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What Is Faiss (Facebook AI Similarity Search)?

Faiss is an open-source library designed for efficient similarity search and clustering of dense vectors, enabling applications like recommendation systems and image search.
Jul 2024  · 8 min read

Traditional search engines do a good job of finding exact matches, but they often fall short when it comes to identifying similar items based on content like images, videos, or text. This limitation can hinder the development of applications such as recommendation systems, image searches, and anomaly detection systems.

To overcome this, Meta AI (formerly Facebook AI) developed Facebook AI Similarity Search (Faiss).

Faiss is a library specifically designed to handle similarity searches efficiently, which it’s especially useful when dealing with large multimedia datasets.

In this article, I’ll explain what Faiss is and guide you on how to start using it for your search applications.

What Is Faiss?

Faiss, short for Facebook AI Similarity Search, is an open-source library built for similarity search and clustering of dense vectors. Faiss can be used to build an index and perform searches with remarkable speed and memory efficiency. Additionally, it enhances search performance through its GPU implementations for various indexing methods.

Core functionality

Faiss makes nearest-neighbor searches fast by indexing vectors using sophisticated algorithms like k-means clustering and product quantization. These methods help Faiss organize and retrieve vectors efficiently, ensuring similarity searches are quick and accurate. Here's a closer look at the indexing algorithms:

  1. K-means clustering: This algorithm breaks the data into clusters, which helps narrow down the search space by focusing on the most relevant clusters during queries.
  2. Product quantization (PQ): PQ compresses vectors into shorter codes, reducing memory usage significantly and speeding up the search without a big drop in accuracy.
  3. Optimized product quantization (OPQ): An enhanced version of PQ, OPQ rotates the data to better fit the quantization grid, improving the accuracy of the compressed vectors.


Faiss is pretty versatile when it comes to measuring similarity between vectors, offering a variety of distance metrics to choose from. The main ones are:

  • Euclidean distance: This measures the straight-line distance between two points, which is great when you care about the geometric similarity of vectors.
  • Cosine similarity: This looks at the cosine of the angle between two vectors, focusing more on their orientation than their size. It's especially handy for text analysis where the direction matters more than the length.

These options allow you to pick the metric that best fits your data and application needs.

Faiss can run on both CPUs and GPUs, using modern hardware to speed up the search process. Faiss is designed for various computing platforms, from personal computers to high-performance computing clusters. It smoothly transitions between CPU and GPU indices, and its Python interface works well with C++ indices, making it easy to switch from testing to deployment. This multi-platform support ensures that Faiss can be efficiently used in various computing environments, optimizing performance and resource use.

Key Features of Faiss

Faiss is a standout tool for similarity search, packed with features designed to handle large and diverse datasets effectively. Here’s a closer look at some of the core capabilities that make it a powerful asset for data-intensive tasks.


Faiss is designed to manage datasets from millions to billions of vectors, which is perfect for applications like large recommendation systems or massive image and video databases. It uses advanced techniques like inverted file systems and hierarchical navigable small world (HNSW) graphs to keep things efficient even with extensive datasets.


Faiss is fast due to its optimized algorithms and data structures. It uses k-means clustering, product quantization, and optimized brute-force searches to speed things up. If you’re using a GPU, Faiss can be up to 20 times faster on newer Pascal-class hardware compared to its CPU versions. This speed is crucial for real-time applications where you need quick responses.


Faiss gives you flexibility in accuracy, balancing speed and precision based on what you need. You can fine-tune it for highly accurate searches or go for quicker, less precise results. There are different indexing methods and parameters to choose from, and you can measure performance with metrics like 1-recall@1 and 10-intersection to see how well it’s doing compared to a brute-force approach.


Faiss can handle different types of data by converting them into vector representations. This means you can use it for images, text, audio, and more, making it useful across various fields and industries. It supports several distance metrics, including Euclidean distance, cosine similarity, and inner-product distance, allowing you to tailor the search process to your needs. Faiss is adaptable for diverse applications like image similarity search, text document retrieval, and audio fingerprinting.

Use Cases of Faiss

Faiss is versatile and efficient, making it a great fit for a variety of applications across different industries. Let’s dive into some of the main use cases where Faiss excels.

