data()
index = VectorStoreIndex.from_documents(
    documents,
    storage_context=storage_context,
    embed_model=OpenAIEmbedding(),
)

# Persist
storage_context.persist()

# Load from disk
index = VectorStoreIndex.from_vector_store(
    vector_store,
    embed_model=OpenAIEmbedding(),
)

Data Loading

Document Loaders

from llama_index.core import SimpleDirectoryReader, Document

# Load from directory
documents = SimpleDirectoryReader(
    input_dir="./data",
    required_exts=[".pdf", ".txt", ".md"],
    exclude=["*.tmp"],
    recursive=True,
).load_data()

# Load specific files
documents = SimpleDirectoryReader(
    input_files=["./file1.pdf", "./file2.txt"]
).load_data()

# Create documents manually
documents = [
    Document(text="Content here", metadata={"source": "manual"}),
]

# With metadata extraction
def custom_metadata_func(file_path: str) -> dict:
    return {
        "file_path": file_path,
        "file_name": os.path.basename(file_path),
    }

documents = SimpleDirectoryReader(
    input_dir="./data",
    file_metadata=custom_metadata_func,
).load_data()

Custom Data Connectors

from llama_index.core import Document
from typing import List

class CustomDataReader:
    """Custom data loader."""

    def load_data(self, source: str) -> List[Document]:
        documents = []

        # Load from custom source
        # Example: API, database, etc.
        data = self._fetch_from_source(source)

        for item in data:
            doc = Document(
                text=item["content"],
                metadata={
                    "source": source,
                    "id": item["id"],
                    "timestamp": item["timestamp"],
                },
            )
            documents.append(doc)

        return documents

    def _fetch_from_source(self, source: str):
        # Implement data fetching
        pass

# Usage
reader = CustomDataReader()
documents = reader.load_data("api://endpoint")

Node Parsing

Chunking Strategies

from llama_index.core.node_parser import (
    SentenceSplitter,
    TokenTextSplitter,
    SemanticSplitterNodeParser,
    HierarchicalNodeParser,
)
from llama_index.embeddings.openai import OpenAIEmbedding

# Sentence splitter (default)
splitter = SentenceSplitter(
    chunk_size=1024,
    chunk_overlap=20,
    paragraph_separator="\n\n",
)

nodes = splitter.get_nodes_from_documents(documents)

# Token splitter
token_splitter = TokenTextSplitter(
    chunk_size=512,
    chunk_overlap=50,
)

# Semantic splitter (uses embeddings)
semantic_splitter = SemanticSplitterNodeParser(
    buffer_size=1,
    breakpoint_percentile_threshold=95,
    embed_model=OpenAIEmbedding(),
)

# Hierarchical node parser
hierarchical_parser = HierarchicalNodeParser.from_defaults(
    chunk_sizes=[2048, 512, 128],  # Parent -> Child -> Grandchild
)

Node Processing Pipeline

from llama_index.core.ingestion import IngestionPipeline
from llama_index.core.extractors import (
    TitleExtractor,
    SummaryExtractor,
    KeywordExtractor,
)

# Create pipeline
pipeline = IngestionPipeline(
    transformations=[
        SentenceSplitter(chunk_size=1024, chunk_overlap=20),
        TitleExtractor(),
        SummaryExtractor(),
        KeywordExtractor(),
        OpenAIEmbedding(),
    ],
)

# Run pipeline
nodes = pipeline.run(documents=documents)

# Access metadata
for node in nodes[:3]:
    print(f"Title: {node.metadata.get('document_title')}")
    print(f"Summary: {node.metadata.get('section_summary')}")
    print(f"Keywords: {node.metadata.get('excerpt_keywords')}")

Vector Stores

ChromaDB

import chromadb
from llama_index.vector_stores.chroma import ChromaVectorStore
from llama_index.core import VectorStoreIndex, StorageContext

# Persistent client
db = chromadb.PersistentClient(path="./chroma_db")
chroma_collection = db.get_or_create_collection("my_collection")

