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new 0.14.0 Jun 6, 2023
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#12 in Database implementations

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Smaller & Faster Single-File
Vector Search Engine

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Euclidean • Angular • Jaccard • Hamming • Haversine • User-Defined Metrics
Linux • MacOS • Windows • Docker • WebAssembly 🔜

Implementation 84 K SLOC in faiss/ 1 K SLOC in usearch/
Supported metrics 9 fixed metrics Any User-Defined metrics
Supported ID types uint32_t, uint64_t uint32_t, uint40_t, uint64_t
Dependencies BLAS, OpenMP None
Bindings SWIG Native
Acceleration Learned Quantization Downcasting

FAISS is the industry standard for a high-performance batteries-included vector search engine. Both USearch and FAISS implement the same HNSW algorithm. But they differ in a lot of design decisions. USearch is designed to be compact and broadly compatible without sacrificing performance.

FAISS, f32 USearch, f32 USearch, f16 USearch, f8
Insertions 76 K/s 105 K/s 115 K/s 202 K/s
Queries 118 K/s 174 K/s 173 K/s 304 K/s
Recall @1 99% 99.2% 99.1% 99.2%

Dataset: 1M vectors sample of the Deep1B dataset. Hardware: c7g.metal AWS instance with 64 cores and DDR5 memory. HNSW was configured with identical hyper-parameters: connectivity M=16, expansion @ construction efConstruction=128, and expansion @ search ef=64. Both libraries were compiled for the target architecture. Jump to the Performance Tuning section to read about the effects of those hyper-parameters.

User-Defined Functions

Most vector-search packages focus on just 2 metrics - "Inner Product distance" and "Euclidean distance". That only partially exhausts the list of possible metrics. A good example would be the rare Haversine distance, used to compute the distance between geo-spatial coordinates, extending Vector Search into the GIS domain. Another example would be designing a custom metric for composite embeddings concatenated from multiple AI models in real-world applications. USearch supports that: Python and C++ examples.

USearch: Vector Search Approaches

Unlike older approaches indexing high-dimensional spaces, like KD-Trees and Locality Sensitive Hashing, HNSW doesn't require vectors to be identical in length. They only have to be comparable. So you can apply it in obscure applications, like searching for similar sets or fuzzy text matching.

Memory Efficiency, Downcasting, and Quantization

Training a quantization model and dimension-reduction is a common approach to accelerate vector search. Those, however, are only sometimes reliable, can significantly affect the statistical properties of your data, and require regular adjustments if your distribution shifts.

USearch uint40_t support

Instead, we have focused on high-precision arithmetic over low-precision downcasted vectors. The same index, and add and search operations will automatically down-cast or up-cast between f32_t, f16_t, f64_t, and f8_t representations, even if the hardware doesn't natively support it. Continuing the topic of memory-efficiency, we provide a uint40_t to allow collection with over 4B+ vectors without allocating 8 bytes for every neighbor reference in the proximity graph.

View Larger Indexes from Disk

Modern search systems often suggest using different servers to maximize indexing speed and minimize serving costs. Memory-optimized for the first task, and storage-optimized for the second, if the index can be served from external memory, which USearch can.

To Build To Serve
Instance u-24tb1.metal is4gen.8xlarge
Price ~ $200/h ~$4.5/h
Memory 24 TB RAM + EBS 192 GB RAM + 30 TB SSD

There is a 50x difference between the cost of such instances for identical capacity. Of course, the latency of external memory access will be higher, but it is in part compensated with an excellent prefetching mechanism.


There are two usage patters:

  1. Bare-bones with usearch/usearch.hpp, only available in C++.
  2. Full-fat version with it's own threads, mutexes, type-punning, quantization, that is available both in C++ and is wrapped for higher-level bindings.



To use in a C++ project simply copy the include/usearch/usearch.hpp header into your project. Alternatively fetch it with CMake:

FetchContent_Declare(usearch GIT_REPOSITORY https://github.com/unum-cloud/usearch.git)


Once included, the low-level C++11 interface is as simple as it gets: reserve(), add(), search(), size(), capacity(), save(), load(), view(). This covers 90% of use-cases.

using namespace unum::usearch;

index_gt<cos_gt<float>> index;
float vec[3] = {0.1, 0.3, 0.2};

index.add(/* label: */ 42, /* vector: */ {&vec[0], 3});
auto results = index.search(/* query: */ {&vec[0], 3}, 5 /* neighbors */);

for (std::size_t i = 0; i != results.size(); ++i)
    results[i].member.label, results[i].member.vector, results[i].distance;

The add is thread-safe for concurrent index construction.


index.load("index.usearch"); // Copying from disk
index.view("index.usearch"); // Memory-mapping from disk

