More seriously, I suspect that plain SQLite by itself, running in process in the main service, writing to a ':memory:' database for ultimate speed or perhaps to a local file with WAL mode for slightly more durability, could achieve pretty nice efficiency and work as the main transaction manager database.
SQLite can achieve this with some help: https://blog.expensify.com/2018/01/08/scaling-sqlite-to-4m-q...
More seriously, I suspect that plain SQLite by itself, running in process in the main service, writing to a ':memory:' database for ultimate speed or perhaps to a local file with WAL mode for slightly more durability, could achieve pretty nice efficiency and work as the main transaction manager database.