Storage Definitions and Storage Classes
The %PersistentOpens in a new tab class provides the high-level interface for storing and retrieving objects in the database. The actual work of storing and loading objects is performed by what is called a storage class.
Every persistent object and every serial object uses a storage class to generate the actual methods used to store, load, and delete objects in a database. These internal methods are referred to as the storage interface. The storage interface includes methods such as %LoadData(), %SaveData(), and %DeleteData(). Applications never call these methods directly; instead they are called at the appropriate time by the methods of the persistence interface (such as %OpenId() and %Save()).
The storage class used by a persistent class is specified by a storage definition. A storage definition contains a set of keywords and values that define a storage class as well as additional parameters used by the storage interface.
A persistent class may contain more than one storage definition but only one can be active at a time. The active storage definition is specified using the StorageStrategy keyword of the class. By default, a persistent class has a single storage definition called Default.
For information on the names of the globals that store the data for a class, see Globals.
Updates to a Storage Definition
The storage definition for a class is created when the class is first compiled. Class projection, such as for SQL, occurs after compilation. If a class compiles properly and then projection fails, InterSystems IRIS does not remove the storage definition. Also, if a class is changed in such a way that might affect the storage definition, it is the responsibility of the application developer to determine if the storage definition has been updated and, if necessary, to modify the storage definition to reflect the change. See Resetting the Storage Definition.
The %Storage.Persistent Storage Class
%Storage.Persistent is the default storage class used by persistent objects. It automatically creates and maintains a default storage structure for a persistent class.
New persistent classes automatically use the %Storage.Persistent storage class. The %Storage.Persistent class lets you control certain aspects of the storage structure used for a class by means of the various keywords in the storage definition.
Refer to the Class Definition Reference for details on the various storage keywords.
Also see Extent Definitions for information on the MANAGEDEXTENT class parameter.
The %Storage.SQL Storage Class
The %Storage.SQL class is a special storage class that uses generated SQL SELECT, INSERT, UPDATE, and DELETE statements to provide object persistence.
%Storage.SQL is typically used for:
%Storage.SQL is more limited than %Storage.Persistent. Specifically, it does not automatically support schema evolution or multi-class extents.
