Many client systems have for a number of years used "in memory" or "private" databases for the engine/app layers. Private ( in memory) databases can be used for the engine and application layer databases which are effective static between releases. Private ( in memory ) databases allow for many performance enhancements and reduces downtime when upgrading servers. Private databases should not be used for data layers ( ones that we want to keep data between releases).
All the data from a stSoftware server can be fetched via the standard ReST API, all the standard ACLs and validation rules are applied whether the data is access via the standard screens or via the ReST API