Special Edition Using SAP R/3, Third Edition

Special Edition Using SAP R/3, Third Edition - CH 3 - Exploring R/3 Architecture Page 18 of 26
elements are not changed by any other application in the period between the dialog operations and the
consequent database updates. For example, an order accepted on the basis of material in stock relies
on that material still being available when the database is updated to mark it as reserved for the
specific order. Another ordering system cannot be allowed to reserve the same material in the interim.
The R/3 system alerts the user if there is going to be potential competition for access to the same
record. The second request is refused until the first is satisfied.
When the log record generated by the dialog is used to update the database, the update program
automatically removes all the locks that were put in place during the transaction.
Memory Management
There are two types of main memory:
Reserved main memory areas exclusively at the disposal of a particular work process
Shared main memory areas used jointly by all work processes
The reserved areas are used to store mode-specific data that needs to be retained for longer than a
single work step. Before R/3 Release 3.0, two areas are distinguished by how they are used: roll and
paging.
Roll Area of Reserved Memory
Data that is made available to a work process automatically at the beginning of a dialog step is rolled
into a reserved area of memory. At the end of the dialog step, this data is rolled out to be returned to
the database, if necessary. The memory released then is available for other processes.
The roll area of memory is reserved for the exclusive use of a specific work process while it is active.
Some of the types of information that use roll memory follow:
Data specific to the particular user, including access privileges and identification
Entry data previously collected by steps of the dialog that have already been completed
Management information required by the two processors for ABAP/4 and dynpro activities
Paging Area of Reserved Memory
The paging areas of reserved memory can be used by applications to store extensive collections of
data that may vary greatly in length. The paging areas are accordingly organized into segments
assigned to a work process when they are needed and withdrawn when they are needed elsewhere.
Managing the Roll and Page Memories
When each processing step is complete, the contents of the reserved roll and paging areas are
buffered in two shared memory pools. The shared memory areas are used as buffers for database,
program, and table operations. Both pools are within the system’s virtual address space, and the
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