# Create vector store
vector_store = ChromaVectorStore(chroma_collection=chroma_collection)
storage_context = StorageContext.from_defaults(vector_store=vector_store)

# Create index
index = VectorStoreIndex.from_documents(
    documents,
    storage_context=storage_context,
)

# Load existing
index = VectorStoreIndex.from_vector_store(vector_store)

Pinecone

import pinecone
from llama_index.vector_stores.pinecone import PineconeVectorStore
from llama_index.core import VectorStoreIndex, StorageContext

# Initialize Pinecone
pinecone.init(
    api_key=os.environ["PINECONE_API_KEY"],
    environment=os.environ["PINECONE_ENV"],
)

# Create index if not exists
if "my_index" not in pinecone.list_indexes():
    pinecone.create_index(
        "my_index",
        dimension=1536,
        metric="cosine",
    )

pinecone_index = pinecone.Index("my_index")
vector_store = PineconeVectorStore(pinecone_index=pinecone_index)
storage_context = StorageContext.from_defaults(vector_store=vector_store)

# Create index
index = VectorStoreIndex.from_documents(
    documents,
    storage_context=storage_context,
)

Qdrant

from qdrant_client import QdrantClient
from llama_index.vector_stores.qdrant import QdrantVectorStore
from llama_index.core import VectorStoreIndex, StorageContext

# Initialize client
client = QdrantClient(host="localhost", port=6333)

# Create vector store
vector_store = QdrantVectorStore(
    collection_name="my_collection",
    client=client,
)

storage_context = StorageContext.from_defaults(vector_store=vector_store)
index = VectorStoreIndex.from_documents(
    documents,
    storage_context=storage_context,
)

Advanced Retrievers

Hybrid Search

from llama_index.core import VectorStoreIndex, SimpleKeywordTableIndex
from llama_index.core.retrievers import VectorIndexRetriever, KeywordTableSimpleRetriever
from llama_index.core.schema import QueryBundle

class HybridRetriever:
    """Combine vector and keyword search."""

    def __init__(self, vector_index, keyword_index, mode="OR"):
        self.vector_retriever = VectorIndexRetriever(
            index=vector_index,
            similarity_top_k=10,
        )
        self.keyword_retriever = KeywordTableSimpleRetriever(
            index=keyword_index,
            similarity_top_k=10,
        )
        self.mode = mode

    def retrieve(self, query: str):
        query_bundle = QueryBundle(query_str=query)

        vector_nodes = self.vector_retriever.retrieve(query_bundle)
        keyword_nodes = self.keyword_retriever.retrieve(query_bundle)

        vector_ids = {n.node.node_id for n in vector_nodes}
        keyword_ids = {n.node.node_id for n in keyword_nodes}

        if self.mode == "AND":
            combined_ids = vector_ids.intersection(keyword_ids)
        else:
            combined_ids = vector_ids.union(keyword_ids)

        combined_dict = {n.node.node_id: n for n in vector_nodes}
        combined_dict.update({n.node.node_id: n for n in keyword_nodes})

        return [combined_dict[nid] for nid in combined_ids]

# Usage
vector_index = VectorStoreIndex.from_documents(documents)
keyword_index = SimpleKeywordTableIndex.from_documents(documents)

hybrid_retriever = HybridRetriever(vector_index, keyword_index)
nodes = hybrid_retriever.retrieve("search query")

Query Fusion Retriever

from llama_index.core.retrievers import QueryFusionRetriever, VectorIndexRetriever
from llama_index.core.postprocessor import SentenceTransformerRerank

# Create retrievers
vector_retriever = VectorIndexRetriever(
    index=vector_index,
    similarity_top_k=20,
)

# Fusion retriever with query expansion
fusion_retriever = QueryFusionRetriever(
    retrievers=[vector_retriever],
    similarity_top_k=10,
    num_queries=3,  # Generate 3 query variations
    mode="reciprocal_rerank",
    use_async=True,
)

