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Sessions and Memory Areas
This section describes the sessions in AS ABAP where ABAP programs are executed and the associated memory areas that can be accessed by an ABAP program. For a more general and technically more detailed description, see Documentation Memory Management in SAP Help Portal.
The following figure shows the general organization of sessions for ABAP programs and their memory areas up to the internal session level:
The application layer of an AS ABAP can be distributed across multiple application servers. If an AS ABAP has more than one application server, these are normally instanced on different computers. It is also possible, however, for multiple application servers of one or more AS ABAPs to be instanced on the same host computer. The transaction SM51 shows an overview of the application server of the current AS ABAP and which computers it is instanced on. The network name of the computer on which the current application server is instanced can be taken from the system field sy-host in an ABAP program.
Every application server has access to the standard database of the AS ABAP. Each application server has its own shared memory. The ABAP runtime environment uses the shared memory for programs, program data, and buffers (for example, in table buffering). ABAP programs can store shared objects and data clusters in the shared memory.
The shared memory is frequently used and may therefore become a sparse resource, which can result in bottlenecks when explicitly storing data from ABAP programs.
Logging on to an application server opens a user session. Logons take place using SAP GUI, the RFC interface, or ICF (including APC). A user session is assigned a standalone memory area, the user memory, which can be used as follows in ABAP programs:
The maximum number of user sessions is configured using the profile parameter rdisp/tm_max_no. The profile parameter rdisp/rfc_max_login determines which percentage of the sessions can be opened using the RFC interface.
For more information, see User Sessions.
An ABAP session is opened for each user session. Each ABAP session is assigned its own memory area of ABAP memory, in which data clusters can be stored. The programs of a call sequence share access to these clusters and the data is retained for as long as the top level transaction of the call sequence.
Further ABAP sessions for a user session can be opened as followed:
A maximum of 16 ABAP sessions are possible per user session. The number of sessions actually possible for a system is determined by the profile parameter rdisp/max_alt_modes, whose default is 6.
An internal session contains a program memory, itself divided into the areas heap and stack. This is where the modifiable objects of the program are held. The non-modifiable program data is stored in the PXA. The program memory contains references to the necessary data in the PXA. Technically speaking, the program memory and the global PXA are stored in the shared memory of the application server where they are the responsibility of SAP Memory Management.
In one ABAP session, there can be a maximum of nine internal sessions, which always belong to a call sequence. Data from the ABAP memory is always assigned to a call sequence.
On 64-bit platforms, an internal session can theoretically require up to 4 TB of memory. On 32-bit platforms, the theoretical upper limit is 4 GB. The practical upper limit is usually below the theoretical limit, since the actual physically installed main memory is the maximum available and divided up among all users.
Further information about the way ABAP programs are organized in an internal session can be found in Programs in the Internal Session.
Instances of classes, with the exception of shared objects, are located in the internal session. It is therefore not possible to store references to objects in the ABAP memory.