User-Defined Metrics in C++

For advanced users, more compile-time abstractions are available.

template <typename metric_at = ip_gt<float>,            //
          typename label_at = std::size_t,              // `uint32_t`, `uuid_t`...
          typename id_at = std::uint32_t,               // `uint40_t`, `uint64_t`...
          typename scalar_at = float,                   // `double`, `half`, `char`...
          typename allocator_at = std::allocator<char>> //
class index_gt;

You may want to use a custom memory allocator or a rare scalar type, but most often, you would start by defining a custom similarity measure. The function object should have the following signature to support different-length vectors.

struct custom_metric_t {
    T operator()(T const* a, T const* b, std::size_t a_length, std::size_t b_length) const;

The following distances are pre-packaged:

  • cos_gt<scalar_t> for "Cosine" or "Angular" distance.
  • ip_gt<scalar_t> for "Inner Product" or "Dot Product" distance.
  • l2sq_gt<scalar_t> for the squared "L2" or "Euclidean" distance.
  • jaccard_gt<scalar_t> for "Jaccard" distance between two ordered sets of unique elements.
  • bitwise_hamming_gt<scalar_t> for "Hamming" distance, as the number of shared bits in hashes.
  • bitwise_tanimoto_gt<scalar_t> for "Tanimoto" coefficient for bit-strings.
  • bitwise_sorensen_gt<scalar_t> for "Dice-Sorensen" coefficient for bit-strings.
  • pearson_correlation_gt<scalar_t> for "Pearson" correlation between probability distributions.
  • haversine_gt<scalar_t> for "Haversine" or "Great Circle" distance between coordinates used in GIS applications.


Most AI, HPC, or Big Data packages use some form of a thread pool. Instead of spawning additional threads within USearch, we focus on the thread safety of add() function, simplifying resource management.

#pragma omp parallel for
    for (std::size_t i = 0; i < n; ++i)
        native.add(label, span_t{vector, dims}, add_config_t { .thread = omp_get_thread_num() });

During initialization, we allocate enough temporary memory for all the cores on the machine. On the call, the user can supply the identifier of the current thread, making this library easy to integrate with OpenMP and similar tools.



pip install usearch


import numpy as np
from usearch.index import Index

index = Index(
    ndim=3, # Define the number of dimensions in input vectors
    metric='cos', # Choose 'l2sq', 'haversine' or other metric, default = 'ip'
    dtype='f32', # Quantize to 'f16' or 'f8' if needed, default = 'f32'
    connectivity=16, # How frequent should the connections in the graph be, optional
    expansion_add=128, # Control the recall of indexing, optional
    expansion_search=64, # Control the quality of search, optional

vector = np.array([0.2, 0.6, 0.4], dtype=np.float32)
index.add(42, vector)
matches, distances, count = index.search(vector, 10)

assert len(index) == 1
assert count == 1
assert matches[0] == 42
assert distances[0] <= 0.001

Python bindings are implemented with pybind/pybind11. Assuming the presence of Global Interpreter Lock in Python, we spawn threads in the C++ layer on large insertions.


index.load('index.usearch') # Copy the whole index into memory
index.view('index.usearch') # View from disk without loading in memory

Batch Operations

Adding or querying a batch of entries is identical to adding a single vector. The difference would be in the shape of the tensors.

n = 100
labels = np.array(range(n), dtype=np.longlong)
vectors = np.random.uniform(0, 0.3, (n, index.ndim)).astype(np.float32)

index.add(labels, vectors, threads=..., copy=...)
matches, distances, counts = index.search(vectors, 10, threads=...)

assert matches.shape[0] == vectors.shape[0]
assert counts[0] <= 10

You can also override the default threads and copy arguments in bulk workloads. The first controls the number of threads spawned for the task. The second controls whether the vector itself will be persisted inside the index. If you can preserve the lifetime of the vector somewhere else, you can avoid the copy.

User-Defined Metrics in Python

Assuming the language boundary exists between Python user code and C++ implementation, there are more efficient solutions than passing a Python callable to the engine. Luckily, with the help of Numba, we can JIT compile a function with a matching signature and pass it down to the engine.

from numba import cfunc, types, carray

signature = types.float32(
    types.uint64, types.uint64)

def python_dot(a, b, n, m):
    a_array = carray(a, n)
    b_array = carray(b, n)
    c = 0.0
    for i in range(n):
        c += a_array[i] * b_array[i]
    return c

index = Index(ndim=ndim, metric=python_dot.address)