# Add reranker
reranker = SentenceTransformerRerank(
    model="cross-encoder/ms-marco-MiniLM-L-6-v2",
    top_n=5,
)

# Use in query engine
query_engine = RetrieverQueryEngine(
    retriever=fusion_retriever,
    node_postprocessors=[reranker],
)

Auto-Merging Retriever

from llama_index.core.node_parser import HierarchicalNodeParser
from llama_index.core.retrievers import AutoMergingRetriever
from llama_index.core.storage import StorageContext

# Create hierarchical nodes
node_parser = HierarchicalNodeParser.from_defaults(
    chunk_sizes=[2048, 512, 128],
)

nodes = node_parser.get_nodes_from_documents(documents)
storage_context = StorageContext.from_defaults(nodes=nodes)

# Create index
index = VectorStoreIndex(nodes, storage_context=storage_context)

# Create auto-merging retriever
auto_merging_retriever = AutoMergingRetriever(
    index.as_retriever(similarity_top_k=6),
    storage_context=storage_context,
    verbose=True,
)

# Use
query_engine = RetrieverQueryEngine(retriever=auto_merging_retriever)

Rerankers

SentenceTransformer Reranker

from llama_index.core.postprocessor import SentenceTransformerRerank
from llama_index.core import VectorStoreIndex

# Create reranker
reranker = SentenceTransformerRerank(
    model="cross-encoder/ms-marco-MiniLM-L-6-v2",
    top_n=5,
)

# Use in query engine
query_engine = index.as_query_engine(
    similarity_top_k=20,  # Retrieve more
    node_postprocessors=[reranker],  # Then rerank
)

response = query_engine.query("Your query")

Cohere Reranker

from llama_index.core.postprocessor import CohereRerank

reranker = CohereRerank(
    api_key=os.environ["COHERE_API_KEY"],
    top_n=5,
    model="rerank-english-v3.0",
)

query_engine = index.as_query_engine(
    similarity_top_k=20,
    node_postprocessors=[reranker],
)

LLM Reranker

from llama_index.core.postprocessor import LLMRerank
from llama_index.llms.openai import OpenAI

reranker = LLMRerank(
    top_n=5,
    llm=OpenAI(model="gpt-4o", temperature=0.0),
)

query_engine = index.as_query_engine(
    similarity_top_k=10,
    node_postprocessors=[reranker],
)

Query Engines

Router Query Engine

from llama_index.core.query_engine import RouterQueryEngine
from llama_index.core.tools import QueryEngineTool, ToolMetadata
from llama_index.core.selectors import LLMSingleSelector

# Create multiple query engines
summary_engine = summary_index.as_query_engine()
vector_engine = vector_index.as_query_engine()

# Create tools
query_engine_tools = [
    QueryEngineTool(
        query_engine=summary_engine,
        metadata=ToolMetadata(
            name="summary_tool",
            description="Useful for summarizing documents",
        ),
    ),
    QueryEngineTool(
        query_engine=vector_engine,
        metadata=ToolMetadata(
            name="vector_tool",
            description="Useful for specific questions about documents",
        ),
    ),
]

# Create router
router_engine = RouterQueryEngine(
    selector=LLMSingleSelector.from_defaults(),
    query_engine_tools=query_engine_tools,
    verbose=True,
)

response = router_engine.query("What is the main topic?")

Sub-Question Query Engine

from llama_index.core.query_engine import SubQuestionQueryEngine

# Create sub-question engine
sub_question_engine = SubQuestionQueryEngine.from_defaults(
    query_engine_tools=query_engine_tools,
    use_async=True,
    verbose=True,
)

# Automatically decomposes into sub-questions
response = sub_question_engine.query(
    "Compare the revenue growth of Company A and Company B"
)

Multi-Step Query Engine

from llama_index.core.query_engine import MultiStepQueryEngine

# Create multi-step engine
multi_step_engine = MultiStepQueryEngine(
    query_engine=base_query_engine,
    llm=OpenAI(model="gpt-4o"),
    max_iterations=5,
    verbose=True,
)

# Breaks down complex questions
response = multi_step_engine.query(
    "What factors contributed to the market cap change?"
)

