Backend RAG Implementation

Master Retrieval-Augmented Generation (RAG) to build LLM applications that provide accurate, grounded responses using external knowledge sources.

When to Use This Skill

• Building Q&A systems over proprietary documents
• Creating chatbots with current, factual information
• Implementing semantic search with natural language queries
• Reducing hallucinations with grounded responses
• Enabling LLMs to access domain-specific knowledge
• Building documentation assistants
• Creating research tools with source citation

Core Components

1. Vector Databases

Purpose: Store and retrieve document embeddings efficiently

Options:

• Pinecone: Managed, scalable, fast queries
• Weaviate: Open-source, hybrid search
• Milvus: High performance, on-premise
• Chroma: Lightweight, easy to use
• Qdrant: Fast, filtered search
• FAISS: Meta's library, local deployment

2. Embeddings

Purpose: Convert text to numerical vectors for similarity search

Models:

• text-embedding-ada-002 (OpenAI): General purpose, 1536 dims
• all-MiniLM-L6-v2 (Sentence Transformers): Fast, lightweight
• e5-large-v2: High quality, multilingual
• Instructor: Task-specific instructions
• bge-large-en-v1.5: SOTA performance

3. Retrieval Strategies

Approaches:

• Dense Retrieval: Semantic similarity via embeddings
• Sparse Retrieval: Keyword matching (BM25, TF-IDF)
• Hybrid Search: Combine dense + sparse
• Multi-Query: Generate multiple query variations
• HyDE: Generate hypothetical documents

4. Reranking

Purpose: Improve retrieval quality by reordering results

Methods:

• Cross-Encoders: BERT-based reranking
• Cohere Rerank: API-based reranking
• Maximal Marginal Relevance (MMR): Diversity + relevance
• LLM-based: Use LLM to score relevance

Quick Start

from langchain.document_loaders import DirectoryLoader
from langchain.text_splitters import RecursiveCharacterTextSplitter
from langchain.embeddings import OpenAIEmbeddings
from langchain.vectorstores import Chroma
from langchain.chains import RetrievalQA
from langchain.llms import OpenAI

# 1. Load documents
loader = DirectoryLoader('./docs', glob="**/*.txt")
documents = loader.load()

# 2. Split into chunks
text_splitter = RecursiveCharacterTextSplitter(
    chunk_size=1000,
    chunk_overlap=200,
    length_function=len
)
chunks = text_splitter.split_documents(documents)

# 3. Create embeddings and vector store
embeddings = OpenAIEmbeddings()
vectorstore = Chroma.from_documents(chunks, embeddings)

# 4. Create retrieval chain
qa_chain = RetrievalQA.from_chain_type(
    llm=OpenAI(),
    chain_type="stuff",
    retriever=vectorstore.as_retriever(search_kwargs={"k": 4}),
    return_source_documents=True
)

# 5. Query
result = qa_chain({"query": "What are the main features?"})
print(result['result'])
print(result['source_documents'])

Advanced RAG Patterns

Pattern 1: Hybrid Search

from langchain.retrievers import BM25Retriever, EnsembleRetriever

# Sparse retriever (BM25)
bm25_retriever = BM25Retriever.from_documents(chunks)
bm25_retriever.k = 5

# Dense retriever (embeddings)
embedding_retriever = vectorstore.as_retriever(search_kwargs={"k": 5})

# Combine with weights
ensemble_retriever = EnsembleRetriever(
    retrievers=[bm25_retriever, embedding_retriever],
    weights=[0.3, 0.7]
)

Pattern 2: Multi-Query Retrieval

from langchain.retrievers.multi_query import MultiQueryRetriever

# Generate multiple query perspectives
retriever = MultiQueryRetriever.from_llm(
    retriever=vectorstore.as_retriever(),
    llm=OpenAI()
)

# Single query → multiple variations → combined results
results = retriever.get_relevant_documents("What is the main topic?")

Pattern 3: Contextual Compression

from langchain.retrievers import ContextualCompressionRetriever
from langchain.retrievers.document_compressors import LLMChainExtractor

compressor = LLMChainExtractor.from_llm(llm)

compression_retriever = ContextualCompressionRetriever(
    base_compressor=compressor,
    base_retriever=vectorstore.as_retriever()
)

# Returns only relevant parts of documents
compressed_docs = compression_retriever.get_relevant_documents("query")

Pattern 4: Parent Document Retriever

from langchain.retrievers import ParentDocumentRetriever
from langchain.storage import InMemoryStore

# Store for parent documents
store = InMemoryStore()

# Small chunks for retrieval, large chunks for context
child_splitter = RecursiveCharacterTextSplitter(chunk_size=400)
parent_splitter = RecursiveCharacterTextSplitter(chunk_size=2000)

retriever = ParentDocumentRetriever(
    vectorstore=vectorstore,
    docstore=store,
    child_splitter=child_splitter,
    parent_splitter=parent_splitter
)

Document Chunking Strategies

Recursive Character Text Splitter

from langchain.text_splitters import RecursiveCharacterTextSplitter

splitter = RecursiveCharacterTextSplitter(
    chunk_size=1000,
    chunk_overlap=200,
    length_function=len,
    separators=["\n\n", "\n", " ", ""]  # Try these in order
)

