This is the default persistence setting in ModeShape 5 and doesn't require any explicit configuration. Any repository without an explicit persistence configuration will use this and will only hold onto data only during the lifetime of the process.
For obvious reasons, it's not suitable in production systems and should only be used for testing and prototyping.
If you want, you can explicitly opt to configure this type of persistence in the JSON configuration like so:
Simply omitting it will have the same effect: