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Buffer Synchronization in Distributed Configurations
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When working in the R/3 system, changes made by users to the information they are working with are not immediately updated in the database. This is particularly true in distributed configurations. In distributed configurations, that is, with several application servers belonging to the same SAP system, every SAP instance has its own buffers. This could result in data inconsistency when users work on the same data from different servers. A locking 103
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Central and Local Storage of Executable Files under UNIX mechanism for data access is provided by the enqueue server. To avoid that, the SAP system provides a way to synchronize the buffers across the application severs. This is known as buffer synchronization, or buffer refresh. If the R/3 system is centralized and is made up of only one SAP instance, then buffer refresh is not needed. Data buffered in the SAP system includes many important tables, programs, screens, and so on. In buffer synchronization, when the system receives any modifying action in the data that is buffered and may have been buffered by another application server, the SAP systems sends synchronization telegrams, which are written in the central database table DDLOG. All application servers read from the DDLOG table to check whether their own buffers must be refreshed. R/3 includes some parameters to control the buffer synchronization. These parameters are rdisp/bufreftime and rdisp/bufrefmode. The rdisp / bufreftime parameter sets the value in seconds for the time between two synchronizations. The rdisp/bufrefmode parameter affects the mode in which the buffers are refreshed. It admits two values, which are sendon / sendoff, and exeauto/exeoff. The first set of values, sendon / sendoff, controls whether synchronization is active (sendon) or not (sendoff). The second set controls whether the central instance reads the DDLOG table (exeauto) or not (exeoff). SAP recommends that when using the transport control program (tp) to perform imports in a centralized SAP system with just one server, to set the buffer mode parameter to rdisp/bufrefmode = sendoff, exeauto. If it is set to exeoff, the instance will not read the DDLOG table, so the changes to repository objects performed by the tp program will not be updated in the repository buffers. Under some circumstances this can cause syntax error messages because the ABAP processor can detect a new version of a program before the buffers are correctly updated. The buffers are completely erased and reconstructed when restarting the SAP instances. There are also some special commands which can be entered in the command field of the R/3 window to synchronize the buffers in the application server where these commands are issued. For example, entering $TAB in the command field refreshes the table buffers, and entering $SYNC refreshes all buffers, except for the program buffer. Warning Handle these options with care and only when instructed by the SAP hotline or an R/3 note. Refreshing the buffers when users are working with buffered tables, programs, or screens might cause some inconsistencies and a decrease in performance.
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When implementing a distributed SAP system consisting of several application servers you have to decide whether to install the SAP executable files either locally on the hard disk of every application server or in a central repository of executables from a single server. There is also the option of having some servers share the executable through NFS and others store them locally. In a standard SAP installation the executables directory is stored on a central host system, usually the central instance. The UNIX directory is /usr/sap/<SID>/SYS/exe/run, where <SID> is the SAP system name. Then, when installing additional application servers (instances), these servers usually share the central directory over the network using standard NFS (network file system). But, after the initial installation, the administrators can decide to store the SAP executables on the local hard disks of the application servers. In 104
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Starting and Stopping SAP R/3 these cases, a special program sapcpe is used to ensure that the instance updates the executables program files when these are updated in the central server. SAP recommends storing the executables locally if there is enough local hard disk space available. Depending on the SAP release version and options installed, the needed size is approximately 400 MB. (Look this information up in the SAP installation manuals.) Each option has its advantages and disadvantages. The choice depends on the hardware, the network, and the operational needs. Central storage of executables presents the following advantages: There is no need for additional storage space on application servers. When there are new or updated executable SAP programs, placing them in the central location makes them immediately available for all application servers. There is no need to maintain and configure the sapcpe program and associated files to synchronize the updated executable files. When the sapcpe program is used, the initial startup of instances is slow because of directories synchronization. The disadvantages of central storage are The network load between the application server and the central server is increased and therefore can result in a decrease in performance, both in the application servers and the central servers. The startup of instances is slower because the executables are loaded through the network. You have to carefully maintain the NFS configuration and daemons, since, if the central server fails the export process for any reason, the program will be unavailable for all servers. If an application server has to make any program swap with active executables because they run out of memory, they actually page over the network, considerably decreasing the system performance. Based on these criteria, you should decide according to the specific needs of your installation. For example, if all servers are connected with a high speed network such as FDDI, FC, ATM, or similar, and they all have enough memory, then having the executables centrally located should not be a major problem. On the other hand, if you have enough local storage space, place the executables on the hard disks of the application servers. If you have servers with less space, you can have a mixed environment. To do this, you must prepare the executables directories and configure the sapcpe program. Defining a master location from which the other servers will update their local files in case there are new or updated programs is required for storing locally. The actual procedure to set up the directories and the sapcpe program can be found in the SAP online documentation in the system services section within the system administration help books. Windows NT systems all share the same copy of executables, located on the \\<hostname>\sapmnt directory and normally installed on the central instance.
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Starting the SAP R/3 system involves starting the underlying database and all the SAP processes configured to run in all application servers. The type and number of processes are configurable with the start profile and 105
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Starting and Stopping SAP R/3 the instance profile parameters. These processes might include the following: The operating system and/or network performance collectors The central system log collection process The CPIC gateway server The message server The dispatcher processes The spool processes The dialog and background processes The SAP R/3 system can be started and stopped by using operating system commands or from within the CCMS utilities. However, for the latter, at least the database server and the central instance must have been started first using the operating system startup commands. In current releases, the database system is not stopped from within R/3 either. In centralized installations, with just one single server, one start and one stop command are enough for starting or stopping the whole system. However, in distributed configurations, some configuration is needed to start and stop the group of application servers of a SAP system. Starting the SAP system first requires starting the database and then the instance processes. Stopping is the opposite process: first you have to stop the instance processes and then the database background processes. For example, you can write a shell script command file that can start the whole system from a single server. In these cases, many people use remote shell commands to execute the start programs in remote computers. Stopping can be done the same way. Remember that using remote commands (for example, rsh, remsh, or similar) can be a security violation in some systems because a list of permitted hosts is necessary. For this, check with your security manager. To start or stop the SAP system in a UNIX environment, you must log on as user <sid>adm, for example, for SAP system DD1, as user dd1adm. The following commands are available. Note The brackets indicate optional parameters where you can choose just one from the list or none at all. 1. startsap [R3] [DB] [ALL] Using the command, startsap R3, only the SAP instance is started. It is assumed that the database is already running. Otherwise, the instance will not start successfully. With the command, startsap DB, only the database is started. Using startsap ALL, the system will first start the database and then the SAP instance. ALL is the default setting and can be omitted. If the database is running, it will just start the instance. 2. stopsap [DB] [R3] [ALL] Using stopsap R3, all the instance processes are stopped. With the command, stopsap DB, the system stops just the database. Make sure you first stop the instance processes; otherwise, the SAP processes will "hang" because no update is possible. Issuing the command, stopsap [ALL], the system stops the SAP instance and then the database. ALL is the default parameter and can be omitted. When in distributed SAP installations with several application servers, pay attention to stopping all the instances before stopping the database, which is only located in the database server. 106
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Starting and Stopping SAP R/3 To check if the system has been correctly started or stopped, you can use standard UNIX operating system utilities such as the ps command. From the UNIX system, the SAP processes are prefixed by dw, so, for example, issuing the command
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