McObject's e X treme DB Fusion combines on-disk and in-memory data storage in a single embedded database system, so developers can optimize applications for speed and persistence, while adopting the most cost-effective and physical space-conserving data storage. It is available in the High Availability, 64-bit and Single-Threaded editions provided by McObject, includes transaction logging, and is compatible with McObject’s e X treme SQL interface. In-memory database systems (IMDSs) such as McObject’s e X treme DB offer superior performance and the possibility of very small RAM, CPU and storage demands. IMDSs (or RAM databases) boost speed and reduce footprint by eliminating mechanical disk I/O, multiple data copies, and redundant logical processes, such as caching. In contrast, on-disk databases cache frequently requested data in memory, for faster access, but write database updates, insertions and deletes through the cache to be stored to disk. Byte-for-byte, disk storage can be cheaper than memory, and can also take less physical space. e X treme DB Fusion extends McObject’s core technology, enabling the developer to combine both database paradigms -- in-memory and on-disk -- in a single database instance. Specifying one set of data as transient (managed in memory), while choosing on-disk storage for other record types, requires a simple database schema declaration. The resulting embedded database system retains in-memory strengths (speed, small database footprint, intuitive native API, etc.), yet leverages the potential cost savings and built-in durability of an on-disk database.