The Appian engines are in-memory, real-time databases based on KDB and the K language. The engines provide extremely fast storage and retrieval of data and also contain low level logic for high volume operations like security and group membership checks.

https://docs.appian.com/suite/help/20.4/Enterprise_Architecture_Overview.html#:~:text=administration%20console%20interface.-,Appian%20Engines,security%20and%20group%20membership%20checks.