Recommendation systems

Faiss is a game-changer for recommendation systems. It can quickly find similar items within huge datasets, like products, movies, or articles.

Imagine an e-commerce platform using Faiss to analyze user behavior and product interactions. It generates high-dimensional vectors for these interactions, and by performing nearest-neighbor searches, Faiss identifies products similar to those a user has viewed or purchased. This personalized approach boosts user engagement and satisfaction, which drives sales and customer retention.

Image and video search

Faiss also powers search engines that retrieve visually similar images or videos by indexing high-dimensional vectors from multimedia content. Think of a photo organization app using Faiss to help users find all images of a specific landmark in their photo library. By converting images into vectors and indexing these vectors with Faiss, the app can quickly perform similarity searches. This capability is just as useful for video platforms, where Faiss can index and search for similar video clips based on visual and audio features, enhancing content discovery and recommendation features.

Anomaly detection

In anomaly detection, Faiss is great at identifying outliers or anomalies in datasets by finding points that deviate significantly from their nearest neighbors.

Take fraud detection, for example: financial transactions can be converted into vectors that include attributes like transaction amount, location, and time. Faiss performs similarity searches to flag transactions that are outliers, indicating potential fraud.

In network security, Faiss can detect unusual network traffic patterns that might signify cyber-attacks.

In quality control, it can identify defective products by comparing their features to those of normal products.

Information retrieval

Faiss is also a fantastic tool for information retrieval, helping find relevant documents or passages based on semantic similarity. This is invaluable for search engines, digital libraries, or any system needing quick and accurate text retrieval.

For example, a search engine can convert documents and queries into high-dimensional vectors using techniques like word2vec or BERT embeddings. By indexing these vectors with Faiss, the search engine can quickly perform similarity searches to retrieve documents that match the user’s query in meaning rather than just keyword matching. This makes search results more relevant, enhancing user experience and efficiency in information retrieval.

Getting Started With Faiss

In this section, I’ll show you how to set up Faiss and use it alongside LangChain and OpenAI embeddings.


To install Faiss, you can use pip to get the CPU or GPU version:

# For CPU
pip install faiss-cpu

# For GPU
pip install faiss-gpu

Additionally, to use Faiss with LangChain and OpenAI embeddings, you’ll need to install the following packages:

pip install -U langchain-community langchain-openai tiktoken

Basic application

Here’s a simple code example demonstrating how to index vectors and perform a nearest neighbor search using Faiss, LangChain, and OpenAI embeddings:

from langchain.document_loaders import WikipediaLoader
from langchain_community.vectorstores import FAISS
from langchain_openai import OpenAIEmbeddings
from langchain_text_splitters import CharacterTextSplitter

# Load content from Wikipedia using WikipediaLoader
loader = WikipediaLoader("Machine_learning")
document = loader.load()

# Chunking
text_splitter = CharacterTextSplitter(chunk_size=1000, chunk_overlap=0)
docs = text_splitter.split_documents(documents)

# Loading embeddings model
embeddings = OpenAIEmbeddings()

# Convert documents to vectors and index vectors
db = FAISS.from_documents(docs, embeddings)

# Search query
query = "What is machine learning?"
docs = db.similarity_search(query)

If you want to see more examples and familiarize yourself with Faiss, check out the official documentation and these tutorials.


Faiss stands out as a powerful tool for efficient similarity search, offering scalability, speed, accuracy, and versatility. It can handle large datasets and perform quick, accurate similarity searches, making it invaluable for various applications like recommendation systems, anomaly detection, and information retrieval.

By using Faiss, we can create smarter and more intuitive systems that enhance user experiences and improve operational efficiency.

For further learning and resources, consider exploring other relevant articles below:

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Ryan Ong

Ryan is a lead data scientist specialising in building AI applications using LLMs. He is a PhD candidate in Natural Language Processing and Knowledge Graphs at Imperial College London, where he also completed his Master’s degree in Computer Science. Outside of data science, he writes a weekly Substack newsletter, The Limitless Playbook, where he shares one actionable idea from the world's top thinkers and occasionally writes about core AI concepts.


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