Retrieval ‑ Augmented Generation (RAG) has become one of the most important design patterns in modern AI because it gives language models direct access to external knowledge. Instead of relying solely on what a model has memorized during training, RAG systems retrieve relevant information from documents, databases, or other sources and feed it into the model at generation time. This idea dramatically improves accuracy, reduces hallucinations, and allows AI systems to stay current without constant retraining. RAG has evolved into a rich ecosystem of architectural layers, addressing different challenges: Core Retrieval layer focuses on improving how information is found, from basic vector search to more advanced techniques like query expansion and hierarchical retrieval. Structure ‑ Aware layer organizes and interprets data based on relationships, formats, or time, enabling retrieval from graphs, tables, or multimodal sources. Reasoning ‑ Enhanced layer st...