RAG: indexing, retrieval, and evaluation

Indexing

The indexing pipeline loads a web document, splits it into chunks with overlap, embeds each chunk with OpenAI's text-embedding-3-small, and persists the result to a local Chroma vectorstore.

Retrieval and generation

The RAG pipeline takes a user question, retrieves the top similar chunks from the vectorstore, and feeds them into a chat model along with a constrained prompt. The chain composes as retriever → prompt → model → string output, idiomatic LangChain.

Evaluation: three lenses

A separate evaluation pipeline grades each answer three complementary ways. The lenses disagree often, which is exactly when evaluation pays off.

• Code-based. Deterministic, free, instant. Length bounds, keyword presence, structural rules. Catches format regressions; blind to meaning.
• Model-as-judge. A separate LLM scores relevance and faithfulness against the retrieved context. Catches semantic problems the code checks cannot, at the cost of an extra API call.
• Human-grade. The ground truth the other two only approximate. Auto-skips when run non-interactively.

What I would change for production

The repo README spells out the migration path candidly. Each topic carries an honest tradeoff.

• Observability. LangSmith tracing so each retrieval and generation becomes an inspectable span.
• Cost guards. Rate limiting, token budget caps, and embedding-call dedupe for unchanged source documents.
• Async indexing. A background worker for large corpora so the request path stays snappy.
• Citations. Returning which retrieved chunks contributed to each answer for trust and debugging.
• Tests. Unit tests for chain composition; a smoke test against a tiny fixed corpus.
• Schema versioning. Versioning the collection by embedding model and chunk size as the vectorstore evolves.

Tech stack

• LangChain v1 for chain composition (retriever → prompt → LLM → string output)
• Python with python-dotenv for environment configuration
• OpenAI for chat (gpt-5.4-nano) and embeddings (text-embedding-3-small)
• Chroma as the local vectorstore via langchain-chroma, with chunks loaded from the web via WebBaseLoader + BeautifulSoup
• Shared config module (rag_common.py) so all three pipelines pull model names, logging, and the vectorstore from one place
• Logging and CLI: structured logging instead of print(), with argparse and environment-variable overrides for the source URL and query