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In-memory database

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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 are faster than disk-optimized databases since the internal optimization algorithms are simpler and execute fewer CPU instructions. Accessing data in memory provides faster and more predictable performance than disk. In applications where response time is critical, such as telecommunications network equipment, main memory databases are often used.[1]

ACID support

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:

  • Snapshot files, which record the state of the database at a given moment in time. These are normally generated when the MMDB does a controlled shut-down, or on request, and thus while they give a measure of persistence to the data (in that not everything is lost in the case of a system crash) they only offer partial durability (as 'recent' changes will be lost). For full durability, they will need to be supplemented by one of the following:
  • Transaction Logging, which records changes to the database in a journal file and facilitates automatic recovery of an in-memory database.
  • High Availability implementations that rely on database replication, with automatic failover to an identical standby database in the event of primary database failure. To protect against loss of data in the case of a complete system crash, replication of a MMDB is normally used in conjunction with one or more of the mechanisms listed above.

Some MMDBs allow the database schema to specify different durability requirements for selected areas of the database - thus, faster-changing data that can easily be regenerated or that has no meaning after a system shut-down would not need to be journalled for durability (though it would have to be replicated for high availability), whereas configuration information would be flagged as needing preservation.

"Hybrid" in-memory/on-disk databases

Technology emerged in 2003 when Solid Information Technology (currently IBM solidDB, acquired by IBM in 2008) released version 4.1 of their database server engine, which combined in-memory and on-disk data storage in a single database system.

The advantage to this approach 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.[2]

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 adopt a hybrid database system at lower cost, and with less code customization, than using separate in-memory and on-disk databases, respectively, for its disk-less and disk-based products.[3]

Commercial products

In recent years, main memory databases have attracted the interest of larger database vendors. TimesTen, a start-up company founded by Marie-Anne Neimat in 1996 as a spin-off from Hewlett-Packard, was acquired by Oracle Corporation in 2005. Oracle now markets this product as both a standalone database and an in-memory database cache to the Oracle database. IBM acquired SolidDB in 2008, and Microsoft is widely rumored to be launching an in-memory solution in 2009.[4]

Products

References

  1. ^ http://findarticles.com/p/articles/mi_m0EIN/is_2002_June_24/ai_87694370
  2. ^ Cole, Bernard."Hybrid embedded database merges on-disk and in-memory data management", Embedded.com, February, 2007. Retrieved on August 15, 2008.
  3. ^ Graves, Steve. "Hybrid Data Management Gets Traction In Set-Top Boxes", Embedded.com, July, 2008. Retrieved on August 15, 2008.
  4. ^ http://www.intelligententerprise.com/channels/business_intelligence/showArticle.jhtml?articleID=210700171
  5. ^ http://www.sybase.com/products/databasemanagement/adaptiveserverenterprise
  • 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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