Agents

ReAct Agent

from llama_index.core.agent import ReActAgent
from llama_index.core.tools import FunctionTool
from llama_index.llms.openai import OpenAI

# Define tools
def add(x: int, y: int) -> int:
    """Add two numbers."""
    return x + y

def multiply(x: int, y: int) -> int:
    """Multiply two numbers."""
    return x * y

def search_knowledge(query: str) -> str:
    """Search the knowledge base."""
    response = query_engine.query(query)
    return str(response)

tools = [
    FunctionTool.from_defaults(add),
    FunctionTool.from_defaults(multiply),
    FunctionTool.from_defaults(search_knowledge),
]

# Create agent
agent = ReActAgent(
    llm=OpenAI(model="gpt-4o", temperature=0.0),
    tools=tools,
    max_iterations=10,
    verbose=True,
)

# Run
response = agent.chat("What is 20 + (5 * 3)?")
print(response)

Function Calling Agent

from llama_index.core.agent import FunctionCallingAgent

# Use for models with native function calling
agent = FunctionCallingAgent(
    llm=OpenAI(model="gpt-4o"),
    tools=tools,
    max_iterations=10,
    verbose=True,
)

response = agent.chat("Calculate and search")

Custom Tools

from llama_index.core.tools import BaseTool
from pydantic import BaseModel, Field
from typing import Optional

# Pydantic input schema
class SearchInput(BaseModel):
    query: str = Field(description="Search query")
    limit: Optional[int] = Field(default=10, description="Max results")
    category: Optional[str] = Field(default=None, description="Filter category")

class CustomSearchTool(BaseTool):
    name = "custom_search"
    description = "Search with structured parameters"

    def __call__(self, input: SearchInput) -> str:
        results = self._search(input.query, input.limit, input.category)
        return "\n".join(results)

    def _search(self, query: str, limit: int, category: str):
        # Implementation
        return ["result1", "result2"]

# Usage
tool = CustomSearchTool()
agent = ReActAgent(llm=llm, tools=[tool])

Query Engine as Tool

from llama_index.core.tools import QueryEngineTool, ToolMetadata

# Wrap query engine as tool
query_tool = QueryEngineTool(
    query_engine=index.as_query_engine(),
    metadata=ToolMetadata(
        name="knowledge_base",
        description="Search the knowledge base for information",
    ),
)

# Use in agent
agent = ReActAgent(
    llm=OpenAI(model="gpt-4o"),
    tools=[query_tool, other_tool],
)

Streaming

Streaming Responses

from llama_index.core import VectorStoreIndex

# Enable streaming
query_engine = index.as_query_engine(streaming=True)

# Stream response
response = query_engine.query("What is the main topic?")

# Print tokens as they arrive
for token in response.response_gen:
    print(token, end="", flush=True)

Async Streaming

import asyncio

async def stream_query(query: str):
    streaming_response = await query_engine.aquery(query)

    content = ""
    async for token in streaming_response.async_response_gen():
        print(token, end="", flush=True)
        content += token

    return content

# Run
asyncio.run(stream_query("Your query"))

Streaming Chat

from llama_index.core.memory import ChatMemoryBuffer

# Create chat engine with memory
memory = ChatMemoryBuffer.from_defaults(token_limit=1500)
chat_engine = index.as_chat_engine(
    chat_mode="context",
    memory=memory,
    streaming=True,
)

# Stream chat
response = chat_engine.stream_chat("Tell me about topic X")
for token in response.response_gen:
    print(token, end="", flush=True)

# Continue conversation
response = chat_engine.stream_chat("Can you elaborate?")
for token in response.response_gen:
    print(token, end="", flush=True)

FastAPI Streaming

from fastapi import FastAPI
from fastapi.responses import StreamingResponse

app = FastAPI()

@app.post("/stream")
async def stream_response(query: str):
    async def generate():
        streaming_response = await query_engine.aquery(query)
        async for token in streaming_response.async_response_gen():
            yield f"data: {token}\n\n"
        yield "data: [DONE]\n\n"

    return StreamingResponse(
        generate(),
        media_type="text/event-stream",
    )