To use Numba JIT, install USearch with extras:

pip install usearch[jit]


from usearch.index import Index
from usearch.io import load_matrix, save_matrix

vectors = load_matrix('deep1B.fbin')
index = Index(ndim=vectors.shape[1])
index.add(labels, vectors)



npm install usearch


var index = new usearch.Index({ metric: 'cos', connectivity: 16, dimensions: 3 })
index.add(42, new Float32Array([0.2, 0.6, 0.4]))
var results = index.search(new Float32Array([0.2, 0.6, 0.4]), 10)

assert.equal(index.size(), 1)
assert.deepEqual(results.labels, new Uint32Array([42]))
assert.deepEqual(results.distances, new Float32Array([0]))





cargo add usearch


let quant: &str = "f16";
let index = new_ip(3, &quant, 0, 0, 0).unwrap();

assert!(index.capacity() >= 10);
assert!(index.connectivity() != 0);
assert_eq!(index.dimensions(), 3);
assert_eq!(index.size(), 0);

let first: [f32; 3] = [0.2, 0.1, 0.2];
let second: [f32; 3] = [0.2, 0.1, 0.2];

assert!(index.add(42, &first).is_ok());
assert!(index.add(43, &second).is_ok());
assert_eq!(index.size(), 2);

// Read back the tags
let results = index.search(&first, 10).unwrap();
assert_eq!(results.count, 2);


assert!(index.add_in_thread(42, &first, 0).is_ok());
assert!(index.add_in_thread(43, &second, 0).is_ok());
let results = index.search_in_thread(&first, 10, 0).unwrap();

Being a systems-programming language, Rust has better control over memory management and concurrency but lacks function overloading. Aside from the add and search, USearch Rust binding also provides add_in_thread and search_in_thread, which let users identify the calling thread to use underlying temporary memory more efficiently.




assert!(new_l2sq(3, &quant, 0, 0, 0).is_ok());
assert!(new_cos(3, &quant, 0, 0, 0).is_ok());
assert!(new_haversine(&quant, 0, 0, 0).is_ok());




Add that snippet to your pom.xml and hit mvn install.


Index index = new Index.Config().metric("cos").dimensions(2).build();
float vec[] = {10, 20};
index.add(42, vec);
int[] labels = index.search(vec, 5);





let index = Index.l2sq(dimensions: 3, connectivity: 8)
let vectorA: [Float32] = [0.3, 0.5, 1.2]
let vectorB: [Float32] = [0.4, 0.2, 1.2]
index.add(label: 42, vector: vectorA[...])
index.add(label: 43, vector: vectorB[...])

let results = index.search(vector: vectorA[...], count: 10)
assert(results.0[0] == 42)



import (


package main

import (

func main() {
	conf := usearch.DefaultConfig(128)
	index := usearch.NewIndex(conf)
	v := make([]float32, 128)
	index.Add(42, v)
	results := index.Search(v, 1)



  • JavaScript: Allow calling from "worker threads".
  • Rust: Allow passing a custom thread ID.
  • C# .NET bindings.

Application Examples

AI has a growing number of applications, but one of the coolest classic ideas is to use it for Semantic Search. One can take an encoder model, like the multi-modal UForm, and a web-programming framework, like UCall, and build a text-to-image search platform in just 20 lines of Python.

import ucall
import uform
import usearch

import numpy as np
import PIL as pil

server = ucall.Server()
model = uform.get_model('unum-cloud/uform-vl-multilingual')
index = usearch.index.Index(ndim=256)

def add(label: int, photo: pil.Image.Image):
    image = model.preprocess_image(photo)
    vector = model.encode_image(image).detach().numpy()
    index.add(label, vector.flatten(), copy=True)

def search(query: str) -> np.ndarray:
    tokens = model.preprocess_text(query)
    vector = model.encode_text(tokens).detach().numpy()
    matches = index.search(vector.flatten(), 3)
    return matches.labels


Research in natural sciences is expensive. To search for properties of different molecules, large databases are used. Comparing the molecules, however, is hard. Instead, binary hash-like fingerprints are produced from the graph structure, using packages like RDKit.

from usearch.index import Index, MetricKind
from rdkit import Chem
from rdkit.Chem import AllChem

import numpy as np

molecules = [Chem.MolFromSmiles('CCOC'), Chem.MolFromSmiles('CCO')]
encoder = AllChem.GetRDKitFPGenerator()

fingerprints = np.vstack([encoder.GetFingerprint(x) for x in molecules])
fingerprints = np.packbits(fingerprints, axis=1)

index = Index(ndim=2048, metric=MetricKind.BitwiseTanimoto)
labels = np.array(len(molecules), dtype=np.longlong)

index.add(labels, fingerprints)
matches = index.search(fingerprints, 10)

RDKit provides Atom-Pair, Topological Torsion, Morgan, and Layered Fingerprints, among other options.

Check that and other examples on our corporate GitHub 🤗


~30K SLoC