An in-memory database (IMDB; also main memory database system or MMDB) is a database management system that primarily relies on main memory for computer data storage. It is contrasted with database management systems which employ a disk storage mechanism. Main memory databases can be faster than databases which rely on disk storage because the MMDB does not have to perform disk I/O to update or query its data.
In their simplest form, main memory databases store data on volatile memory devices. These devices lose all stored information when the device loses power or is reset. In this case, MMDBs can be said to lack support for the durability portion of the ACID properties. Volatile memory-based MMDBs can, and often do, support the other three ACID properties of atomicity, consistency and isolation.
Many MMDBs add durability via the following mechanisms:
Transaction Logging, which records changes to the database in a journal file and facilitates automatic recovery of an in-memory database.
Non-volatile RAM, usually in the form of static RAM backed up with battery power (battery RAM), or an electrically erasable programmable ROM (EEPROM). With this storage, the MMDB system can recover the data store from its last consistent state upon reboot.
High Availability implementations that rely on database replication, with automatic failover to an identical standby database in the event of primary database failure.
"Hybrid" In-Memory/On-Disk Databases
Technology emerged in approximately 2007 that combined in-memory and on-disk data storage in a single database system. The advantage to this approach, as stated by companies including McObject, Raima or Altibase, is flexibility: the developer can strike a balance between performance (which is enhanced by sorting, storing and retrieving specified data entirely in memory, rather than going to disk); cost, because a less expensive hard disk can be substituted for more memory; persistence; and form factor, because RAM chips cannot approach the density of a small hard drive.[1]
Manufacturing efficiency is another reason a combined in-memory/on-disk database system may be chosen. Some device product lines, especially in consumer electronics, include some units with permanent storage, and others that rely on memory for storage (set-top boxes, for example). If such devices require a database system, a manufacturer can adapt a hybrid database system at lower cost, and with less code customization, than using separate in-memory and on-disk databases, repectively, for its disk-less and disk-based products.[2]
Jack Belzer: Encyclopedia of Computer Science and Technology - Volume 14: Very Large Data Base Systems to Zero-Memory and Markov Information Source, Marcel Dekker Inc., ISBN 0-8247-2214-0
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