• NMSLIB is generic but fast, see the results of ANN benchmarks.
• A standalone implementation of our fastest method HNSW also exists as a header-only library.
• All the documentation (including using Python bindings and the query server, description of methods and spaces, building the library, etc) can be found on this page.
• For generic questions/inquiries, please, use the Gitter chat: GitHub issues page is for bugs and feature requests.

Non-Metric Space Library (NMSLIB) is an efficient cross-platform similarity search library and a toolkit for evaluation of similarity search methods. The core-library does not have any third-party dependencies. It has been gaining popularity recently. In particular, it has become a part of Amazon Elasticsearch Service.

The goal of the project is to create an effective and comprehensive toolkit for searching in generic and non-metric spaces. Even though the library contains a variety of metric-space access methods, our main focus is on generic and approximate search methods, in particular, on methods for non-metric spaces. NMSLIB is possibly the first library with a principled support for non-metric space searching.

NMSLIB is an extendible library, which means that is possible to add new search methods and distance functions. NMSLIB can be used directly in C++ and Python (via Python bindings). In addition, it is also possible to build a query server, which can be used from Java (or other languages supported by Apache Thrift (version 0.12). Java has a native client, i.e., it works on many platforms without requiring a C++ library to be installed.

Authors: Bilegsaikhan Naidan, Leonid Boytsov, Yury Malkov, David Novak. With contributions from Ben Frederickson, Lawrence Cayton, Wei Dong, Avrelin Nikita, Dmitry Yashunin, Bob Poekert, @orgoro, @gregfriedland, Scott Gigante, Maxim Andreev, Daniel Lemire, Nathan Kurz, Alexander Ponomarenko.

NMSLIB started as a personal project of Bilegsaikhan Naidan, who created the initial code base, the Python bindings, and participated in earlier evaluations. The most successful class of methods--neighborhood/proximity graphs--is represented by the Hierarchical Navigable Small World Graph (HNSW) due to Malkov and Yashunin (see the publications below). Other most useful methods, include a modification of the VP-tree due to Boytsov and Naidan (2013), a Neighborhood APProximation index (NAPP) proposed by Tellez et al. (2013) and improved by David Novak, as well as a vanilla uncompressed inverted file.

If you find this library useful, feel free to cite our SISAP paper [BibTex] as well as other papers listed in the end. One crucial contribution to cite is the fast Hierarchical Navigable World graph (HNSW) method [BibTex]. Please, also check out the stand-alone HNSW implementation by Yury Malkov, which is released as a header-only HNSWLib library.

The code is released under the Apache License Version 2.0 http://www.apache.org/licenses/. Older versions of the library include additional components, which have different licenses (but this does not apply to NMLISB 2.x):

Older versions of the library included the following components:

• The LSHKIT, which is embedded in our library, is distributed under the GNU General Public License, see http://www.gnu.org/licenses/.
• The k-NN graph construction algorithm NN-Descent due to Dong et al. 2011 (see the links below), which is also embedded in our library, seems to be covered by a free-to-use license, similar to Apache 2.
• FALCONN library's licence is MIT.

Leonid Boytsov was supported by the Open Advancement of Question Answering Systems (OAQA) group and the following NSF grant #1618159: "Matching and Ranking via Proximity Graphs: Applications to Question Answering and Beyond". Bileg was supported by the iAd Center.

Most important related papers are listed below in the chronological order:

• L. Boytsov, D. Novak, Y. Malkov, E. Nyberg (2016). Off the Beaten Path: Let’s Replace Term-Based Retrieval with k-NN Search. In proceedings of CIKM'16. [BibTex] We use a special branch of this library, plus the following Java code.
• Malkov, Y.A., Yashunin, D.A.. (2016). Efficient and robust approximate nearest neighbor search using Hierarchical Navigable Small World graphs. CoRR, abs/1603.09320. [BibTex]
• Bilegsaikhan, N., Boytsov, L. 2015 Permutation Search Methods are Efficient, Yet Faster Search is Possible PVLDB, 8(12):1618--1629, 2015 [BibTex]
• Ponomarenko, A., Averlin, N., Bilegsaikhan, N., Boytsov, L., 2014. Comparative Analysis of Data Structures for Approximate Nearest Neighbor Search. [BibTex]
• Malkov, Y., Ponomarenko, A., Logvinov, A., & Krylov, V., 2014. Approximate nearest neighbor algorithm based on navigable small world graphs. Information Systems, 45, 61-68. [BibTex]
• Boytsov, L., Bilegsaikhan, N., 2013. Engineering Efficient and Effective Non-Metric Space Library. In Proceedings of the 6th International Conference on Similarity Search and Applications (SISAP 2013). [BibTex]
• Boytsov, L., Bilegsaikhan, N., 2013. Learning to Prune in Metric and Non-Metric Spaces. In Advances in Neural Information Processing Systems 2013. [BibTex]
• Tellez, Eric Sadit, Edgar Chávez, and Gonzalo Navarro. Succinct nearest neighbor search. Information Systems 38.7 (2013): 1019-1030. [BibTex]
• A. Ponomarenko, Y. Malkov, A. Logvinov, and V. Krylov Approximate nearest neighbor search small world approach. ICTA 2011
• Dong, Wei, Charikar Moses, and Kai Li. 2011. Efficient k-nearest neighbor graph construction for generic similarity measures. Proceedings of the 20th international conference on World wide web. ACM, 2011. [BibTex]
• L. Cayton, 2008 Fast nearest neighbor retrieval for bregman divergences. Twenty-Fifth International Conference on Machine Learning (ICML). [BibTex]
• Amato, Giuseppe, and Pasquale Savino. 2008 Approximate similarity search in metric spaces using inverted files. [BibTex]
• Gonzalez, Edgar Chavez, Karina Figueroa, and Gonzalo Navarro. Effective proximity retrieval by ordering permutations. Pattern Analysis and Machine Intelligence, IEEE Transactions on 30.9 (2008): 1647-1658. [BibTex]