How do I use LlamaIndex with pre-trained embeddings?

To use LlamaIndex with pre-trained embeddings, you’ll need to configure the library to generate and store vector representations of your data using a model of your choice. LlamaIndex simplifies connecting external data to large language models (LLMs) by organizing documents into structured indices, and pre-trained embeddings enable efficient semantic search. Start by installing LlamaIndex and an embeddings library (e.g., sentence-transformers), then define an embedding model, load your data, and build an index that leverages these embeddings for retrieval.

First, set up your environment and choose an embedding model. For example, you might use Hugging Face’s all-MiniLM-L6-v2 or OpenAI’s text-embedding-ada-002. LlamaIndex provides wrapper classes like HuggingFaceEmbedding or OpenAIEmbedding to integrate these models. Here’s a basic workflow: After loading documents (e.g., PDFs, text files), create a ServiceContext object that specifies your embedding model. This context is passed to the VectorStoreIndex class, which automatically generates embeddings for each document chunk and stores them in a vector database (e.g., FAISS, Pinecone). For instance, using a local model with sentence-transformers, you’d initialize the index with VectorStoreIndex.from_documents(documents, service_context=service_context).

Next, customize the process for your use case. If you’re working with domain-specific data (e.g., medical text), you might select a specialized pre-trained model like PubMedBERT. LlamaIndex also lets you adjust how documents are split into “nodes” (text chunks) before embedding. For example, setting chunk_size=512 ensures text is processed in manageable segments. When querying the index, LlamaIndex uses the embeddings to find the most relevant nodes based on semantic similarity, which are then fed to the LLM for answer generation. To optimize performance, consider caching embeddings or using batch processing for large datasets. If you’re using a GPU, models like bge-large-en from Sentence Transformers can speed up inference.

Finally, balance trade-offs between speed, accuracy, and resource usage. Smaller models (e.g., all-MiniLM-L6-v2) are faster but may sacrifice nuance, while larger models (e.g., text-embedding-3-large) capture more detail at the cost of higher latency. For production, pair LlamaIndex with a dedicated vector database like Chroma or Weaviate to scale beyond in-memory storage. By combining pre-trained embeddings with LlamaIndex’s indexing pipeline, you create a flexible system for grounding LLMs in your data without retraining models from scratch.