Question

You are building a RAG chain with LangChain Expression Language (LCEL) on Databricks. Your chain must call two independent retrievers — one against a product-documentation vector index and another against a support-ticket vector index — then merge their results before passing everything to the prompt. Profiling shows the two retrieval calls currently run one after the other, adding significant latency. Which LCEL construct should you use to invoke both retrievers concurrently and combine their outputs into a single dictionary for the downstream prompt?

Accepted Answer

Wrap the two retrievers in a RunnableParallel (a dict of runnables) so both execute concurrently and their outputs are collected under named keys.

Answer

Chain the two retrievers sequentially with the pipe operator and rely on LCEL to automatically parallelize independent steps.

Answer

Use a RunnablePassthrough to forward the query to both retrievers, which guarantees they run in parallel.

Answer

Place both retrievers inside a RunnableBranch, which routes the query to each retriever concurrently based on content.