HoloDB is an RDBMS seemingly filled with random data. This data does not actually take up any space in memory or on a volume (to use an analogy, it is as if the data set is projected as a hologram from a simple configuration). The base layer is an arbitrarily large, read-only data set that is readable and searchable, and yet fully consistent. Any pieces of data and index lookups are calculated on-the-fly. An optional second layer is built on top of this, allowing read-write access (stores differences while maintains consistency and searchability). You can start an arbitrarily large database in moments, with minimal effort; all you need is a YAML configuration file or some JPA entites.