Special Edition Using SAP R/3, Third Edition

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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 The paging areas are accordingly organized into segments assigned to a work process when they are needed and withdrawn when they are needed elsewhere.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 file://J:\prodinfo\MEMBERS\MA\ir057.html 3/23/01
Special Edition Using SAP R/3, Third Edition - CH 3 - Exploring R/3 Architecture Page 19 of 26 memory manager can coordinate external paging through the operating system and internal paging through the dispatcher. Extended Memory Management and R/3 User Session Contexts R/3 Release 3.0 introduced an extended memory capability that permits a more intensive use of virtual memory. Reservations for roll and paging areas are no longer necessary and have been replaced by the R/3 user session context system. When a work process is conducting activities as part of a step in a user dialog, it needs ready access to a set of data elements. These elements are the user session context. If another work process is assigned to the next step in the dialog, the information now in some or all of the data elements of the first user session context must be made available to the second work process. The second user session context may also require additional information. Before Release 3.0, user session context information was transferred by the roll-in, roll-out method of copying information. This took a great deal of time and many system resources. In Release 3.0 and later releases, the data needed by subsequent user session contexts is accessed not by copying, but by mapping. The information transferred to the subsequent dialog step is not the actual data, but a set of pointers that indicate where that data can be found. Thus, a very complex data structure can be addressed or mapped for use by a work process by the transfer of a simple mapping pointer message. NOTE: Data swapping is handled by the operating system. Using mapping rather than copying in this context is expected to provide a much faster response time and, in certain installations, the possibility of handling larger loads without adding application servers. Developments in virtual memory made possible by 64-bit architecture are expected to extend the benefits of the mapping concept. Open Interfaces to Third-Party Products Three types of interfaces are used: Collectors of data about the operating system performance, the database, and the network Service interfaces to supervise backup, recovery, spooling, and security Management interfaces for handling messages and events related to application monitoring and control These systems are discussed in Part VIII, "Maintaining and Enhancing the Implementation." file://J:\prodinfo\MEMBERS\MA\ir057.html 3/23/01
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