What is a database engine?

What exactly is a database engine?

• I have gone through the definition on http://en.wikipedia.org/wiki/Database_engine several times: 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. What I do not understand is what is left to do, isn't CRUD all that the databases do? If the database engine performs these functions, what does the rest of the database do?

• Answer:

  CRUD is meant to define the characteristics necessary for a database as it relates to persistent storage. It is not meant to describe everything that could be done by a database engine. To make a comparison, fundamentally a vehicle is a device used for transport. While true, this definition certainly doesn't include all the detail entailed in a modern automobile. A database engine might handle multiple users, http://en.wikipedia.org/wiki/Database_transaction, http://en.wikipedia.org/wiki/Multi-Version_Concurrency_Control, buffers and caches, http://en.wikipedia.org/wiki/ACID, as well as different http://en.wikipedia.org/wiki/Isolation_level. A read may pull data from memory, remote databases, and multiple tables on disk processing it using SQL through multiple explicit and/or implicit code paths in order to present it to the requesting application. A create may allocate storage, provision structures, assign values, and do it's own processing before storing data. Etc.