Token-Based Splitting

from langchain.text_splitters import TokenTextSplitter

splitter = TokenTextSplitter(
    chunk_size=512,
    chunk_overlap=50
)

Semantic Chunking

from langchain.text_splitters import SemanticChunker

splitter = SemanticChunker(
    embeddings=OpenAIEmbeddings(),
    breakpoint_threshold_type="percentile"
)

Markdown Header Splitter

from langchain.text_splitters import MarkdownHeaderTextSplitter

headers_to_split_on = [
    ("#", "Header 1"),
    ("##", "Header 2"),
    ("###", "Header 3"),
]

splitter = MarkdownHeaderTextSplitter(headers_to_split_on=headers_to_split_on)

Vector Store Configurations

Pinecone

import pinecone
from langchain.vectorstores import Pinecone

pinecone.init(api_key="your-api-key", environment="us-west1-gcp")

index = pinecone.Index("your-index-name")

vectorstore = Pinecone(index, embeddings.embed_query, "text")

Weaviate

import weaviate
from langchain.vectorstores import Weaviate

client = weaviate.Client("http://localhost:8080")

vectorstore = Weaviate(client, "Document", "content", embeddings)

Chroma (Local)

from langchain.vectorstores import Chroma

vectorstore = Chroma(
    collection_name="my_collection",
    embedding_function=embeddings,
    persist_directory="./chroma_db"
)

Retrieval Optimization

1. Metadata Filtering

# Add metadata during indexing
chunks_with_metadata = []
for i, chunk in enumerate(chunks):
    chunk.metadata = {
        "source": chunk.metadata.get("source"),
        "page": i,
        "category": determine_category(chunk.page_content)
    }
    chunks_with_metadata.append(chunk)

# Filter during retrieval
results = vectorstore.similarity_search(
    "query",
    filter={"category": "technical"},
    k=5
)

2. Maximal Marginal Relevance

# Balance relevance with diversity
results = vectorstore.max_marginal_relevance_search(
    "query",
    k=5,
    fetch_k=20,  # Fetch 20, return top 5 diverse
    lambda_mult=0.5  # 0=max diversity, 1=max relevance
)

3. Reranking with Cross-Encoder

from sentence_transformers import CrossEncoder

reranker = CrossEncoder('cross-encoder/ms-marco-MiniLM-L-6-v2')

# Get initial results
candidates = vectorstore.similarity_search("query", k=20)

# Rerank
pairs = [[query, doc.page_content] for doc in candidates]
scores = reranker.predict(pairs)

# Sort by score and take top k
reranked = sorted(zip(candidates, scores), key=lambda x: x[1], reverse=True)[:5]

Prompt Engineering for RAG

Contextual Prompt

prompt_template = """Use the following context to answer the question. If you cannot answer based on the context, say "I don't have enough information."

Context:
{context}

Question: {question}

Answer:"""

With Citations

prompt_template = """Answer the question based on the context below. Include citations using [1], [2], etc.

Context:
{context}

Question: {question}

Answer (with citations):"""

With Confidence

prompt_template = """Answer the question using the context. Provide a confidence score (0-100%) for your answer.

Context:
{context}

Question: {question}

Answer:
Confidence:"""

Evaluation Metrics

def evaluate_rag_system(qa_chain, test_cases):
    metrics = {
        'accuracy': [],
        'retrieval_quality': [],
        'groundedness': []
    }

    for test in test_cases:
        result = qa_chain({"query": test['question']})

        # Check if answer matches expected
        accuracy = calculate_accuracy(result['result'], test['expected'])
        metrics['accuracy'].append(accuracy)

        # Check if relevant docs were retrieved
        retrieval_quality = evaluate_retrieved_docs(
            result['source_documents'],
            test['relevant_docs']
        )
        metrics['retrieval_quality'].append(retrieval_quality)

        # Check if answer is grounded in context
        groundedness = check_groundedness(
            result['result'],
            result['source_documents']
        )
        metrics['groundedness'].append(groundedness)

    return {k: sum(v)/len(v) for k, v in metrics.items()}

Resources

• references/vector-databases.md: Detailed comparison of vector DBs
• references/embeddings.md: Embedding model selection guide
• references/retrieval-strategies.md: Advanced retrieval techniques
• references/reranking.md: Reranking methods and when to use them
• references/context-window.md: Managing context limits
• assets/vector-store-config.yaml: Configuration templates
• assets/retriever-pipeline.py: Complete RAG pipeline
• assets/embedding-models.md: Model comparison and benchmarks

Best Practices

1. Chunk Size: Balance between context and specificity (500-1000 tokens)
2. Overlap: Use 10-20% overlap to preserve context at boundaries
3. Metadata: Include source, page, timestamp for filtering and debugging
4. Hybrid Search: Combine semantic and keyword search for best results
5. Reranking: Improve top results with cross-encoder
6. Citations: Always return source documents for transparency
7. Evaluation: Continuously test retrieval quality and answer accuracy
8. Monitoring: Track retrieval metrics in production

Common Issues

• Poor Retrieval: Check embedding quality, chunk size, query formulation
• Irrelevant Results: Add metadata filtering, use hybrid search, rerank
• Missing Information: Ensure documents are properly indexed
• Slow Queries: Optimize vector store, use caching, reduce k
• Hallucinations: Improve grounding prompt, add verification step