SAP SHM OVIEW documentation, setup help and example usage

ARTICLE

Shared Objects - Overview
Shared objects are instances of classes or anonymous data objects in shared memory . Named data objects can be stored as attributes of classes. The memory area of the shared memory for shared objects is the shared object memory. Its size is determined by the profile parameter
abap/shared_objects_size_MB .
Shared objects are managed with the help of areas
. You create and maintain areas and their properties using transaction SHMA .

Shared Memory-Enabled Classes
The prerequisite for storing an instance of a class in the shared memory is that the class of the object has to be defined with the SHARED MEMORY ENABLED addition of the CLASS statement, or that the property Shared memory-enabled
has to be selected in the Class Builder. Each area is linked to a global area root class , which can contain separate data and references to other instances of shared memory-enabled classes or to anonymous data objects in its attributes.

Areas and Area Instances
An area is the template for area instances in the shared memory. Several area instances can be created from one area instance, which are distinguished by name. In addition, there can be several versions of an area instance ( area instance versions ). Version IDs are used to differentiate between the versions. All area instance versions of the same area instance name make up an area instance. In the simplest case (without versioning), an area instance consists of a single area instance version.

Area Classes and Area Handles
When an area is defined in transaction SHMA , a global and final area class (with the same name) is generated as a subclass of
CL_SHM_AREA . Class CL_SHM_AREA itself is a direct subclass of CL_ABAP_MEMORY_AREA . In an ABAP program, an area is accessed exclusively using the methods of the generated area class. There are static methods for binding an ABAP program (or its internal session) to area instances in the shared memory (attach methods). Binding creates an instance of the area class as an area handle , and creates an area lock at the same time. The ABAP program can access the bound area instance version using the area handle and thus access the shared objects that are stored there. The area handle also contains the methods for deleting the connection or the lock (detach methods).

Access to Objects
The shared objects in an area instance version are addressed by references , which are obtained using the area handle. The shared objects within an area instance version can refer to each other without any restrictions. An ABAP program can contain references to shared objects provided that it is bound to an area instance version. Area instances themselves are self-contained so that they cannot contain any references to other area instance versions or be contained in an internal session. There is one exception to this rule, provided that an area instance is bound to an internal session for writing or updating. During this time, there can be references in the area instance to objects in the internal session or to other area in
stances that are similarly bound.

Note
Do not confuse area instances or area instance versions with instances of classes. In particular, an area instance is not an instance of an area class. Area instances are managed areas (the actual areas) in the shared memory in which shared objects can be stored, and that are addressed by area handles.