I’ve used LMDB as a simpler alternative to SQLite as “an alternative to fopen”. The goal was simply robust file writes in the face of unpredictable server reboots for a tiny Python program writing data to be processed later by a tiny C++ program.
That’s harder than it sounds to roll by hand with fopen. SQLite with write ahead logging is pretty much as good as it gets for reliablity, but SQL at all was overkill for the task. LMDB is a close second and it’s memory mapped key-value interface is much simpler. . Would write again.
LMDB is a storage engine whereas SQLite is a small database. There is even a version of SQLite that used LMDB as the underlying storage engine: https://github.com/LMDB/sqlightning.