Create, read, update and delete are the four basic functions of persistent storage.
CRUD operations are idempotent, meaning that multiple applications of the same operation have the same effect on a storage as a single application.
Storage management is the direct manipulation of the contents of storage locations by users.
Data can be put in a location of a storage. The fundamental feature of a storage location is that it has a readable and updatable content (state). These read and update operations are two basic operations on a storage and re known as the load-update pair (LUP).
Before a storage location can be read or updated, it needs to be available. A storage location can be made either available or unavailable for usage. These create and delete operations are the two other basic operations on a storage.
Together they make up the four basic operations of storage management.
Each letter in the acronym can map to a standard SQL statement, HTTP method or DDS operation.
| CRUD | SQL | HTTP | DDS |
|---|---|---|---|
| Create | Insert | PUT | write |
| Read | Select | GET | read |
| Update | Update | PUT | write |
| Delete | Delete | DELETE | dispose |
Although a relational database provides a common persistence layer in software applications, numerous other persistence layers exist. CRUD functionality can for example be implemented with object databases, XML databases, flat text files, or custom file formats.
The POST method in HTTP is not a CRUD operation as GET, PUT and DELETE, which have storage management semantics, meaning that they let user agents directly manipulate the states of starget resources. It is a process operation that has target-resource-specific semantics, so it does not let user agents directly manipulate the states of target resources. Contrary to CRUD operations, POST method is not necessarily idempotent.