Evaluation

Faithfulness Evaluation

from llama_index.core.evaluation import FaithfulnessEvaluator
from llama_index.llms.openai import OpenAI

# Create evaluator
llm = OpenAI(model="gpt-4o", temperature=0.0)
evaluator = FaithfulnessEvaluator(llm=llm)

# Evaluate response
query_engine = index.as_query_engine()
response = query_engine.query("What are the key points?")

eval_result = evaluator.evaluate_response(response=response)

print(f"Passing: {eval_result.passing}")
print(f"Feedback: {eval_result.feedback}")

Relevancy Evaluation

from llama_index.core.evaluation import RelevancyEvaluator

evaluator = RelevancyEvaluator(llm=llm)

eval_result = evaluator.evaluate_response(
    query="What is the main topic?",
    response=response,
)

print(f"Passing: {eval_result.passing}")
print(f"Score: {eval_result.score}")

Ragas Integration

from llama_index.core.evaluation import RagasEvaluator
from llama_index.core.evaluation.ragas import RagasMetric

# Create evaluator
evaluator = RagasEvaluator(
    metric=RagasMetric.FAITHFULNESS,
)

# Evaluate
result = evaluator.evaluate_response(
    query=query,
    response=response,
    contexts=[node.text for node in response.source_nodes],
)

print(f"Score: {result.score}")

Batch Evaluation

from tqdm import tqdm

def batch_evaluate(queries, responses, evaluator):
    results = []

    for query, response in tqdm(zip(queries, responses)):
        result = evaluator.evaluate_response(
            query=query,
            response=response,
        )
        results.append(result)

    passing_rate = sum(1 for r in results if r.passing) / len(results)
    avg_score = sum(r.score for r in results if r.score) / len(results)

    return {
        "passing_rate": passing_rate,
        "average_score": avg_score,
        "results": results,
    }

Observability

Callbacks and Token Tracking

from llama_index.core import Settings
from llama_index.core.callbacks import (
    CallbackManager,
    LlamaDebugHandler,
    TokenCountingHandler,
)
import tiktoken

# Create handlers
token_counter = TokenCountingHandler(
    tokenizer=tiktoken.encoding_for_model("gpt-4o").encode,
)
debug_handler = LlamaDebugHandler(print_trace_on_end=True)

# Set callback manager
Settings.callback_manager = CallbackManager([token_counter, debug_handler])

# Run query
response = query_engine.query("Your query")

# Get token counts
print(f"Total LLM tokens: {token_counter.total_llm_token_count}")
print(f"Embedding tokens: {token_counter.total_embedding_token_count}")

LlamaIndex Debugging

from llama_index.core.callbacks import LlamaDebugHandler

debug_handler = LlamaDebugHandler()
Settings.callback_manager = CallbackManager([debug_handler])

# Run query
response = query_engine.query("Your query")

# Get events
events = debug_handler.get_event_pairs()

for event in events:
    print(f"Event: {event[0].type}")
    print(f"Duration: {event[1].time - event[0].time}")

LangSmith Integration

import os

os.environ["LANGCHAIN_TRACING_V2"] = "true"
os.environ["LANGCHAIN_API_KEY"] = "your-langsmith-key"
os.environ["LANGCHAIN_PROJECT"] = "my-project"

# LlamaIndex automatically logs to LangSmith
response = query_engine.query("Your query")

Chat Engines

Basic Chat

from llama_index.core import VectorStoreIndex

# Create chat engine
chat_engine = index.as_chat_engine(
    chat_mode="condense_question",
    verbose=True,
)

# Chat
response = chat_engine.chat("Tell me about X")
print(response)

response = chat_engine.chat("Can you elaborate?")
print(response)

Chat with Memory

from llama_index.core.memory import ChatMemoryBuffer

# Create memory
memory = ChatMemoryBuffer.from_defaults(token_limit=2000)

chat_engine = index.as_chat_engine(
    chat_mode="context",
    memory=memory,
    system_prompt="You are a helpful assistant.",
    verbose=True,
)

