Summary of key value stores out there.
https://github.com/linkedin/PalDB
Made by LinkedIn. Pure Java. Write once store. Uses single DB file. Looks good 1st but no commits since 4 years.
https://github.com/jankotek/mapdb
Nice key value store written in Kotlin
Small memory footprint, runs embedded within your Java program. Encryption support. Con: All data in memory.
https://github.com/indeedeng/mph-table https://engineering.indeedblog.com/blog/2018/02/indeed-mph/
Looks promising. But unfortunately a lot of dependencies. Also it requires some mmap JNI library which is not available for all platforms. Windows possible at all?
https://github.com/malyn/sg-cdb
Java implementation of D. Bernsteins CDB algorithm. Well coded, but inactive since 7 years.
https://github.com/sleepy-cat/cdb-java
Another Java implementation of CDB.
https://github.com/JetBrains/xodus
A Jetbrains embedded transactional database written in Java and Kotlin. https://github.com/JetBrains/xodus/tree/master/entity-store
https://github.com/spotify/sparkey
Made by spotify engineers as fun project. No commits anymore. Always index and data file. Addresses large files.
https://github.com/google/leveldb
Made by some Googlers. Actively maintained.
https://github.com/discoproject/discodb
Discodb is comprised of a low-level data structure implemented in C with a OO C++ API.
Embedded one C++ File. Low memory.
Facebook Opensource. GPL and Apache2 Seems to have encryption provider API
https://sqlite.org/src4/doc/trunk/www/lsmusr.wiki
Embedded key/value DB like BerkeleyDB, LevelDB, KyotoCabinet
https://fallabs.com/kyotocabinet/
Kyoto Cabinet is a library of routines for managing a database. Has compression (zlib) API which also offers RC4 encryption too. GPL and commercial license. Von last.fm gibt es besseren Java Layer: https://github.com/lastfm/lastcommons-kyoto
https://github.com/Microsoft/FASTER
C/C# implementation. Concurrent key-value store + cache that is designed for point lookups and heavy updates.