How to build a RAG model from scratch?

Recent advancements in AI, particularly in the domain of large language models (LLMs) and generative models, have transformed how we interact with data. One such innovation is the Retrieval-Augmented Generation (RAG) model, which combines the benefits of retrieval-based methods and generative models. RAG enhances traditional generation methods by retrieving relevant information from a knowledge source (like a search index or document database) to augment and guide the generation process. This hybrid approach has shown to be more effective in tasks such as question answering, summarization, and knowledge-grounded dialogue.
In this article, we'll cover the following:

1. Overview of RAG Models: What are RAG models, and why do we need them?
2. Architectural Components of a RAG Model: An in-depth look at the building blocks.
3. Building the RAG Model: Step-by-step guide on implementing a RAG model from scratch.
4. Training the RAG Model: Fine-tuning both the retrieval and generation components.
5. Use Cases and Applications: Real-world applications of RAG models. By the end of this article, you'll understand how to build, train, and fine-tune your RAG model from scratch, as well as how to deploy it for real-world applications.

1. Overview of RAG Models

What is a RAG Model?

A RAG model is a hybrid system combining two AI techniques:

• Retrieval-based models: Systems that search for relevant pieces of information from a large set of documents or knowledge bases.
• Generative models: AI systems, such as GPT-3, that can generate coherent text from a prompt. The idea behind RAG is to combine the strengths of both approaches. Rather than generating answers from a fixed model that has limitations in knowledge scope and can hallucinate or invent facts, RAG allows the generative component to retrieve real, external knowledge first and then generate answers grounded in those retrieved facts. For instance, in question answering, a RAG model would:

1. Use a retriever to gather relevant documents from a database.
2. Use a generator to synthesize the final answer by combining the retrieved documents with the input query. RAG models thus excel in knowledge-intensive tasks because they can access a large and constantly updated repository of information rather than relying purely on what the language model was trained on.

2. Architectural Components of a RAG Model

The RAG model has two main architectural components: the retriever and the generator. Each of these plays a crucial role in ensuring the model’s effectiveness in generating accurate, knowledge-grounded responses.

2.1 Retriever

The retriever is responsible for searching and fetching relevant documents, passages, or snippets from a pre-built knowledge base. It typically involves the following steps:

• Indexing: Before querying, we need a pre-built index of documents or knowledge pieces, created using methods such as:

1. - Dense passage retrieval (DPR)
2. - BM25 (classic term-based retrieval model)
3. - Sentence Transformers for embedding-based retrieval

• Embedding the Query: The input query is first transformed into a vector representation using an embedding model, typically a bi-encoder (like a BERT-based retriever). This vector is then used to search for the most relevant documents in the index.
• Document Scoring: Once the embeddings are obtained, the retriever computes a similarity score between the query embedding and the document embeddings. The top k documents are then selected.

2.2 Generator

The generator is responsible for producing the final output based on the input query and the retrieved documents. This component is usually a pre-trained language model such as BART or T5, fine-tuned to take the retrieved documents and generate responses.
Key elements of the generator:

• Input Formatting: The retrieved documents are concatenated with the query to form a single input sequence, which is passed to the language model.
• Text Generation: The model uses techniques such as beam search or nucleus sampling to generate the final output based on the input sequence.

3. Building the RAG Model from Scratch

Building a RAG model from scratch involves multiple steps, from constructing the retriever to integrating the generator. Below is a step-by-step guide on building a simple RAG model using the Hugging Face Transformers library and FAISS for efficient retrieval.

Step 1: Set Up the Environment

To start, you'll need to install the necessary dependencies:

These libraries are essential:

• transformers: For pre-trained language models.
• faiss: For building and querying the document index.
• sentence-transformers: For embedding queries and documents.
• datasets: For accessing and processing data.

Step 2: Prepare the Data

For the retriever, you need a large corpus of documents. Let’s assume you have a set of documents stored as text files. Each document should ideally be small (e.g., a few paragraphs) to make retrieval efficient.
We will use FAISS to index and query these documents.

Step 3: Implement the Retriever

The retriever uses the FAISS index to retrieve relevant documents for a given query. Here's how to perform the retrieval:

Now, the retriever is ready to use. Let’s test it with a query:

Step 4: Implement the Generator

The generator will be based on a pre-trained sequence-to-sequence model like BART or T5, which is capable of generating text from a query and additional context (retrieved documents).
We will concatenate the retrieved documents and the query into a single input for the generator:

Step 5: Putting it All Together

Finally, we combine the retriever and the generator into a single pipeline that accepts a query and outputs a generated response:

You should see the generator outputting an answer based on the retrieved documents, grounded in real-world knowledge.

4.1 Fine-Tuning the Retriever

Fine-tuning the retriever can significantly improve its performance. A bi-encoder model (such as DPR) is often fine-tuned on a dataset of question-answer pairs to optimize the retrieval process.
To fine-tune a retriever:

• Gather a dataset containing queries and their relevant documents.
• Use contrastive loss to train the model to maximize the similarity between queries and relevant documents while minimizing the similarity to irrelevant ones.
• Update the FAISS index with embeddings from the fine-tuned retriever.

4.2 Fine-Tuning the Generator

Similarly, the generator can be fine-tuned on datasets like SQuAD, TriviaQA, or custom question-answer pairs. The fine-tuning process involves training the model to generate coherent answers given both a query and retrieved documents.

Key steps:

1. Gather (or create) a dataset of query, document, and answer triples.
2. Fine-tune the sequence-to-sequence model on this dataset using cross-entropy loss.
3. Validate and adjust hyperparameters such as learning rate and batch size.

5. Use Cases and Applications

RAG models are versatile and can be applied to numerous real-world tasks, including:

• Open-domain Question Answering: RAG models can handle complex, knowledge-intensive questions by retrieving information from large corpora and generating answers.
• Knowledge-grounded Dialogue: Conversational agents can use RAG to access external knowledge during a conversation, enabling more informed and accurate responses.
• Document Summarization: RAG can be used to summarize lengthy documents by retrieving key information and generating concise summaries.
• Product Recommendations: Retrieval-augmented generation can assist in generating personalized recommendations based on retrieved user data and product descriptions.

Conclusion

Building a RAG model from scratch involves constructing both a retriever and a generator, fine-tuning them for optimal performance, and integrating them into a pipeline. The combination of these components results in a powerful model capable of answering knowledge-intensive questions, generating grounded responses, and interacting with external knowledge bases.
With tools like Hugging Face Transformers, FAISS, and Sentence Transformers, the process of building a RAG model is accessible to AI practitioners and enthusiasts. By following the steps outlined in this guide, you can develop a fully functional RAG model tailored to your specific use cases, whether it’s open-domain question answering, knowledge-grounded dialogue, or document summarization.