# Chat maintains context
response = chat_engine.chat("What is X?")
response = chat_engine.chat("Give me more details")

# Reset memory
chat_engine.reset()

Chat Modes

# condense_question - Condenses chat history + question
chat_engine = index.as_chat_engine(chat_mode="condense_question")

# context - Uses context from index
chat_engine = index.as_chat_engine(chat_mode="context")

# condense_plus_context - Both condense and context
chat_engine = index.as_chat_engine(chat_mode="condense_plus_context")

# react - ReAct agent mode
chat_engine = index.as_chat_engine(chat_mode="react", verbose=True)

# openai - OpenAI function calling
chat_engine = index.as_chat_engine(chat_mode="openai")

Common Issues

Chunk Size Problems

# ❌ BAD: Too small chunks lose context
splitter = SentenceSplitter(chunk_size=64)

# ❌ BAD: Too large chunks dilute relevance
splitter = SentenceSplitter(chunk_size=8192)

# ✅ GOOD: Balanced chunk size
splitter = SentenceSplitter(
    chunk_size=512,  # For embeddings
    chunk_overlap=50,  # ~10% overlap
)

Retrieval Quality

# ❌ BAD: No reranking, few results
query_engine = index.as_query_engine(similarity_top_k=3)

# ✅ GOOD: Retrieve more, rerank
query_engine = index.as_query_engine(
    similarity_top_k=20,
    node_postprocessors=[
        SentenceTransformerRerank(top_n=5),
    ],
)

Memory Issues

# ❌ BAD: Load all documents at once
documents = SimpleDirectoryReader("./huge_folder").load_data()

# ✅ GOOD: Process in batches
from llama_index.core import StorageContext

for batch in document_batches:
    nodes = splitter.get_nodes_from_documents(batch)
    index.insert_nodes(nodes)

Embedding Dimension Mismatch

# ❌ BAD: Index created with different embedding
# Pinecone index: 1536 dimensions
# Using: 768 dimension embeddings

# ✅ GOOD: Match dimensions
embed_model = OpenAIEmbedding(model="text-embedding-3-small")  # 1536 dims
# Create Pinecone index with same dimensions

Best Practices

1. Use appropriate chunk sizes (512-1024 for most use cases)
2. Always add overlap (5-10% of chunk size)
3. Use rerankers for better retrieval quality
4. Enable streaming for better UX
5. Use async for parallel queries
6. Implement caching for repeated queries
7. Monitor token usage with callbacks
8. Test with evaluation before production
9. Use hybrid search for better recall
10. Keep context window in mind for chat

Resources

• Documentation: https://docs.llamaindex.ai/
• GitHub: https://github.com/run-llama/llama_index
• Examples: https://github.com/run-llama/llama_index/tree/main/docs/examples
• Discord: https://discord.gg/dGcwcsnxhU
• Blog: https://blog.llamaindex.ai/

Quick Reference

Common Imports

from llama_index.core import (
    VectorStoreIndex,
    SimpleDirectoryReader,
    Document,
    Settings,
    StorageContext,
)
from llama_index.core.node_parser import SentenceSplitter
from llama_index.core.query_engine import RetrieverQueryEngine
from llama_index.core.retrievers import VectorIndexRetriever
from llama_index.llms.openai import OpenAI
from llama_index.embeddings.openai import OpenAIEmbedding

Common Patterns

# Basic RAG
index = VectorStoreIndex.from_documents(documents)
query_engine = index.as_query_engine()
response = query_engine.query("query")

# With reranking
query_engine = index.as_query_engine(
    similarity_top_k=20,
    node_postprocessors=[reranker],
)

# Streaming
query_engine = index.as_query_engine(streaming=True)
for token in query_engine.query("query").response_gen:
    print(token)

# Chat
chat_engine = index.as_chat_engine()
response = chat_engine.chat("message")

# Agent
agent = ReActAgent.from_tools(tools, llm=llm)
response = agent.chat("message")