Enqueue Work Process in Software

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Enqueue Work Process
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The enqueue work process, also known as the lock work process, is in charge of the lock management system. It allows multiple application servers to synchronize their access to the database and maintain the data consistency. In order for the system to run in a consistent manner, it must ensure that when a transaction's dialog steps are handled by different work processes, they retain the assigned locks until the end of the transaction or the intentional release of the lock, even when switching work processes.
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Enqueue Work Process Commonly there is only one enqueue work process for a single SAP system; however there are circumstances where, for performance reasons, it might be useful to configure up to four enqueue work processes. R/3 note 127773 contains details. The profile parameter that controls the number of enqueue work processes is:
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rdisp/wp_no_enq
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The function of this work process is to protect applications from blocking among themselves during data access. For that reason, a locking/unlocking mechanism must be present. This is the function of the enqueue work process. The locks (enqueues) are managed by the enqueue work process using a lock table which resides in the main memory. When the processes receive a locking request, the enqueue work process verifies whether the requested lock object interferes with other existing entries in the lock table. The ABAP applications logic considers that data modifications are usually done when a previous reading has taken place. For that reason, the locking requests are made before the data reading requests. SAP designed the locking mechanism so that each lock not only needs to be respected by the application server executing the transaction but also by all other servers within the SAP system. The name of the SAP instance running the enqueue service is included in the common parameter profile, the DEFAULT.PFL file. The parameter is rdisp/enqname = <instance_name>, for example, rdisp/enqname = adminix_C12_00. Lock objects The lock objects are special types of objects defined in the ABAP dictionary. The blocking type can be shared (type S), exclusive (type E), or exclusive but not cumulative (type X). The exclusive locks are used to avoid parallel modification of the data, which means that exclusively locked data can be displayed or modified by only one user. With the shared mode, several users can access the same data at the same time in display mode. As soon as any user processes the data, the remaining users do not have further access to them. Locks of type exclusive but not cumulative can only be called once. So a lock request will be rejected if an exclusive lock already exists. When the lock objects are defined in the dictionary, there are two ABAP function modules automatically generated for them: one to lock the object (enqueue) and another function to unlock it (dequeue). These functions are called at the beginning and at the end of a transaction respectively. If for some reason there are problems between the locking and unlocking of an object, it remains locked until the administrator manually deletes the lock. Refer to Chap. 10 on how to proceed with the locking mechanism management. The locking object mechanism is intimately related with the SAP logical units of work (SAP LUWs).
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Update Work Process
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Update Work Process
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The update work process is in charge of executing database changes when requested by the dialog or background work processes. Figure 2 11 shows a simple scheme of how the update process works.
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Figure 2 11: Update work process. (Copyright by SAP AG.) The dialog work processes can generate database modifications with the corresponding instructions to the database server, independently of whether these work processes run on the same or different machines as the database. However, when the ABAP language element CALL FUNCTION IN UPDATE TASK is executed, it raises the order for the modification to occur in the update server. Specific update work processes then modify the database accordingly. It is recommended to have the update service on the same server as the database for better performance. However, with fast network controllers, it does not make much difference having the update server on a different host than the database. The update is an asynchronous process, which means that the update requests are processed at the moment and in the order they arrive at the work process. This makes a more homogeneous response time. The drawback is that the transaction might not have finished when another update transaction is waiting. If for any reason the update transaction cannot be completely accomplished, the user will get a system message and an express mail. Sometimes this is due to database problems, such as tablespaces becoming full and the like. If the transaction could not finish correctly, the system rolls it back. The rollback of a transaction is possible by having a separate dialog part from the update part. The dialog program first generates log records in the VBLOG table, which are then processed by the update program (run within the update process) once the dialog is finished. The log records, read by the update work process, contain all the necessary information to make the modifications. During the update phase, the database is modified. The update of a log record can have several parts, known as the update components. This division permits the system to structure the objects that make up the update transaction components according to their importance.
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Message Server An update request can contain a primary update component (V1) and several secondary ones (V2). The time critical processes are held inside the V1, the less critical within the V2. In order to be able to initiate the V2 components of the log record, the V1 component must have finished. However, the V2 components can be executed in any order and even in parallel if there are enough update processes defined. The execution of primary components (V1) corresponding to different log records can be also done in parallel using several update work processes. Before release 3.0 of R/3, there was only one type of update work process taking care of both V1 and V2 components. With the release of version 3.0, a new profile parameter was established to indicate the number of update work processes for secondary components, also. The important profile parameter is rdisp/vbname = <instance name>. This is a common parameter for the full SAP system and therefore is normally in the DEFAULT.PFL file. The other parameters, rdisp/wp_no_vb and rdisp/wp_no_vb2, indicate the number of update work processes of types V1 and V2, respectively. These are defined inside the instance specific profile parameter file. If there are error situations during the update, these cannot be solved with user online actions. The active update process component is then stopped. If the errors occurred in the primary component (V1) of a log record, the modifications are rolled back. The log record receives a corresponding status flag and is not taken out of the VBLOG table. Subsequent V2 update actions are not executed. However, if the interrupted or error component is a type V2, only the modifications done by this particular component are rolled back. The corresponding log record is marked with a status flag and is not deleted from the table. The other components can follow normal update processing. After an error situation or update interruption, the system automatically notifies the user by express mail about the aborted update and creates an error log entry in the system log. Then it is possible to evaluate and treat the update according to the error message received. Refer to the section entitled "Monitoring Update Records" in Chap. 10 for how to proceed under such circumstances.
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