Recently, we published a Market Guide for In-Memory Computing. The document covers all forms of IMC, including Database Management Systems (DBMS). Gartner defines In-Memory Computing (IMC) as a computing style where applications assume all the data required for processing is located in the main memory of their computing environment. Although we define many styles of IMC (Application Servers, Data Grids, Messaging and Complex Event Processing), I want to concentrate specifically on DBMS technology in-memory. Why? There appears to be some level of misconception about what does and does not qualify as an In-Memory DBMS (IMDBMS).
Our definition of IMDBMS requires the database structure to be in-memory, specifically the main memory of the server. Data in the database is accessed through instructions for accessing memory and not using I/O instructions. This should not be confused with products that buffer data in a disk-block cache. Disk-block caching has been used in the industry for many years, pre-dating relational technology. For example, IBM’s IMS DBMS was, from its introduction in 1968, able to cache data in memory, also referred to as pre-fetch or read-ahead; however, it is not an IMDBMS. While we agree that caching does improve performance, over accessing disk or flash, it is not IMC.
One major difference between traditional disk-based DBMS engines and IMDBMS is the implementation of the consistency model. IMDBMS covers all DBMS consistency models from ACID consistency to eventually consistent models, the latter found in many of the noSQL DBMS engines. However, regardless of the consistency model, a commit operation will be performed. Disk-based systems, even if all the data is cached in memory buffers, require the transaction to be written to disk or flash. Regardless of the length of time taken to perform this operation, it is greater than zero. With IMDBMS products, the commit operation takes place in memory. Although this requires unique methods or assuring the persistence of the data, due to the volatility of memory, such as synchronous writing of data to a second server using Remote Direct Memory Access (RDMA), the latency is less than writing to external media. This illustrates why the performance of IMDBMS is higher, even over using a disk-block buffer.
With our precise definition of true IMDBMS, we seek to dissipate the hype in the market over IMDBMS and claims made by some vendors that their technology is IMDBMS when, in fact, it is not.