head to head · open source
faiss vs langchain4j
faiss has 40,929 GitHub stars, 4,526 forks, 327 open issues and last shipped yesterday. langchain4j has 13,121 stars, 2,557 forks, 878 open issues and last shipped 3 days ago. faiss leads on adoption by 212% (40,929 vs 13,121 stars). faiss is written in C++ under MIT; langchain4j is written in Java under Apache-2.0. faiss has attracted 11% as many forks as stars, langchain4j 19%. faiss was the more recently maintained of the two, and both are self-hostable with no licence fee.
Two open source projects, one decision. Both are free and self-hostable — the differences are community size, license terms, language stack and release pace.
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Side by side
| faiss | langchain4j | |
|---|---|---|
| GitHub stars | ★ 41K | ★ 13K |
| License | MIT | Apache-2.0 |
| Written in | C++ | Java |
| Last push | 2026-09-18 | 2026-09-16 |
| Forks | ⑂ 4.5K | ⑂ 2.6K |
| Self-hosting | Yes | Yes |
| Data ownership | Your server | Your server |
pick faiss if
- You weight community size — 41K stars and counting
- You want the MIT license terms
- Your stack matches C++
- You value the larger contributor base for long-term maintenance
pick langchain4j if
- You want the langchain4j feature set and don't need the biggest community
- You prefer the Apache-2.0 license terms
- Your stack matches Java
- You evaluated both and langchain4j fits your workflow better
About faiss
Faiss is an MIT licensed C++ library for efficient similarity search and clustering of dense vectors, with complete Python and NumPy wrappers, developed primarily at Meta's Fundamental AI Research group, and it is intended for engineers and researchers who need nearest neighbour search over vector sets that range from small collections to sets that possibly do not fit in RAM.
read the full faiss overview →
About langchain4j
LangChain4j is an Apache 2.0, open source, idiomatic Java library for building LLM powered applications on the JVM, aimed at Java developers and enterprise teams who want one unified API over many LLM providers and embedding stores without leaving the Java ecosystem.
read the full langchain4j overview →
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Frequently asked questions
Is faiss or langchain4j more popular?
faiss has 40,929 GitHub stars and langchain4j has 13,121. faiss has the larger community by that measure.
Are faiss and langchain4j free?
Both are open source. faiss is licensed under MIT and langchain4j under Apache-2.0. Neither carries a licence fee.
What is the difference between faiss and langchain4j?
faiss is written in C++ and langchain4j in Java. The practical differences are community size, licence terms, language stack and release cadence — all compared in the table above.
Which should I choose, faiss or langchain4j?
Choose faiss if you want the larger community (40,929 stars) or its MIT licence terms. Choose langchain4j if its feature set, stack or Apache-2.0 licence fits better. Both are self-hostable.