What is routing in modular RAG?

Routing uses a classifier or language model to direct queries to different retrieval strategies, data sources, or prompt templates based on query intent. For example, factual questions might go to a FAQ index, code questions to a codebase index, and complex reasoning to an agentic workflow. Routing improves efficiency by avoiding unnecessary retrieval for conversational turns that do not need external knowledge, and quality by matching each query to its best-suited retrieval method. LangChain and LlamaIndex both support routing as a first-class pattern.