What is the role of the retriever versus the generator in RAG?

The retriever finds relevant documents from a large corpus given a query, optimizing for recall and precision on the top-k results. The generator is a language model that reads the retrieved documents along with the query and produces a coherent natural-language answer, optimizing for faithfulness and fluency. The two components have different failure modes: a bad retriever returns irrelevant context and the generator either hallucinates or refuses to answer, while a bad generator ignores good context or fabricates additions. Both must be evaluated separately. ---