Retrieval-Augmented Generation: Overview – DZone

By Jessie Hobb On Nov 17, 2023

This article is intended for data scientists, AI researchers, machine learning engineers, and advanced practitioners in the field of artificial intelligence who have a solid grounding in machine learning concepts, natural language processing, and deep learning architectures. It assumes familiarity with neural network optimization, transformer models, and the challenges of integrating real-time data into generative AI systems.

Introduction

Retrieval-Augmented Generation (RAG) models have emerged as a compelling solution to augment the generative capabilities of AI with external knowledge sources. These models synergize neural retrieval methods with seq2seq generation models to introduce non-parametric data into the generative process, significantly expanding the potential of AI to handle information-rich tasks. In this article we’ll look into a technical exposition of RAG architectures, delve into their operational intricacies, and provide a quick evaluation of their utility in professional settings and an overview of RAG models, highlighting their strengths, limitations, and the computational considerations intrinsic to their deployment.

Generative AI has traditionally been constrained by the static knowledge encapsulated within its parameters at the time of training. Retrieval-Augmented Generation models revolutionize this paradigm by leveraging external knowledge sources, providing a conduit for AI models to access and utilize vast repositories of information in real-time.

Technical Framework of RAG Models

A RAG model functions through an orchestrated two-step process: a retrieval phase followed by a generation phase. The retrieval component, often instantiated by a Dense Passage Retriever (DPR), employs a BERT-like architecture for encoding queries and documents into a shared embedding space. The generation component is typically a Transformer-based seq2seq model that conditions its outputs on the combined embeddings of the input and retrieved documents.

The Retriever: Dense Passage Retrieval

The retrieval phase is crucial for the RAG architecture. It employs a dense retriever, which is fine-tuned on a dataset of (query, relevant document) pairs. The DPR encodes both queries and documents into vectors in a continuous space, using dual-encoder architecture.

# Define tokenizers for the question and context encoders
question_tokenizer = DPRQuestionEncoderTokenizer.from_pretrained('facebook/dpr-question_encoder-single-nq-base')
context_tokenizer = DPRContextEncoderTokenizer.from_pretrained('facebook/dpr-ctx_encoder-single-nq-base')

# Encode and retrieve documents
question_tokens = question_tokenizer(query, return_tensors="pt")
context_tokens = context_tokenizer(list_of_documents, padding=True, truncation=True, return_tensors="pt")

# Encode question and context into embeddings
question_embeddings = question_encoder(**question_tokens)['pooler_output']
context_embeddings = context_encoder(**context_tokens)['pooler_output']

# Calculate similarities and retrieve top-k documents
similarity_scores = torch.matmul(question_embeddings, context_embeddings.T)
top_k_indices = similarity_scores.topk(k).indices
retrieved_docs = [list_of_documents[index] for index in top_k_indices[0]]

The Generator: Seq2Seq Model

For the generation phase, RAG employs a seq2seq framework, often instantiated by a model like BART or T5, capable of generating text based on the enriched context provided by retrieved documents. The cross-attention layers are crucial for the model to interweave the input and retrieved content coherently.

from transformers import BartForConditionalGeneration

# Initialize seq2seq generation model
seq2seq_model = BartForConditionalGeneration.from_pretrained('facebook/bart-large')

# Generate response using the seq2seq model conditioned on the input and retrieved documents
input_ids = tokenizer(query, return_tensors="pt").input_ids
outputs = seq2seq_model.generate(input_ids, encoder_outputs=document_embeddings)
response = tokenizer.decode(outputs[0], skip_special_tokens=True)

Performance Optimization and Computational Considerations

Training RAG models involves optimizing the dense retriever and the seq2seq generator in tandem. This necessitates backpropagating the loss from the output of the generator through to the retrieval component, a process that can introduce computational complexity and necessitate high-throughput hardware accelerators.

from torch.nn.functional import cross_entropy

# Compute generation loss
prediction_scores = seq2seq_model(input_for_generation).logits
generation_loss = cross_entropy(prediction_scores.view(-1, tokenizer.vocab_size), labels.view(-1))

# Compute contrastive loss for retrieval
# Contrastive loss encourages the correct documents to have higher similarity scores
retrieval_loss = contrastive_loss_function(similarity_scores, true_indices)

# Combine losses and backpropagate
total_loss = generation_loss + retrieval_loss
total_loss.backward()
optimizer.step()

Applications and Implications

RAG models have broad implications across a spectrum of applications, from enhancing conversational agents with real-time data fetching capabilities to improving the relevance of content recommendations. They also stand to make significant impacts on the efficiency and accuracy of information synthesis in research and academic settings.

Limitations and Ethical Considerations

Practically, RAG models contend with computational demands, latency in real-time applications, and the challenge of maintaining up-to-date external databases. Ethically, there are concerns regarding the propagation of biases present in the source databases and the veracity of information being retrieved.

Conclusion

RAG models represent a significant advancement in generative AI, introducing the capability to harness external knowledge in the generation process. This paper has provided a technical exploration of the RAG framework and has underscored the need for ongoing research into optimizing their performance and ensuring their ethical use. As the field evolves, RAG models stand to redefine the landscape of AI’s generative potential, opening new avenues for knowledge-driven applications.