The document database model is used to keep state and there are a few kinds available:
diesel.db.inmem- in memory, per user
diesel.db.memshared- in memory, per app
diesel.db.col- persisted in the built-in MongoDB, available for paid accounts, depending on volume
diesel.db.postgres- persisted in a Postgres DB
diesel.db.elk- persisted in an Elastic DB
See the Default executors for details.