Database engine

A database engine (or storage engine) is the underlying software component that a database management system (DBMS) uses to create, read, update and delete (CRUD) data from a database. Most database management systems include their own application programming interface (API) that allows the user to interact with their underlying engine without going through the user interface of the DBMS.

The term "database engine" is frequently used interchangeably with "database server" or "database management system". A 'database instance' refers to the processes and memory structures of the running database engine.

Many of the modern DBMS support multiple storage engines within the same database. For example, MySQL supports InnoDB as well as MyISAM.

Some storage engines are transactional.

Additional engine types include:

Database bits are laid out in storage in data structures and groupings that can take advantage of both known effective algorithms to retrieve and manipulate them and the storage own properties. Typically the storage itself is designed to meet requirements of various areas that extensively utilize storage, including databases. A DBMS in operation always simultaneously utilizes several storage types (e.g., memory, and external storage), with respective layout methods.

In principle the database storage can be viewed as a linear address space, where every bit of data has its unique address in this address space. In practice, only a very small percentage of addresses are kept as initial reference points (which also requires storage); most data is accessed by indirection using displacement calculations (distance in bits from the reference points) and data structures which define access paths (using pointers) to all needed data in an effective manner, optimized for the needed data access operations.

A database, while in operation, resides simultaneously in several types of storage, forming a storage hierarchy. By the nature of contemporary computers most of the database part inside a computer that hosts the DBMS resides (partially replicated) in volatile storage. Data (pieces of the database) that are being processed/manipulated reside inside a processor, possibly in processor's caches. These data are being read from/written to memory, typically through a computer bus (so far typically volatile storage components). Computer memory is communicating data (transferred to/from) external storage, typically through standard storage interfaces or networks (e.g., fibre channel, iSCSI). A storage array, a common external storage unit, typically has storage hierarchy of its own, from a fast cache, typically consisting of (volatile and fast) DRAM, which is connected (again via standard interfaces) to drives, possibly with different speeds, like flash drives and magnetic disk drives (non-volatile). The drives may be connected to magnetic tapes, on which typically the least active parts of a large database may reside, or database backup generations.

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