Rethinking Knowledge Retrieval for Generation: A Survey on RAG Architectures and Applications

Abstract

Large Language Models (LLMs) have demonstrated remarkable fluency and versatility across natural language tasks but remain fundamentally limited by their static knowledge and susceptibility to hallucinations, especially in domains requiring up to date or attribute grounded information. Retrieval Augmented Generation (RAG) addresses these challenges by integrating external retrieval mechanisms with generative models, enabling dynamic, context aware generation grounded in verifiable data sources. This survey presents a comprehensive examination of RAG as a modular and evolving paradigm that enhances factual reliability, adaptability, and task alignment in LLM based systems. We formalize the RAG framework through its three foundational components retrieval, generation, and augmentation and survey state of the art methods spanning dense and sparse retrievers, fusion strategies, embedding optimizations, and reinforcement learning based retrieval policies. Anchored around four emerging axes efficiency, security, user centric interactivity, and complex reasoning we categorize recent innovations and highlight their implications for scalability, robustness, and personalization. The paper also reviews advances in evaluation protocols, domain specific applications, and architectural variants such as Na\"ive RAG, Advanced RAG, and Modular RAG. Finally, we identify persistent challenges and outline future directions aimed at advancing the integration of retrieval with LLMs for more grounded, interpretable, and controllable generation.